Apple Compressor (4.0) - Manual (Page 123)

Chapter 5 Custom settings and output formats 123
Video codecs can be divided into the following two types:
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Lossless codecs: Lossless codecs completely preserve the data they compress and are usually
used to transfer footage from one editing suite to another. Lossless codecs can require high
data rates and high-end computers with specialized hardware. Examples of lossless codecs are
the Animation, 8-bit, and 10-bit uncompressed 4:2:2 codecs.
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Lossy codecs: In comparison to lossless codecs, lossy codecs return only an approximation
of the data and are usually used to deliver finalized video to an end user. The level of
approximation is determined by the quality of the codec. Typically there is a trade-off between
the output quality of the codec and its achievable level of compression. With some codecs, a
compression ratio of at least 5:1 can be reached without visible degradation. An example of a
lossy codec is the Photo-JPEG codec.
Low-quality lossy compression introduces unnatural visual artifacts that may become
noticeable to the human eye. Using a lossy codec, you should compress your media file only
once, because compressing a file multiple times adds more artifacts.
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 the 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.
Choosing an audio codec
AAC is a good general-purpose audio delivery codec and is compatible with a wide variety
of playback devices. For QuickTime movies playing on slower computers, IMA 4:1 is a better
choice because it’s less processor intensive, thus allowing the computer to focus on the more
challenging video stream.
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.