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ReClock 0.99k by OGO (ogo@nerim.net)
Web site: http://ogo.nerim.net/reclockfilter
Please take time to test ReClock, and report success or failures to make it work with your PC. Have a look on changelog.txt for latest modifications.
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What is ReClock?
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Said simply the purpose of ReClock is to definitely get rid (I hope) of jerky playback of AVI and MPEG material on a PC (or a PC connected to a TV).
ReClock is born from my own frustration. I have a fast PC, a good video card, and when I play a DIVX on my brand new Home Cinema, I get dropped frames here and there for no reason, or a completely jerky and un-watch able DIVX. This is very annoying.
The following sections will give you a complete and I hope clear explanation of what cause jerky playback, and how ReClock will try to solve these problems.
The last section will give you the instructions to install and use ReClock.
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The not so great history of frame rates
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In basic terms, a video can be thought of as being made up of numerous snapshots, called frames. The frame rate, or the number of frames displayed each second
As you know there exist 3 common broadcasting formats: cinema, TV, and computers
Cinema is the oldest. It is the format of the movies you see in your favourite theater. Cinema movies play at 24 frames per second (fps). It’s very simple: every 1/24 th of a second, your see a new frame. This is also called “progressive scan”.
A television, however, does not deal with video in terms of frames. Instead, it displays video using half-frames, called fields. Each frame contains exactly two fields. One field is made up of the odd horizontal lines in a frame. This is called the odd field or the top field since it contains the top line of the image. The other field is made up of the even horizontal lines in a frame. This is called the even field or bottom field. This way to broadcast video is called “interlaced scan”
Now there are three common TV standards: PAL, SECAM and NTSC. All of them use “interlaced scan”.
Let’s start with PAL, which is the European TV standard. It is also used for DVD and DIVX material. PAL material is played at 25 fps (or 50 fields per second). You see the first problem here: how can we play a 24fps movie on a 50 fields per second PAL TV? Well first of all, we can present each movie frame two consecutive times to make the movie play at 48 fields per second. But playing the movie like this would give jerky playback every second because one movie picture would be missing. So the movie is just played 50/48 times faster to match 50 fields per second. So a cinema movie that has duration of 60mn plays on PAL in 57mn36 seconds. That’s why movies you watch on your PAL TV are always a bit shorter in time.
SECAM is the French brother of PAL and works exactly the same way. It is still used in France because it gives better colours when broadcasted by radio waves (less sensitive to noise).
And now here is NTSC, the American brother of PAL. NTSC is also used for DVD and DIVX material, and plays somewhere near 29,97 fps (to be exact it is 4,5 Mhz/286/525). You see a bigger problem here. How can we play cinema movies on NTSC. Accelerate them? Sure no, because you would notice that the film plays much too fast (a 60mn movie would play in 48mn3s). So NTSC engineers came with a solution called “telecine” or “3:2 pulldown” which is quite complicated. To convert a film that runs at 24 fps to run at 29.97 fps, it is first necessary to slow down the video by 0.1% to 23.976 fps. Then approximately 6 frames are added to the video each second, bringing the frame rate to 29.97 fps. This is done by adding one extra frame to each group of 4 film frames. Although they could simply duplicate 1 out of every 4 frames to produce the extra frame, this method is not used. This is because the duplication of one frame would cause that frame to be displayed for twice as long as the other 3 frames, which leads to jerkier motion. Fortunately, film producers can make use of the field-based nature of video to more gradually introduce the extra frame. Instead of adding a whole new frame at once, 2 fields are introduced separately to each group of 4 film frames. Since 2 fields make up a frame, this method is equivalent to adding 1 new frame. However, since the 2 duplicated fields are not added at the same time, this reduces the jerkiness of the video.
Let’s finish with computers. Computers are quite simple, they just do work like cinema and use “progressive scan”. But they use many more frame rates: 60 Hz, 75 Hz, 85 Hz, 100 Hz when watching your monitor, 50hz when connected to a PAL TV, and 60hz when connected to a NTSC TV. To obtain a smooth playback on a PC you just have to make sure that your video card uses a refresh rate that is an exact multiple of the movie you play. You can already see that PAL can be played fine at 50 Hz, 75 Hz or 100 Hz, but NTSC cannot be played without begin jerky because all we have is 60fps which is not a multiple of 29,97fps.
