Programmer 7500/MAX_PROGRAMMERS.iso/PROGRAMS/UTILS/BENCHMAR/TIMING.ZIP/TIMING.GVG

Text File | 1988-04-19 | 58.9 KB | 1,459 lines

NTSC Studio Timing: Principles and Applications

Copyright Grass Valley Group, Inc. 1987

Editorial Staff: Mike Guess & David Colborn

The most critical design in every teleproduction facility is the system
timing.  The final video product will always reflect the quality of the
system design.  This booklet will review the principles of video, discuss
system timing, and offer approaches to system timing design.  A
definition of subcarrier to horizontal phase and an explanation of how to
achieve and maintain SC/H phase is also included.

SECTION 1

Video Basics

The Camera and Pickup Tube

Light from a scene enters the camera through the lens and creates a
pattern of electrical charges on the pickup tube's target.  An electron
beam scans across the target and completes an electrical  circuit with the
pattern of electrical charges on the target. Electrons representing the
scene in lightness or darkness flow from the target and become the video
signal.  In this way, the pickup tube inside the camera changes the varying
brightnesses of light that it "sees" into varying electrical voltages called
video.

Scanning

In order to accurately reproduce a scene, the scanning must be done in an
organized way.  In both the camera and the television receiver, the
scanning of the target or screen is done by an electron beam moving in
horizontal lines across the target plate or screen.  At the same time, the
electron beam gradually moves down the scene.  Between horizontal scans
the beam returns to the left side of the viewer's screen (called "horizontal
retrace" or "line flyback").  When the beam reaches the bottom of the
scene, the beam is sent back to the top (called "vertical retrace" or "field
flyback").  There are 525 horizontal lines in a complete picture.


Fields and Frames

Each scan of the scene is called a field and only involves half of the total
525 lines or 262.5 lines.  Two complete scans of the scene (525 lines) is
called a frame.  Because the fields are scanned in rapid sequence (60 per
second), the viewer only perceives the completed picture.

Field one is scanned as the beam moves from the top to the bottom of the
screen.  The beam is then quickly returned to the top of the screen and
field two is scanned.  The lines of the second field are "interleaved"
between the lines of the first field.  This is called "interlaced scanning".

By the way, vertical retrace ( field flyback ) takes a certain amount of
time so the actual number of lines displayed on the screen is less than
525.


Horizontal and Vertical Sync

For accurate reproduction, both the camera and the television receiver
must be synchronized to scan the same part of the scene at the same time.
At the end of each horizontal line the beam must return to the left side of
the scene. This is called "horizontal retrace".  Coordination of the
horizontal retrace is handled by the horizontal sync pulse.

At the bottom of the scene, when 262.5 horizontal lines have been
scanned, it is time for the beam to return to the top of the scene.  The
start of vertical retrace is signaled by the vertical sync pulse which is
different in width than horizontal sync pulses.  Since the vertical retrace
takes much longer than the horizontal retrace, a longer vertical
synchronizing interval is employed.


Blanking

During the time when horizontal and vertical retrace are taking place, the
electron beams in the camera and home TV are cut off.  This time period is
called blanking.  Blanking means that nothing will be written on the
television receiver screen.

During horizontal blanking, sync and "burst" (to be described in more detail
later) occur.  During vertical blanking, vertical sync, vertical equalizing
pulses, and vertical serrations occur. The equalizing pulses are inserted to
cause the video fields to begin at the proper points to achieve interlace.
The vertical serrations keep the television receiver's horizontal sync
circuitry from drifting off frequency during the time when no horizontal
picture information is present.



Black and White Vs. Color

Black and white (monochrome) television was the first system to be
successfully transmitted and many television sets had been purchased by
the time color was being considered.  One of the constraints placed on the
new color system was that it had to be compatible with the monochrome
system.  Everything that we have considered so far is applicable to both
monochrome and color television systems.  Both systems use interlace
scanning, synchronizing, and blanking pulses.

In the illustration above you will recognize the sync pulses and the active
video that we have already discussed.  You will also see a small segment
identified as "burst".  We will be looking into the color system, along with
the concept "burst", in the next few pages of the Video Basics section.

Carrier Waves

Ever since radio was invented, a "carrier" wave has been used to "carry"
electrical information through the atmosphere.  A carrier wave is a signal
that goes up and down in voltage very rapidly and evenly.  One complete up
and down is a "cycle".

The carrier wave can be sent through the air for considerable distances
and it can be easily picked up by a receiver like a radio or TV.

 The carrier wave is somewhat like a rapid and exaggerated ocean wave

Subcarrier

In the video signal, a subcarrier wave is included to carry the color
information.  The subcarrier, with its color information, is combined with
the black and white information and together they modulate (impress) the
main carrier.

The subcarrier has a particular frequency (cycles up and down a certain
number of times per second) and that frequency is abbreviated as 3.57MHz
(Mega Hertz).  Mega means a million times and Hertz means per second.


Encoding Color Onto the Subcarriers

The color signal is composed of luminance (Y), red (R), green (G), and blue
(B).  To make the total video signal more compact for broadcasting
purposes, the Y, R, G, and B components of the color signal are combined as
Y and two color difference signals called R-Y (V) and B-Y (U).  Each color
difference signal contains the information for hue (color) and saturation
(brightness of the color).  The amplitude values of the two color
difference signals are modulated onto two subcarriers which have the
same frequency but are 90 degrees apart in phase (see section on Phase
below).  These two modulated subcarriers are then further combined to
form one chrominance signal that changes in amplitude and phase (see
illustration in margin).  The final transmitted signal contains both a
lumininance (Y) and a chrominance component.  The original Y, R, G, & B
components of the scene are decoded by the television receiver from the
transmitted signals.


Phase

For our purposes, phase relates to a time comparison between two signals.
For example, signal A starts at zero microseconds and signal B begins
several microseconds later.  Both signals are a sine wave like the carrier
wave on the previous page.  A sine wave travels through a path that is
described as 360 degrees (see sidebar below).

