Selecting Compression Parameters

About the sound quality / compression rate compromise

High quality of the compressed sound and high degree of compactness of the MPEG audio file, determined by the selected bitrate (a number of kilobytes of data needed to represent one second of the encoded audio), are contradictory requirements which should be compromised according to ones needs.

First of all - how good is good enough? It is widely accepted that the sound encoded with the 128 kbits/s (kilobits per second) bitrate retains high quality referred to as a CD-quality. This means that if you compare the original music played from an audio CD with it’s MPEG-encoded version you will hardly hear any difference. Of course, if you have more demanding ears you can choose higher bitrates: up to 192 kbits/s for the VBR encoding and up to 320 kbits for the (less efficient) CBR encoding.

When storing CD-quality audio (44kHz, 16 bit, stereo) in an uncompressed WAVE format, the bitrate is approximately 1.4 Mbits/s (megabits per second). This means that using the 128 kbits/s MPEG encoding you can compress your wave audio files 11 times. Using the Constant Bitrate (CBR) encoding you can enforce even higher compression rates: up to 22 (producing a good quality stereo output) or up to 44 (producing a good quality mono output). The highest compression rates - up to 88 for the 16 kbits/s CBR bitrate - are recommended for speech encoding only.

Presetting the VBR encoder

To preset the VBR (recommended) encoder:

1. Select one of five quality levels which correspond to five standard VBR parameter settings: 1, 30, 50, 75 and 100. The corresponding approximate bitrates are shown next to the VBR parameter display. If you really insist on doing something nonstandard you can edit the number shown in the VBR parameter window and set it to any value between 1 and 100 (higher number means better quality). Most users, however, stick to VBR settings listed above.

2. Select the preferred channel mode (you can choose between mono and stereo).

Presetting the CBR encoder

To preset the CBR encoder:

1. Select one of four quality levels

2. If you need a better control over the compression rate select the preferred bitrate (note: each quality level has it’s own bitrate range). Remember that higher bitrates correspond to higher sound quality but result in larger files.

3. Select the preferred channel mode (you can choose between mono and stereo). Note that if the two lowest quality levels are selected the mono output is obligatory.

A note for inquisitive users

If you compress the 44 kHz audio file and obtain the 22 kHz output (to check details of the created MPEG file press the Info button situated in the File Panel) don’t be concerned! Given the preferred quality\bitrate and channel mode the MPEG encoder optimizes the sample rate so as to produce the best quality output. This is why sometimes the input and output sample rates don’t match.