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The nightmare of badly born DIVX
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DIVX for the most are created from DVD material, and DVD material generally comes from movie material.
DIVX created from PAL material are generally fine, because they were created from 24 fps material simply accelerated to 25 fps or from direct 25 fps material. PAL is a good guy.
And now you have telecine for NTSC. Since telecine can only be displayed correctly on “interlaced scan” hardware, it must be removed for PC playback. This operation is called “inverse telecine”. Doing this on a film will revert the frame rate of the movie to 23,976 fps. Funny, but … jerky on every PC you will use to play the movie.
Some people that create DIVX from NTSC don’t even know that inverse telecine must be done. So those DIVX stays at 29,97fps and will have artefacts when watched on your PC because interlacing artefacts do not compress well at all.
And to add another thing to the story, some DIVX has sound and movie sync problems that are solved by … modifying the frame rate a little bit so the movie duration matches the sound track duration. This is a quick and dirty way to do it. Imagine a 25 fps DIVX that is modified to play at 25,001 fps ; well every 1000 frames (that’s only 40 seconds) the playback will become jerky.
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The bigger nightmare of PC hardware
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When you connect your PAL DVD player to your TV, things are really simple. Your player read the movie from the disc at 25 fps, and sends the signal to the TV at 25 fps.
Now imagine that your player is a little too fast and send the movie to the TV at 25,01 fps. What will happen? Jerkiness? No … In fact your TV is smart, and will stay synchronised with your player as long at the player plays near 25 fps. To be more precise no player in the world plays the movie exactly at 25 fps because clocks are never accurate. So every player play the movie “near” 25 fps, but this is no problem for your TV since it is locked to the video signal it receive.
Now when you play a DIVX on your PC and watch it on your PC what happens? First of all, if the refresh rate of your monitor is not a multiple of the frame rate of your DIVX, jerkiness will happen for sure. Do you remember that DIVX can be 24 fps, 25 fps, 23,976 fps, 29,97 fps, or even 25,001 fps? This is the first and main cause of jerky playback.
The other source of jerkiness is much more subtle and harder to understand. Suppose you play a 25 fps movie on you PC. Well your PC isn’t smarter that any DVD player, it will play it “near” 25 fps because it uses its internal clock, which is inaccurate. But it should not be a problem because DVD players also do that? Bad luck, it is a very big problem, because your video card does not synchronise its speed with the speed of the movie like a TV would do with a DVD player. In a PC the clocks used in the video card (to show the video) and in the PC (to play the video) are completely distinct, thus there always is deviation between them, and then jerkiness is inevitable.
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The better history of DirectShow
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DirectShow is the DirectX component that plays or record video and audio on your PC. DirectShow contains many modules called “filters” and connect them in a “filter graph” to finally render the movie.
Let’s take an example, with the steps needed to play a DIVX movie:
- First you need to demux the audio and video from the AVI file: one filter will do that and will produce two streams (audio + video)
- Then you need to decode audio stream (MP3 for example): a MP3 decoder filter will do that
- You also need to decode video: a DIVX decoder filter will do that
- You must render the decoded movie in a window: a video renderer filter is needed
- You must play decoded audio: an audio renderer is used.
DirectShow was cleverly designed because it will automatically search and find what filters are the best to render a movie. For example, the audio and video renderers are completely generic (they are provided by Microsoft) and will eat the output of every decoder filter in the world.
That’s one reason why there exist many DIVX players out there. A player is only a nutshell where filter graphs are built and run.
Now, DirectShow has another interesting feature. When building a filter graph, it set up a “reference clock” that is used to provide a unique time to all the filters in the graph. All filters will play their stuff at the speed of the reference clock.
The default reference clock in DirectShow is provided by the Microsoft sound renderer. Why? Because in order to play sound correctly, your sound card must receive samples exactly at the speed they will be played. So the default reference clock is in fact synced to a hardware clock somewhere in your sound card. Video frames just follow this clock, making jerkiness inevitable.
You may ask why Microsoft made this choice. They could have chosen to sync the reference clock to the video and all would be nice. I suppose they didn’t for at least two reasons:
- Video cards do not have a high-resolution hardware timer available to make a clock.