Signal A's starting point is 0 degrees and, depending on the time lag for
the start of signal B, signal A's phase relationship to signal B is described
in terms of degrees.  In the sidebar below the phase relationship between
the first signal and the second is 90 degrees.  It is this phase relationship
between two signals that is used to derive the color information in the
television system.


Burst/Color Burst

Burst or color burst is produced from a small section of the 3.57MHz
subcarrier wave and is transmitted separately as the reference for the
color information (hue).  The subcarrier is first subjected to a process
called "phase splitting" which splits it into two signals, E'V and E'U, that
have a 90 degree phase difference but an equal amplitude.

E'U is permanently inverted to -E'U.  Components -E'U and E'V are summed
to form burst. Burst has a resultant degree position of 135 degrees.

Burst then acts as a constant reference.  The chrominance signal is
compared to burst to determine the exact color that is being transmitted.
The color is determined by the number of degrees the hue information is
shifted in relation to reference burst.

Burst is placed, as we've seen before, in the horizontal blanking interval
following sync.  When a video signal contains burst and horizontal and
vertical sync, it is said to be "composite video".


Summary

All you have learned so far has stressed the point that all the equipment
used in a television system must be synchronized together.  This is
achieved by feeding each item one or more reference signals from a sync
pulse generator (SPG).  When used as a master timing reference, the SPG
must meet the performance specifications as required by the governing
body.  (In the case of the NTSC system,  this is the National Television
Standards Committee.)  The SPG must also be extremely reliable.  Many
studios will use two SPGs combined with an automatic changeover switch
that will switch to the backup SPG if the master should fail.

The outputs of sync pulse generators are as follows:

Mixed Syncs - Also known as Comp Sync or just Sync

This signal consists of combined horizontal and vertical synchronizing
pulses.


Mixed Blanking - Comp Blanking or Blanking

Combined horizontal and vertical blanking pulses.

Horizontal Drive - Line Drive

A signal that can be used to trigger the horizontal flyback in older
cameras.

Vertical Drive - Field Drive

A pulse that can be used to trigger the vertical flyback in older cameras.

Burst Gate - Burst Flag

Pulse used to gate the burst onto the color black output.

Note:  All pulses should be selectable for 2 or 4 volt amplitude

Subcarrier - SC

A 3.57MHz sine wave that is used  as a phasing reference for chrominance
signals.  Subcarrier should be selectable between 1 or 2 volts.


Color Black - Black Burst

A signal that contains mixed syncs (H and V) and burst.  The color black
signal should have provision for a white flag that indicates line one of
field one.  This is explained in more detail in the SC/H Phase section.

Nearly all equipment designed today is designed to lock to color black.
This equipment will have controls for horizontal timing and subcarrier
phasing and also provide the system designer with the opportunity to
design flexible and expandable television systems.

SECTION 2

System Timing

It is imperative that all video signals arrive at the video switcher (the
central combining point) in synchronization. This means that the scanning
sequence of every source must start and stay in time.  Without this, the
picture on the television receiver or monitor will roll, jump, tear, and/or
have incorrect colors when the source video signals  are combined. Careful
system design is necessary to assure synchronization at the point of input
to a video switcher.

The degree of accuracy with which these events must occur requires a
precision reference.  In all television facilities, this timing reference is
provided by a synchronizing pulse generator.  Establishing and maintaining
precise timing involves a multitude of variables that will be described in
detail in this booklet.

Advance and Delay

Defining advance or delay between two video signals is dependent on
which signal is defined as the reference. Advance on Camera 1 means its
output occurs earlier in time than Camera 2's output. If viewed from the
other perspective, Camera 2 is delayed when referenced to Camera 1.

It must be understood that advance is not  really possible.  Advance or
negative time delay does not exist.  Video  signals take time to move just
as you and I do.  A marathon  runner wins because he had the least delay in
his running time.  On the other hand, he is the most advanced at the finish
line,  but only because the other runners had more delay in their running
times. Video frame synchronizers make video advance  appear possible,
but in reality they introduce delay to achieve the apparent advance.  This
is proven by the fact that the audio  associated with the video going
through a frame synchronizer must also be delayed to avoid lip-sync
errors.


Studio Planning

Before the actual assembly of a teleproduction facility can  begin, a
system plan must be completed.  This can only be  accomplished upon
definition of studio timing requirements.  It  will be necessary to know
the timing requirements of the  equipment to be installed.  This
information is usually available from the manufacturer's published
specifications.

Most newer source equipment locks to color black.  This  implies the
device has its own internal sync generator.  Typically this source
equipment will have adjustments to allow the video  output timing to be
adjusted relative to the reference color  black.  You should verify that the
adjustment range is sufficient for your requirements.


Planning For Timing Advances

The ability to lock to color black has not always existed.  In the early
years of television, cameras needed separate  horizontal and vertical
drive pulses from the sync generator to drive their scanning circuits.
Sync, blanking, and subcarrier were also needed.  System design required
that all drive pulses be advanced by the path length of the camera.  The
delay from pulse input to video output may have been as long as one
microsecond (a very long delay).

These older cameras would receive pulses directly from the sync
generator.  Drive pulses to other pieces of source equipment  would then
have to be delayed to time that equipment.  This delay  could be several
hundred feet of coaxial cable or some equivalent lumped delay.

There are still cameras in use today that require sync, blanking and
subcarrier (horizontal and vertical drive are now virtually obsolete).
These cameras have no internal timing adjustments so it is necessary to
adjust the advanced pulse drives to time the camera.  One way to resolve
this timing  requirement is to drive the camera with a source
synchronizing generator.  New cameras lock to color black and have
internal timing adjustments available.

Until now, most character generators have required pulse drives and
external adjustment of timing.  This is often done by dedicating a source
synchronizing generator to the character generator. Newer character
generator models, like other devices, are beginning to lock to color black.

Digital video devices such as digital effects generators, time base
correctors, and frame synchronizers work on the basis of storing digital
video data.  This allows timing to be easily adjusted and as such, digital
video devices are inherently able to time internally.  Color black locking is
very common.

