QuickTime Audio Codecs

If you have the disk space and bandwidth, the best thing you can do is leave your audio uncompressed. Uncompressed audio normally uses 8-bit (phone quality) or 16-bit (CD quality) samples. (Compressor supports up to 64 bits per sample floating point and a maximum sample rate of 192 kHz.) If you have disk space and bandwidth limitations, you must compress your audio. However, audio takes up less disk space than video, so it doesn’t need to be compressed as much as video.

The two most important components of audio codecs are sample rates and size. The sample rate sets the sound quality, and sample size sets the dynamic range of the sound. QuickTime audio codecs allow you to set both the sample rate and size of your source media file.

Sound Sample Rates

Digitized sound consists of sound samples captured at different frequency rates. The more sound samples per second, the higher the sound quality. For example, audio CDs use a 44.1 kHz sample rate, DVDs sample at 48 kHz, and telephone networks sample voices at 8 kHz. The sample rate you choose depends on the nature of the sound. Music requires a higher sample rate than voice, because music contains a wider range of frequencies. Spoken voice has a more limited range of frequencies, so you can choose a lower sample rate and still maintain acceptable audio quality. In most cases, you should choose the highest sample rate available.

Reducing the sample rate can shrink a media file by as much as 5:1. The audio quality will be affected, but not as much as it would be if you used 8-bit sampling. The following table shows common sample rates and the audio device quality to expect at each rate:

Sample rate
Audio device quality
48 kHz
44 kHz
22 kHz
FM radio
8 kHz

Sound Sample Size

The sound sample size determines the dynamic range of the sound. 8-bit sound provides for 256 possible values, whereas 16-bit sound allows for more than 65,000 possible values.

Choose 16-bit sound for music that has both soft and loud sections, such as orchestral music. For spoken voice or music that has a more or less constant volume level, choosing 8-bit sound can still yield good results.

If you need to shrink your media file, you can reduce the sample size from 16 bits to 8 bits. This cuts the file size in half but also degrades the audio quality.