- Sound playback would become problematic since the sound card would not receive its samples at the speed they would be played. So the sound would not stay synchronised with video
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Here comes ReClock
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Now I think you understand what does ReClock. It provides a reference clock that is synchronised with your video hardware. How? By replacing entirely the default sound renderer with a new rewritten DirectShow filter that is somehow cleverer.
But by doing so, ReClock must solve the two problems we saw in the last section:
Video cards do not have a high-resolution hardware timer available to make a clock: well, this is not completely true, since many of them have something that will help us. ReClock provides a reference clock based on a high-resolution timer based on hardware on your motherboard or your processor. Let’s call this clock the “system clock”. Then, ReClock will correct the system clock with information gathered in real time from your video card if they are available. How it does that will be my little secret :)
Sound playback would become problematic: this is true. How can we solve that? By varying the speed of the sound in real time to force the sound to stay in sync with the video. This is another reason why we replace the sound renderer with our own one. There are two ways to change sound speed: pitch and playback speed. Changing the pitch in real time is very hard to do and I was too lazy, so I went with the second solution. ReClock change the play back rate of the sound in real time by adjusting the audio clock. Doing so will change the pitch of the sound, but your ears should not notice that with such a low correction.
Another thing that does ReClock is to change the global playback rate of the movie in order to match its frame rate to a multiple of the refresh rate of your monitor. Here is how it does that:
- CINEMA mode is detected if the monitor has a refresh rate that is a multiple of 24 Hz (72 Hz, etc…) and if the movie has a frame rate between 23.75 and 24.25. Then the playback rate of the movie (including sound) is modified to match exactly 24fps.
- PAL mode is detected if CINEMA mode wasn’t possible and if the monitor has a refresh rate that is a multiple of 25 Hz (50 Hz, 75 Hz, 100 Hz, etc…) and if the movie has a frame rate between 23.75 and 25.25. Then the playback rate of the movie (including sound) is modified to match exactly 25fps.
- NTSC mode is detected if the monitor has a refresh rate that is a multiple of 30 Hz (60 Hz, 120 Hz, etc…). Then the playback rate of the movie (including sound) is modified to match exactly 30fps.
- CUSTOM mode is detected when the frame rate of the movie +/- 2% is a multiple of the monitor refresh rate. Then the playback rate of the movie (including sound) is modified to match exactly a multiple of the monitor refresh rate.
Finally if nothing matched, the playback rate is not modified, but the reference clock is still synchronised to the video card in order to obtain a stable clock.
Side note: you may ask if NTSC will work well with TV since PC does 30fps where the TV wants 29,97fps. That’s a good question to which I do not have a complete response, but strangely I observed that when I use ReClock with the excellent TvTool in NTSC mode with a GeForce 3, I get a clock correction that make the reference clock work near … 29,97fps. So it seems that when the PC uses TV out in NTSC mode, the refresh rate is not 60 Hz, but 29,97*2 Hz, but I’m not sure at all of that, since it may be a coincidence. Anyway playing NTSC at 30 fps on a NTSC TV should not be a problem; as you may know NTSC was originally a black/white standard at 30 Hz that became 29,97 Hz for technical reasons when colour appeared.
So by adjusting the reference clock to match your video card, adjusting the sound to stay in sync with picture, and modifying the global playback rate, ReClock should allow a smooth movie experience on your monitor and even on your TV.
But as I said before, ReClock will not work with all PC because it uses some functions that are not available on all of them. Firstly your PC must support high-resolution timers (nearly every modern PC has them). Secondly, your video card must support some special calls. I know that most nVidia, ATI, Intel i815 and Matrox G200 do support those calls, but some cheaper cards may not. If those functions are not supported, ReClock will not load and the Microsoft audio renderer will be used.
Moreover, ReClock has some constraints you need to understand:
- ReClock requires DirectX 8.0 or higher. DirectX 9.0 is suggested.
- ReClock will load and work only if your movie has sound, because ReClock is an audio renderer.
- ReClock will not always find the frame rate of the material. For example streaming will never be supported.
- When your run ReClock for the first time for a specific resolution and refresh rate, the reference clock will be inaccurate during the first minutes of playback while being corrected. This can cause jerky playback at the beginning of the movie. The time to obtain an accurate clock will depend on many factors (initial error of clock, PC speed …). After this adjustment, ReClock will store the correction in the registry, so when you will open another media, the clock correction parameters will be retrieved.