Nearly all production switchers require sync, blanking and subcarrier.
Some switchers have some limited adjustment of horizontal (H) delay but
still require advanced pulse drives.  Subcarrier phasing is normally built
in and allows for color timing of the switcher.  Dedication of a source
synchronizing generator to a switcher will simplify system design.  Some
switcher designs now incorporate color black locking.


Planning For Timing Delays

Coaxial cable is necessary for the proper distribution of video, pulse and
subcarrier signals.  Coax has an inherent  delay of up to 1.5 nanoseconds
per foot. This is cumulative and must be considered in system design. Very
long runs can  introduce significant delay.  Coaxial cable can be used for
delay  but it should be remembered that coax introduces frequency
response loss that increases with frequency and length.

Distribution amplifiers (DAs) introduce delay that will need to be planned
for. This can vary from 25 to 70 nanoseconds depending on  the model.
Variable cable equalization adjustment will also affect electrical delay.
Equalization should be adjusted prior to final system timing. Special
purpose video distribution amplifiers are available to provide delay
beyond 1 microsecond. These should be used because they have frequency
response compensation that is superior to coax and  passive video delay
lines. Pulse DAs are available to allow for adjustment of pulse delay of up
to 4 microseconds and regenerate the pulse to eliminate distortion.

Video processing amplifiers have a fixed electrical path  length even
though regenerated sync and color burst are adjustable. The propagation
delay of the GVG 3240 Video Processing Amplifier is about 225
nanoseconds.

Sometimes multiple studio facilities have the output of one  switcher
feeding a second and both share some common video  sources.  In this
instance, the common video sources to the second switcher will need to be
delayed by the path length of the first switcher. This delay may be as
little as 50 nanoseconds for a small routing switcher to 700 nanoseconds
for a large production  switcher.


Final Considerations

There are products available to aid in system design.  One such product is
the 3230 Isophasing System.  This is an automatic  delay distribution
amplifier that will correct source timing  errors of up to 115nS.  The
Isophasing System can provide  up to 32 channels with 5 outputs each,
keeping all outputs within one degree of subcarrier phase.  The 3230
simplifies  system design and daily system maintenance.

Once all the timing requirements of the equipment are known, you can
begin to actually  lay out a system plan on paper.  It is important  that a
specific piece of equipment be defined as the zero timing point.  It will
become the timing reference by which all calculations and measurements
are made.  It is desirable to have this be a source in the plant that is not
easily altered, such as the test output from the master reference sync
generator.

It should be remembered that all equipment signal levels and frequency
responses must be correct before the timing process is begun.


System Design Using Delay

The illustration below shows a small system that will use cumulative
delay to achieve system timing.  This system consists of a camera, a
character generator, two 1/2" and one 3/4" video cassette  recorders
(VCRs). All of the video cassette recorders have time base correctors that
lock to the color black reference.  The color black signals come from the
master synchronizing generator and is distributed by a DA.  These time
base correctors provide ample timing adjustment for the VCRs.

The sources in this system that are to be mixed, keyed or wiped with the
video switcher must be exactly in time at the  switcher input.  Hence the
obvious point of reference for this system is at the switcher input. This
point is therefore designated the zero timing point, or time 0.

In the illustration below the timing requirements of the equipment have
been  plotted relative to Time Zero.

Camera 1 has 850 nanoseconds delay from its composite sync input  to its
composite video output and represents the longest signal path of any
source device in the system.  The character generator, switcher, and color
bars will need delay added to make their total delays the same as the
camera.  Since the camera has the longest path length, the pulse drives
will be provided directly from the sync generator so that the camera gets
the most advanced pulses.  The camera has a subcarrier phase control for
color timing adjustment. The Camera 1 output  becomes the reference
input at the switcher.

To make the video switcher internal color black and color background
generator synchronize with the camera, both sync and blanking drives
must be delayed to the switcher by 400 nanoseconds. This is accomplished
with two adjustable pulse delay distribution amplifiers.  The switcher has
a subcarrier phase control for color timing adjustment.

Timing of the character generator can be handled in two ways.  Delay can
be introduced either in the pulse drives, or in the video and key outputs of
the character generator.  In this system, video delay  distribution
amplifiers are added to the character generator video and key outputs.
This method provides six timed outputs.  The amount of delay necessary is
250 nanoseconds as calculated in the illustration below.

The last source to be timed is the color bars from the  master sync
generator. The color bar output is 30 nanoseconds later than the sync
output from the sync generator. With the camera as a reference, we can
calculate that 820 nanoseconds delay  to the color bar output is required
to match the camera's delay.

The sync and subcarrier required as external reference inputs for the
video processing amplifier should come from the distribution amplifiers
feeding the switcher.  The video processing amplifier has sufficient
timing range for both sync and subcarrier.

The sync generator is a known SC/H phased source, and the color bar
output will be SC/H phase correct.  Fine system timing can now begin by
adjusting the color bars and the camera.  Measurements are made at the
switcher output by selecting between the reference source and the source
under adjustment on the switcher.  An externally locked waveform
monitor and vectorscope should be connected to the switcher output.

The following steps, in this order, are necessary to ensure correct timing
and SC/H phase of all sources.

The first step will be to adjust the color bar delay DA. Adjust so that
the timing of the half amplitude 50% point of the color bar horizontal sync
leading edges match the timing of the camera sync.  A timing match
within 10nS is desirable.


Camera 1 subcarrier phase needs to be adjusted to match its burst phase
to the color bar burst phase.

  The switcher sync and blanking pulse delay DAs must be adjusted so
that the switcher color background sync 50% point and blanking are in
time with the sync and blanking of Camera 1's output.

Switcher color timing (internal color black and background) is
matched to Camera 1 with the switcher subcarrier phase control.

The character generator video delay DA should be adjusted to match
the character generator and Camera 1 horizontal sync leading edges.

Adjust the internal subcarrier phase to color time the character
generator.

The key delay will be adjusted to center the character generator fill
video within the hole produced by the key signal.

Finally, adjust the VCR time base corrector H and SC phase controls to
match each VCR to Camera 1 at the switcher.

The procedure will result in all sources being SC/H phase correct only if
the color bar video signal is SC/H phase correct.  If an SC/H phase meter
is available, the SC/H phase of all sources can be verified. This approach
to system design is usually the least  expensive but does have serious
deficiencies. We are distributing sync and subcarrier to equipment through
many different paths.  This will make establishing and maintaining SC/H
phase very difficult.  With the many variables in this system, SC/H phase
may drift with time and temperature.  Additional source equipment may be
difficult to integrate in the  future and could require major system design
changes.


System Design Using Source

Synchronizing Generators

Most of the difficulties encountered in system design can be  avoided with
a master/source sync generator system.  This system  provides maximum
flexibility and the best SC/H phase stability.  The approach below will be
used with the same equipment employed in the previous delay system.

This time, rather than using the camera as the reference at the  switcher
input, the master synchronizing generator's color bars will be used. These
color bars are fixed in their time relationship to the other outputs of the
master sync  generator and thus make a rock solid, SC/H phase-correct
reference. All the sources still need to be in exact time at the  switcher
input. This time SC/H phased pulse drives will be provided to the camera
and character generator by their own dedicated source sync generators.

The source synchronizing generator has the convenience of a  single-line
locking signal and output advance or delay relative to the lock reference
provided.  This results in a much simpler system to design and maintain
that uses far less cabling.  There is also redundancy in the system since
the source sync generators will continue to freerun if the master should
fail.

Camera 1 still requires drives which are advanced 850 nanoseconds to
produce a timed, composite video output, but this advance will now come
from the source synchronizing generator.  The same is true for the
character generator and video switcher, provided they each have a
dedicated source synchronizing generator.

Final system timing is now a matter of looking at the switcher output and
comparing each of the sources to the master sync generator's color bars.
Each source sync generator is adjusted to time  the source it is driving.  If
the source device has a subcarrier phase control built in, you should
adjust horizontal phase using the source sync generator and subcarrier
with the source device's SC phase control. This will establish correct
SC/H phase and afterwards only the source sync generator should need
adjustment. A SC/H phase meter will allow the source to be SC/H phased
prior to adjustment of the source synchronizing generator for final timing.

Sync and subcarrier for the 3240 Video Processing Amplifier  should come
from the switcher source sync generator.  The source  synchronizing
generator on the video switcher could be removed  and the video switcher
and processor could be driven directly from the master sync generator.
This would require that about 430  nanoseconds of delay be placed in the
color bar path going to the  switcher.  This is the amount of delay required
to generate  switcher color black and background from the applied drives.


Master/Source Reference Selection

The single line reference signal for this master/source  synchronizing
generator system can be color black or encoded  subcarrier.  Grass Valley
Group developed encoded subcarrier  to improve and simplify the locking
of source synchronizing generators.  The encoded subcarrier signal
consists of a continuous 3.579545MHz sine wave which contains two
phase inverted cycles, once per color frame.  This brief phase inversion is
very precisely positioned on the front porch of blanking preceding line 11
on field 1 of the four field sequence.  The phase inversion thus
communicates horizontal, vertical, and color frame information to the
source synchronizing generators.  Encoded subcarrier provides a number of
advantages over color black as a locking signal.  Subcarrier does not have
to be regenerated form the  periodic color burst, so jitter becomes much
less of a problem.  Non-ambiguous color frame lock is guaranteed.  Since
encoded subcarrier is a single frequency, the group delay problem
encountered with color black traveling through coaxial cables does not
exist.  (Group delay will cause large SC/H phase errors if the coax is not
equalized, and some SC/H phase error even when the coax is properly
equalized.)

A color black reference sync generator must first regenerate subcarrier
from the color burst.  Jitter can result if this is not done precisely.
Second, it must very precisely compare the regenerated subcarrier with
the exact 50% point on the leading edge of horizontal and vertical sync to
determine color frame.  If this process is not done precisely, the result
may be SC/H phase instability, jitter, and color frame error.  A cheaper,
less acceptable approach is to independently lock to sync and subcarrier.
An independent locking sync generator cannot provide a color frame output
because the color frame was never determined.  The output SC/H phase
will track reference input SC/H phase error.  Sometimes SC/H error
indicators are provided to help overcome these deficiencies.


Multiple Studio Timing

The illustration on the facing page shows a three studio system in which
the timing of entire source clusters and studios can be changed.  This will
allow one studio to feed any other studio which is in time.  It will also
allow for the priorities to change very easily.

This entire system is being driven by a dual master reference synchronizing
generator with an automatic changeover switch.  This provides additional
security since each master sync generator is powered from a different
circuit.  The master sync generators can have ovenized crystal oscillator
options for higher frequency stability against temperature variations.  An
external frequency reference option allows a rubidium or cesium
frequency standard to be used as the frequency standard, with the internal
oscillator as a backup.

Each of the three studios are similar to the one just designed.  The studios
have some dedicated source devices, with additional cameras and/or video
tape machines that can be assigned.  A routing switcher is used to assign
these sources to the studios.  Every studio output is fed to a routing
switcher input for assignment as a timed input to another studio.  Every
studio is being driven by a reference synchronizing generator which will
adjust the timing of that entire studio.  Each source cluster is driven by a
source synchronizing generator so the source cluster timing will stay
together.  The reference output from each studio reference synchronizing
generator is sent to the routing switcher.  The reference input to any
source cluster synchronizing generator can be assigned to any studio.  This
automatically times the source cluster to the studio using it.  If the
reference synchronizing generator has a phase preset option, the phase
setting for every configuration can be stored and recalled.  A typical
configuration could be Source Cluster 1 timed into Studio 1, the output of
Studio 1 and Source Cluster 3 timed into Studio 2, Source Cluster 2 timed
into Studio 3, which is also a timed input to Studio 2.   These timing
assignments can easily be interchanged with the phase preset option and
routing switcher once the initial timing is completed and stored in each
reference synchronizing generator.