- Changing the playback speed can be noticeable on sound especially when 23,976 fps material is up rated to 25 fps. Doing so will accelerate the sound by 4%, and sound will seem to be a little high pitched.
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Installing and Using ReClock
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Installation is straightforward; simply launch setup.exe. This will install the product in C:\Program files\ReClock by default. You will also notice a new menu in the start menu. This menu will give you access to this readme and uninstall utility.
Un-installation is simple too. Simply uninstall ReClock from the ‘add/remove program’ panel of Windows or from the ReClock menu. Do not delete ReClock folder, it would not uninstall it correctly!
Now you can use the configuration application to modify some settings. You can launch it from the Start menu.
Here is an overview of the configuration application:
If you don’t know what to choose or if you messed up things, simple click on “Default settings” and the recommended settings will be used.
Now let’s have a look on the options:
- “Enable logging”: when checked enable debug logging in c:\reclock_log.txt
- “Use DirectSound for PCM”: when checked use DirectSound to play normal sounds (PCM). If unchecked WaveOut interface will be used. DirectSound is the preferred choice for PCM.
- “Use WaveOut for SPDIF”: when checked use WaveOut interface to send AC3 over SPDIF. If unchecked DirectSound interface will be used. WaveOut is the preferred choice for SPDIF.
- “Devices to use with DirectSound/Waveout”: You can choose here what specific soundcard must be used when using DirectSound or WaveOut. Use this if you have more than one soundcard, or if you use a specific card for SPDIF.
- “Sound pre-buffer size”: this is the amount of sound to bufferize before to play it. The lower it is, the faster you can seek inside media files. But high values also give room to handle properly sound speed variations and speed problems. Values under 100ms are not suggested.
- “Max latency for PCM/SPDIF”: You can specify here, how much sound desynchronisation is authorized on PCM or SPDIF mode. If you experience some sound drops here and there you can try to increase the values here. Values are expressed in percentage of pre-buffer time.
- “Video device to use”: You can specify here what videocard will be used as hardware video clock source.
- “Default frame rate for NTSC DVD”: As ReClock can’t detect frame rate for NTSC DVD, it will use the setting you put here when reading a NTSC DVD.
- “Default frame rate for other files”: As ReClock can’t detect frame rate for a media file, it will use the setting you put here when reading this file.
- “Enable VSYNC Monitor”, “Target VSYNC position”: Control VSYNC monitor operation. Please see paragraph “Using VSYNC Monitor”
- “Give high CPU priority to player”: if enabled, higher CPU priority will be given to the player currently hosting ReClock. It is likely to give smoother playback if some background tasks are running on your PC while you watch a video.
Now how do you use ReClock? Simply by launching your favourite Media player (like BSPlayer, ZoomPlayer, or WMP). If all goes well, your movies should play normally and sound should be working and synced to the video.
You can see what is doing ReClock by accessing its control panel during playback. You have access to this panel from the tray icon or from the filter list in your player (it’s easy with BSPlayer and ZoomPlayer).
Speaking of the tray icon you will see it can have three colors:
- green: all is fine, ReClock found the frame rate (or it was specified manually), and is currently adapting the speed if necessary
- yellow: ReClock cannot find a suitable speed adaptation (for example the video card refresh rate is not a multiple of the frame rate).
- red: ReClock didn’t found the frame rate, or clock correction has been disabled
Here is an overview of the control panel:
The “Media infos” group box will give you information about the media that is currently loaded in the player. If the filename or the video stream frame rate can’t be determined, no adaptation will be made on the frame rate to match your video card refresh rate.
The “Video hardware” group box gives you information about your video card. You should check that the detected refresh rate does match the real one.
The “Renderers infos” group box gives you information about which renderer you player is using for video, and what mode is selected for audio (either WaveOut or DirectSound).