This system provides maximum flexibility in tailoring each studio for the
production it is to be used for.  The cameras would be assigned to a studio
doing live production and the video tape machines could be used for post
production in another studio.  Many more sources can be added when using
this design without causing major system design problems.

Distributed synchronizing generator systems also provide what may be an
important advantage: redundancy.  Should a failure occur in the master
generator, the reference and source generators will freerun and thus the
equipment being driven by them will continue to function.


SECTION 3

Definition of SC/H Phase

In the late 1940s the Electronic Industries Association  (EIA) established
monochrome television standard RS-170. In recent years proposed color
standard RS-170A has received increasing acceptance.  RS-170A fully
outlines the phase relationship of the color subcarrier to horizontal sync.
A graphic representation of this standard is included on pages 12 - 13.  If
we look at the equation that relates horizontal sync to subcarrier and
consider the number of lines in each frame, several conclusions can be
made.

                  H = 2 X 3579545 /455

First, there are 227.5 subcarrier cycles per horizontal line, so subcarrier
phase reverses every line.  This is desirable to reduce the visibility of
color subcarrier on monochrome receivers.  Second, with 525 lines per
frame, there are 119437.5 subcarrier cycles each frame. This causes
subcarrier phase to reverse every frame.  Because of the extra half cycle
of subcarrier, it takes two frames to complete one full four field color
sequence, called a color frame.  It is clear from the horizontal frequency
equation above that horizontal is frequency locked to subcarrier, but it
does not define the phase  relationship between them. Proposed color
standard RS-170A clearly defines SC/H phase as: the zero crossing of the
extrapolated subcarrier of color burst shall align with the 50% point of
the leading edge of horizontal sync.  For color field one, the extrapolated
subcarrier zero crossing will be positive going on even lines.  This
definition of sync to subcarrier phase (SC/H) is required for the
unambiguous identification of the four field color sequence.  The
operational  ramifications of these definitions are not obvious and require
further explanation.


Operational Importance Of SC/H Phase

The importance of SC/H phase lies primarily in the video tape editing
environment.  If during playback the video signal coming off the tape is
not of the same color frame as the house reference, the video at the
machine's time base corrector output must be  shifted horizontally. The
shift can be in either direction and be up to 140 nanoseconds (one half
subcarrier cycle). This may result in narrowing of active picture and a
widening of blanking since the output processor blanking is referenced to
the house.  Even if the off-tape video is of the correct color frame, the
machine-output video will be shifted horizontally to a smaller degree in
an amount equal to any SC/H phase difference between the off-tape and
house video. These horizontal shifts are troublesome in a tape editing
environment, especially when editing scenes together of similar  content.
At the edit point the background will appear to jump horizontally.  This is
unacceptable and thus dictates the need for an entirely SC/H phased
facility.

To ensure the proper operation of the tape machine color framing circuits
(to avoid incorrect color frame operation), the SC/H phase relationship of
the video recorded on tape and house video must match.  As a matter of
uniformity correct SC/H phase is defined by RS-170A. It is important that
all recorded video have a constantly correct SC/H phase relationship.  The
reference input to the tape machine should also be a stable SC/H phase
source.

Problems Achieving and Maintaining SC/H Phase

Subcarrier timing in a studio is a well understood concept  in the industry;
if it is not correct, there will be color hue shifts between sources.  If
sync timing is not correct,  horizontal shifts will occur at the video
switcher.  The concept of SC/H phasing in a studio requires a higher level
of thought regarding each element within the studio.

First, and most obvious, is the house sync generator. If the sync generator
cannot generate consistent SC/H phased outputs, maintaining SC/H phase
in the plant will never be possible. It is equally  important that all the
sync generators in a multiple sync generator facility maintain correct
SC/H phase and color frame  relationships.

Once SC/H phase has been defined by the sync generator none of the
elements in the system should alter the SC/H phase. Some elements are
obvious such as the video processor which regenerates sync and burst.  If
the phase of the regenerated sync or burst is different from the incoming
video, the SC/H phase is altered.  Less obvious are sources which derive
timing from externally applied sync and subcarrier.  If sync and subcarrier
are fanned out through DAs, then their phase can be altered independently.
This dictates that the output of each source device be SC/H phased prior
to or at the input of the switcher.  There are many distortions which make
the determination of color frame and SC/H phase difficult. The most
prominent is sync to subcarrier time base error. This can be generated by
many devices, such as sync generators with noise in the horizontal sync
circuits, linear and regenerative pulse DAs which suffer from pick-off
jitter or low frequency response problems, or any  device that has
separate sync and subcarrier regeneration  circuitry.

Smear due to poor low frequency response, noise, hum, and power glitches
are distortions that may occur in signal transmissions.  If these are not
removed prior to sync separation, determination of the exact 50% point of
sync will be difficult.

Video time base error is different than sync to subcarrier time base error.
Sync to subcarrier time base error is seen when triggering a scope on the
leading edge of sync and viewing color burst. What should be seen are two
overlapping cycles of subcarrier that are not blurred. An example of sync
to subcarrier time base error is shown in the accompanying oscilloscope
photos.

If sync to subcarrier time base error occurs either on the reference pulses
to a tape machine, or exists on the recorded video tape, color frame lock
will be difficult.  In the normal playback mode, excessive sync to
subcarrier time base error will cause the tape machine to shift horizontal
lines by 279nS (subcarrier cycle) increments.  This phenomenon is seen as
a tearing of the picture.


Building An SC/H Phased Plant

The first consideration must be the heart of every system, the
synchronizing generator. The requirements for the sync generator should
include the following:

     1)  Less than 1nS sync to subcarrier timebase error

     2)  Less than 10nS long term SC/H phase stability

     3)  Consistent SC/H phase regardless of operational mode or initial
         conditions

     4)  Compatibility with other equipment


Many studios use multiple sync generators to provide advanced drive
pulses and subcarrier to various source equipment.  Every  source
synchronizing generator must meet these requirements as  well as being
able to precisely color frame lock to the master  reference synchronizing
generator.

This need has been met by the 9500 Series Synchronizing Generators.  The
9500 Series can address any system requirement,  including both ultra
precise encoded subcarrier locking and color  black locking source
synchronizing generators. Every model in  the 9500 Series is
unconditionally SC/H phased, whether locked or  freerunning.

Every video locking 9500 Series Synchronizing Generator will lock to a
non SC/H phased reference and produce correctly SC/H phased outputs.
This is done by identifying color frame of the incoming video and
assigning the nearest color frame. Once this has been achieved, the sync
generator will tolerate SC/H phase drift beyond 100 degrees for the
source synchronizing generator and over 330 degrees for the master.
Should the incoming SC/H phase exceed the 100 degree limit, the source
synchronizing generator will shift its horizontal phase by one-half
subcarrier cycle to maintain color frame match to its reference.  The
master generator will shift its horizontal timing by a full subcarrier
cycle should the 330 degree limit be exceeded and thus not change color
framing.  A stable reference is ensured under any condition.

The 9510 and 9520 Master Synchronizing Generators feature protected
video genlock.  In this mode the generator achieves color frame lock and
then maintains frequency lock to the color burst of the incoming video
only. This mode provides immunity to incoming jumps in video sync which
would otherwise cause severe disturbances in the generator's output. The
transition out of protected video genlock into freerun will occur if the
burst abruptly changes phase, disappears, or there is a complete loss of
video. The transition to freerun will be smooth and not disturb the plant.

Every model in the 9500 Series has a wide retiming range of  2.5 lines
advance to 1.5 lines delay.  Output SC/H phase is correct at any timing
setting.  A one line wide color frame pulse which occurs on line 11 of
field one of the color frame is available on every model.  This color frame
pulse provides absolute, positive identification of color framing to all
equipment in the plant that will accept it.  All pulse outputs are negative
going four volt peak-peak and are shaped.

The 9505 Source Synchronizing Generator is available for either color
black reference or ultra precise encoded subcarrier reference.  Both
models are unconditionally SC/H phased and have  superior performance
specifications.

Test signals are optional in every 9500 Series Synchronizing Generator.
An optional Source Identification submodule can be added to the Test
Signal Generator module to place up to a 14  character identification over
the test signal output.  This identification is also positionable both
vertically and horizontally.

The 9520 will accept two Test Signal Generator/Source Identification
options, a High Stability Ovenized Crystal Oscillator option, and an
External Frequency Reference option. The External Frequency Reference
option permits the 9520 to frequency lock to an external 3.579545, 5.0, or
10.0MHz atomic frequency source for superior timebase stability.

  The 9510 has a phase preset option that will store 16 different phase
settings in a non-volatile memory.  These can be recalled via local or
remote control.  This option permits retiming of a source device or entire
studio with a single binary control.

Conclusions

To achieve an SC/H phased plant, the timing of sync becomes as important
as subcarrier, and each element should be viewed in that light.  To aid
video tape editing, it is important to record video with proper SC/H phase
and also supply SC/H phased reference to the machine in playback.  These
criteria do not have to be compromised with the system approach offered
by the Grass Valley Group.

Measuring SC/H Phase

The SC/H (subcarrier-to-horizontal) phase is the time relationship
between the subcarrier and the leading edge of horizontal sync.  A properly
adjusted SC/H phase occurs when the 50% points of the leading edge of
sync and the subcarrier zero crossings are coincident.

The color frame pulse (V1) appears on line 11 of field 1.  V1 identifies
field 1 of the 4 field color sequence.

Test Equipment Required

The following test equipment is required to perform the SC/H phase
measurement procedure.  Equivalent test equipment may be substituted
but must be equal to or superior in performance.


Dual Trace Oscilloscope                  Tektronix 465

(with delayed sweep and
one channel input inversion)



Switchable Delay Line                     Mathey 511

             or

Subcarrier Delay DA

(360! range)


Test Procedure

     SC/H  Phase Measurement

1.  Connect a video source requiring SC/H phase measurement to the
inverting channel of the oscilloscope.

2.   Connect subcarrier (3.58 MHz continuous) to  the second channel of
oscilloscope.

3.  While observing the oscilloscope (triggered at a horizontal rate),
adjust subcarrier to match amplitude of burst.

4.  At the oscilloscope, invert the video display and set mode to alternate
sweep.

Figure A shows inverted video (top) and continuous subcarrier (bottom).

5.  Adjust the oscilloscope for A plus B mode.

6.  Adjust subcarrier phase and fine level at the generator or delay line for
a null burst as shown in Figure B.

7.  Adjust the oscilloscope for chop mode, noninverted video, and adjust
vertical positions to exactly overlay subcarrier and sync.

8.  Adjust the oscilloscope delayed sweep for a display showing the
leading edge of sync and the subcarrier.  A proper phase relationship
requires coincidence at the 50% points of the leading edge of sync and the
subcarrier zero crossings.  See Figure C.  An improper phase relationship
is shown in Figure D.

Color Frame Pulse (V1) Identification

9.  Adjust the SC/H phase as described in steps 1 through 8, for proper
coincidence.

10.  Trigger the oscilloscope on the leading edge of the V1 pulse with
video and subcarrier connected to the two input channels.  See Figure E.

11.  Increase the oscilloscope sweep rate and, using the delayed sweep
option, view a display showing the first leading edge of sync following the
trigger.

12.  If the negative transition of the subcarrier is coincident with the
leading edge of sync, the triggering V1 pulse is a color frame
identification pulse that occurs on line 11 of field 1.  See Figure F.

NOTE:  The SC/H phase is easiest to observe on a display that is
horizontally triggered.  Because of the low repetition rate of V1 and the
fast sweep rates (50nS/div.) required, only the direction of subcarrier
signal can be easily observed by triggering on V1.

GLOSSARY OF TIMING TERMINOLOGY

APL

Abbreviation for average picture level.  The average luminance level of the
part of a television line between blanking pulses.

Active Picture Period

That part of the video signal that produces the television picture, as
distinguished from that portion of the video signal that occurs during
blanking (horizontal and vertical retrace).