The “Clock corrections” group box will tell you what is done by ReClock to correct the system and audio clock. The “ppm” indication gives you indications on how accurate your system clock is (the closer to 0, the best). A 1 ppm means 0.0001% of clock deviation. For example, if your clock is 3.5 Mhz, and if you play a 25 fps movie you will loose a frame every (1000000/25/3.5) second (that’s around 3 hours). For the sound part, the “sync” indicator should always be close to 0ms. You also have the ability to disable completely the system and audio clocks adaptation using the checkbox “slave reference clock to audio”. Doing so, ReClock will work much like the default DirectSound audio renderer, but rate adaptation will still function and reference clock will be slaved to audio clock with a smooth algorithm. Anyway, remember that you will probably experience jerky playback again by checking this checkbox, so the recommended way to use ReClock is to leave it unchecked.
The “Media adaptation” group box will tell you what adaptation is done to the video to match your hardware. You have the choice between different modes:
- Auto: this is the normal way to use ReClock. In this mode, ReClock will correct the speed of the media to match a multiple of your video refresh rate. If your video refresh rate does not correspond to the video, ReClock will indicate you what to do under the combo box.
- Original speed: in this mode, you can override the speed chosen by ReClock and play the media at its original speed.
- Nearest integer speed: this will round the frame rate to the nearest integer value.
- Other speeds: you can choose any target frame rate you want based on original media speed or on the refresh rate of your monitor. Audio will stay in sync (quite funny at 50 fps).
That’s being said I repeat that the best and normal way to go is to choose “Auto”.
The "Sound adaptation” group box will give you more control on sound:
- You can enable hardware resampling (by your soundcard) of sound instead of software resampling. Hardware resampling gives better sound quality (no degradation in quality at all), but due to some limitations of some soundcards, it may not work well on all soundcards (you may experience sound desynchronisation). That’s why it is not recommended.
- You can enable or disable the “dynamic range compression” for sound. This is an algorithm that will try to make the sound volume more constant. You will see which gain is applied to sound, and how many samples were clipped (saturated). You can also choose how strong the effect of the compressor is with the combo box (light, normal, and strong). “Normal” level is suggested.
The “Advanced settings” group box will give you control over the “VSYNC Monitor”. Please see the paragraph “Using VSYNC Monitor” for more instructions.
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Using TV out with ReClock
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First of all, if you have a dual head video card (a card capable to display different images on your monitor and on your TV at the same time like a Radeon for example), setup your TV as the primary display. Don’t use cloning, or theater mode. Then activate TV-out.
If your card needs a utility (like TV Tool), launch it and activate TV-out.
Then, launch your media player and open a media file, and check that the hardware video refresh rate detected by ReClock is:
- 50 Hz if your TV is in PAL
- 60 Hz if your TV is in NTSC
If the refresh rate detected by ReClock is not equal to the refresh rate of your TV, then either your TV is not your primary display, or ReClock is not compatible with your video card.
Important notice: if you need to adjust the size or the position of the TV image with the control panel of your video card (or TV Tool), remember to quit and restart your media player after you have made your adjustments, because ReClock will need to estimate how the hardware video clock has been affected. If you don’t do that, you will experience jerky playback.
As of now ReClock is known to work fine with the following cards:
- GeForce 2/3 cards with TV Tool, or with built-in drivers
- Radeon cards with built-in TV-out
If you are not sure if ReClock does correct the good video clock on your hardware, you can use the “VSYNC Monitor” to check that if this is the case or not. See paragraph below.
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Using the VSYNC Monitor
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ReClock is able to make sure that the video frames are rendered at a speed which is an exact multiple of the video refresh rate. But we have another problem to face: vertical synchronisation/movie frames synchronisation.
Each time the renderer draw a frame on screen it usually try to start drawing if it know it will be able to draw the entire frame during a video frame period. If it’s not the case, it will wait the next video frame (also called vertical sync or vertical blank or VSYNC), before to continue drawing. If by bad luck each frame comes near by a VSYNC, you will get permanent jerky playback because sometimes the renderer will choose to wait next VSYNC and sometimes not depending on how late the frame arrived.
The VSYNC monitor tries to eliminate this problem by making sure that VSYNC will not happen at the same time as frame arrival in the renderer. It can only be enabled and used is precise conditions:
- If you want to use it, you will have to check that it is enabled in the configuration application (check “Enable VSYNC Monitor”).