Amplitude Modulation (am)

Modulation in which the amplitude of a wave is made to vary.  In
television, the color video signal modulates the subcarrier, causing its
amplitude to vary.

Automatic Changeover Switch

Equipment that receives the outputs of two sync generators and
automatically switches to the backup sync generator should there be a
failure of the sync generator in use.

Backporch

The blanking signal portion which lies between the trailing edge of a
horizontal-sync pulse and the trailing edge of the corresponding blanking
pulse.  The color burst is located on the back porch.

Bandwith

The complete range of frequencies over which the television system can
function.  The information carrying capability of a particular television
channel.

Blanking

The time period when picture information is shut off.  Blanking is a
voltage level at black picture level and acts as a signal to turn off the
scanning beam.  synchronizing pulses which control invisible retrace of
scanning are active during the blanking period.

Breezeway

That portion of the "back porch" between the trailing edge of the sync
pulse and the start of the color burst.

Burst (Color Burst)

Nine cycles of 3.57MHz subcarrier, placed near the end of horizontal
blanking, which is the color reference for the color signal.  Color timing
refers to adjustment of the phase of the subcarrier.

Carrier Wave

A single frequency wave which is transmitted and modulated by another
wave which contains the information.

Character Generator

A device used to generate text or captions for television broadcast.

Chrominance

That portion of the video signal that contains the color information
(saturation and hue).

Clamping

The process that establishes a fixed reference level for the picture signal,
normally keyed off the horizontal synchronizing pulses.  A major benefit
of a clamp is the removal of low-frequency interference, especially power
line hum.

Coaxial Cable

A cable with a noise shield around a signal-carrying conductor.  In
television, the cable impedance is 75 ohms.

Color Background Generator

Circuit that generates a full-field solid color for use as a background in a
video picture.

Color Bars

A video test  signal widely used for system and monitor setup.


Color Black (Black  Burst)

A composite video signal that produces a black screen when viewed on a
television receiver.  Composite video is a video signal that contains
horizontal, vertical, and color synchronizing information.

Color Frame

In NTSC color television it takes four fields to complete one color frame.
For a detailed definition, see the SC/H PHASE section.

Composite Sync (CS)

Horizontal and vertical sync pulses combined.  Often referred to simply as
"sync".  Sync is used by source and monitoring equipment.

Composite Video

A video signal that contains horizontal, vertical, and color synchronizing
information.

Color Track Frame Pulse

A pulse laid down on video tape by a video tape recorder to enable the
machine to lock up correctly when played back.

Cut

A transition between two video pictures which is nearly instant, without
any gradual change.

DC Restoration

The reestablishment by a sampling process of the DC and the
low-frequency components of a video signal which have been suppressed
by AC transmission.

DC Signal Bounce

Overshoot of the proper DC voltage level of the blanking pulse due to
multiple AC couplings in a signal path.  Causes sudden brightness in
picture.

Delay Distribution Amplifier

An amplifier that can introduce adjustable delay in a video signal path.

Distribution Amplifier

Device used to multiply (fan out) a video signal.  May also include cable
equalization and/or delay.  Referred to as a DA.

Drive Pulse(s) (Pulse Drives)

A term commonly used to describe a set of signals needed by source
equipment such as a camera.  This signal set may be composed of any of
the following: sync, blanking, subcarrier, horizontal drive, vertical drive,
and burst flag.

Equalization

Process of altering the frequency response of a video amplifier to
compensate for high-frequency losses in coaxial cable.


Equalizing Pulses

A series of pulses occurring at twice the line frequency before and after
the serrated vertical synchronizing pulse.  Their purpose is to adjust the
scanning sequence for proper interlace.

Fade

A gradual transition of the video picture (and signal) to black.


Field

Half of the horizontal lines (262.5 in NTSC system) needed to create a
complete picture.  Two interlaced fields create a complete frame.


Fill

The video information that fills the "hole" cut in the video picture by the
key signal.


Flyback (retrace)

The movement of the camera or television monitor electron beam back to
the starting point for the next line or field.


Frame

A complete picture composed of two fields.  In the NTSC system, 525
interlaced horizontal lines of picture information.


Frame Synchronizer

A digital buffer that by storage, comparison of sync information to a
reference, and timed release of video signals, can continuously adjust the
signal for any timing errors.


Frequency

The number of cycles of a waveform in a given length of time.

Frequency Modulation (fm)

Modulation in which the frequency of a carrier wave is made to vary.


Front Porch

The blanking signal portion which lies between the end of the active video
picture information and the leading edge of sync.

Frequency Response

The maintenance of a uniform video signal level (amplitude) over a range
of frequencies.

Gate

A signal used to trigger the passage of other signals through a circuit.


Group Delay

A defect in a video signal caused by different frequencies having differing
propagation delays (delay at 1MHz is different than delay at 5MHz).  In the
television picture, group delay will cause an object's color to shift
outside the object's outline.

Harmonic

A wave having a frequency that is an integral multiple of the fundamental
frequency.  For example, a wave with twice the frequency of the
fundamental is called the second harmonic.

Hertz

Unit of measurement for the number of cycles of a  waveform in one
second.


Horizontal Sync Pulse

The synchronizing pulse at the end of each line that determines the start
of horizontal retrace.


Hue

The actual color that appears on the screen.  Hue defines color on the basis
of its position in the spectrum - i.e., whether red, blue, green, or yellow,
etc.  One of the three characteristics of television color.  See Saturation
and Luminance.

Impedance

The total opposition (resistance and reactance) a circuit offers to the
video signal at a given frequency.

Key

A signal that can electronically "cut a hole" in the video picture to allow
for insertion of other elements such as text or a smaller video picture.

Linear and Regenerative Pulse DAs

Linear pulse DA will handle up to 4V p-p signals (pulses) but is limited to
amplifying and fanning out the signal.  Regenerative pulse DA reconstructs
the signal and allows for adjustment of delay.

Luminance (brightness)

The brightness of the picture or area of the television screen being
considered.  See Hue and Saturation.


Master Reference Synchronizing Generator

A synchronizing pulse generator that is the precision reference for an
entire teleproduction facility.

Microsecond (uS)

One millionth of a second: 1 x 10 to the negative sixth or 0.000001 second.


Modulator/Demodulator

Modulator is a circuit that modulates or impresses the carrier wave by
amplitude and/or frequency.  Demodulator is a circuit that demodulates or
decodes the amplitude and/or frequency information from the carrier
wave.  In television, the information typically modulated and demodulated
are the hue and saturation components of the color signal.

Monochrome (black and white)

The video signal which represents the brightness values (luminance) in the
picture, but not the color (chrominance) values in the picture.

Nanosecond (nS)

One-billionth of a second: 1 x 10 to the negative ninth or 0.000000001
second.

NTSC

National Television System Committee which worked on formulation of
standards for present United States television system.  Now describes the
American system of color telecasting which is used mainly in North
America, Japan, and parts of South America.

Ovenized Crystal Oscillator

A crystal oscillator that is surrounded by a temperature regulated heater
(oven) to maintain a stable frequency in spite of external temperature
variations.


Overshoot

Amplitude of the first maximum excursion of a pulse beyond the 100%
level.  Pulse exceeds its defined level temporarily, before settling to the
correct level.  Overshoot amplitude is expressed as a percentage of the
defined level.


PAL

Abbreviation for Phase Alternating Line.  PAL is the name for the color
television system in which the E'V component of burst is inverted in phase
from one line to the next in order to minimize hue errors that may occur in
color transmission.  PAL-B is a European color TV system featuring 625
lines per frame, 50 fields per second, and a 4.43361875 MHz subcarrier.
Used mainly in Europe, China, Malaysia, Australia, New Zealand, and parts
of Africa.  PAL-M is a Brazilian color TV system with phase alternation by
line, but using 525 lines per frame, 60 fields per second, and a
3.57561149 MHz subcarrier.


Phase

The relative timing of a signal in relation to another signal.  If the time
for one cycle of a signal is represented as 360 degrees along a time axis,
the phase position for the second signal is called phase angle, expressed
in degrees.


Path Length or Propagation Delay

The time it takes for a signal to travel through a piece of equipment or a
length of cable.


Phasing (Timing)

Adjusting the delay of a video signal to a reference video signal to ensure
they are synchronous.  This includes horizontal timing and subcarrier
phasing.


Pick-Off Jitter

Jitter is a random aberration in the time period due to noise or time base
instability.  Pick-off means sample point.


Processing Amplifier/Proc Amp

(See Video Processing Amplifier)


Pulse Width

Measured between the 50% amplitude points of the leading and trailing
edges.


Reference Video Signal

A composite video signal used to compare all other video signals to, for
timing purposes.


Rise Time

Time required for a pulse edge to rise from 10% to 90% of the final value.


Return Loss

At a connecting point in a video system, the difference between the signal
amplitude on the connection and the signal amplitude reflected from the
connection.  The difference is measured in decibels (dB).


Saturation (chroma, chroma gain)

The degree of purity of a color.  Adding white to a color reduces its degree
of saturation.


SC/H Phase

The phase relationship of the subcarrier to the leading edge of horizontal
sync.  Alignment of the zero degree crossing of subcarrier with the 50%
point of the leading edge of sync.


SECAM

Abbreviation for sequential couleur a'memorie (sequential with memory).
A color-tv system with 625 lines per frame and 50 fields per second
developed by France and the U.S.S.R. and used in some countries that do not
use either NTSC or PAL systems.


Source

Video producing equipment such as a camera, tape recorder, or character
generator.


Source Synchronizing Generator

A synchronizing pulse generator used to drive a specific piece of source
equipment.  It is referenced to a master reference synchronizing
generator.


Spurious Signals

Any portion of the signal that is not part of the fundamental video signal
and its harmonics.  Spurious signals include transients and noise.


Switcher, Production Switcher, Vision Mixer

Device that allows transitions between different video pictures.  May also
contain special effects generators.


Subcarrier

A continually cycling waveform at 3.57MHz on which color information is
added or encoded; subcarrier is added to the monochrome signal to carry
color information.


Synchronizing Pulse Generator (SPG)

Equipment that generates synchronizing pulses needed by source
equipment.  Also known as a sync generator.


Time Base Corrector

Device used to stabilize the video picture on replay from a tape machine.


U

Color difference signal (B-Y) used to modulate "U" component of
subcarrier.


V

Color difference signal (R-Y) used to modulate "V" component of
subcarrier.


Vertical Serrations

A vertical synchronizing pulse contains a number of small notches called
vertical serrations.  These serrations provide horizontal synchronization
during the vertical interval.


Vertical Sync Pulse

The synchronizing pulse at the end of each field which signals the start of
vertical retrace.


Video Processing Amplifier

A device that stabilizes the composite video signal, regenerates the
synchronizing pulses, and can make other adjustments to the video signal.


Zero Timing Point

The point at which all the video signals must be in synchronization
(typically the switcher input).


                               NTSC REFERENCE TIMING DATA


Subcarrier Frequency           3.579545MHz

Subcarrier Period         279.37nS

Horizontal Frequency          15.734264KHz

Horizontal Period          63.5565S

Vertical Frequency          59.94Hz

Vertical Period                  16.683mS

Vert. Equalizing Pulse Width   2.35S

Horizontal Sync Width           4.75S

Horizontal Blanking Width     10.95S

Vertical Sync Width          27.15S

Vertical Blanking Width          21 lines

Front Porch Width           1.55S

Breezeway                   0.65S

Burst Width                   2.55S

Color Back Porch Width           1.65S


Color Timing Data:


   1! = .776nS

   1nS = 1.289!

For Cable With 66% Propagation Factor

   1! = 6.035" = .503'

   1nS = 7.778" = .648'

Zero SC/H phase is the coincidence of the zero crossing of a subcarrier the
same phase as color burst with the 50% point of the leading edge of
horizontal sync.  On color frame one, the subcarrier zero crossing will be
negative going on odd numbered lines.


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