- It will activate only if the adapted media speed is an exact multiple of the hardware refresh rate (for example a 23.976 fps movie on 75 Hz TV). Since VSYNC/Frame synchronisation issues can only happen if one is an exact multiple of the other, there is no point to use the VSYNC monitor when it’s not the case.
The VSYNC monitor has 3 distinct purposes in ReClock:
1) It can show you on screen when does VSYNC happen. To enable this function check “Show VSYNC on screen” in the “Advanced settings” group box of the properties panel. You should then see on screen some points making a vertical pattern. Each point corresponds to a VSYNC that occurred during the last 100 frames played. You will also see a little horizontal line in the middle of this packet (I will call it the VSYNC bar). This is the current estimation of VSYNC position made by ReClock based on the 100 last frames. Now what to learn from this? Many things:
- The height of the packet gives you an indication on how regularly frames are delivered to the renderer. If the packet height is low (10-15% of the height of your movie excluding black bars), then your playback is likely to be good.
- The position (given by the VSYNC bar) of the packet is important. There are some positions of the bar that are likely to give jerky playback because these positions correspond to instants when frames are presented to the renderer during VSYNC. Please see (3) for a solution to this
- The movements of the little VSYNC bar are very instructive. If it stay quite steady during playback (not traversing the screen completely to come back on the other side), it means that frames are presented to the renderer at a very exact multiple of the refresh rate. So it means that ReClock use the good hardware clock to make its work. This is the way to validate ReClock on multi-headed displays. If you want to see what happens when ReClock does not correct the clocks, then check “Slave reference clock to audio” and then you should see the VSYNC bar moving and crossing the screen, because it’s no more synced to the video frames (using audio hardware clock instead).
2) It can tell you if your playback is jerky or not. For this enable “Tearing test” in the properties panel. You will then see a vertical bar moving from left to right incrusted in the video. If you see the bar broken somewhere in two parts, then it means that the renderer is displaying 2 frames at the same time (buffering problem). You should then choose another renderer (see your player options) or try another player. If you see the bar changing speed, or stopping/restarting, or having jerky movement, then it means your video is jerky. There can be many reasons for this
- Your CPU is not powerful enough to play the video smoothly
- Frames are presented during VSYNC. See (3) for a solution to this
3) It can tweak the reference clock to shift frame presentation in the renderer when frame come at a bad moment (during VSYNC). If you check “Enable VSYNC correction” in the “Advanced settings” group box of the properties panel, you will enable this function. Doing so, you will see two more little horizontal bars on the left of your video. Those are the limits that are given to the VSYNC bar. Each time the VSYNC will go outside these limits, ReClock will fine tweak the reference clock to make it go back within limits. This micro-correction has no impact on the overall smoothness of the movie playing back and will ensure that you won’t get jerky playback. As said in the previous paragraphs, some VSYNC bar positions are likely to give jerky playback. From my experience, those positions depend on your system. That’s why you can choose the limits you will give to the VSYNC bar by tweaking the slider called “Target VSYNC position” in the configuration app.
Here is the procedure to find the good settings:
- Launch configuration app, check “Enable VSYNC Monitor”, and choose left position as target for VSYNC position
- Launch your player, choose a video to playback, and enable “Tearing test”. If the test bar does not stutter, then quit your player and move the target VSYNC slider a little further on the right. Repeat this until you get a stuttering bar
- You have now found a target position that give jerky playback. Now simply move the slider one half of its size away from the current position (left or right, it’s not important), and you should be fine. For example: ¼ would become ¾, ½ would become full left or full right, etc…
However there are currently some caveats with the VSYNC monitor. Depending on the renderer filter that is used, the VSYNC monitor may fight against similar algorithms implemented inside the renderer itself. Here is the situation I saw with the common renderers (but I believe it can be all different with other hardware than mine):
- Internal renderer of BSPlayer will not work with the VSYNC monitor
- Overlay mixer: does work fine
- VMR7: usually works fine too, but I did see some issues sometimes depending on the player used.
- VMR9: this renderer seems to have an internal way (and a broken way I think) to do the same thing and will somehow work against the VSYNC correction algorithm of ReClock. So you may see the VSYNC bar staying outside of the limits fixed.
You may want to try the VSYNC monitor with different players. The two best players I know of are ZoomPlayer and Media Player Classic. They both allow you to choose which renderer to use.