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MPEG-4 Audio Support

QuickTime 6 plays audio files of AAC and handles ISMA profile levels 0 and 1, with the exception of CELP audio. In the current release both encode and decode of AAC are supported. (Note that encode is AAC [Low Complexity] only.) QuickTime 6 conforms to the MPEG-4 audio specification.

QuickTime 6 audio can handle reading in MP4 files, and can export them to QuickTime movies.

A few current limitations:

• Audio can only handle ISMA Profile 0 and Profile 1 for AAC.

• Audio cannot handle multichannel AAC.

In this section:

Defining AAC
QuickTime AAC Encoder

Defining AAC

The characteristics of AAC include

• Perceptual audio codec, similar to MP3

• Multichannel capability

• “Indistinguishable” audio quality––that is, you can take an encoded file and the source from the encoded file and you should not be able to tell the difference over a stereo system. From a CD source:

  • AAC Low Complexity requires 96 kbps per channel.

  • MP3 requires at least 128 kbps per channel.

QuickTime AAC Encoder

The characteristics of the QuickTime AAC encoder include

• AAC-Low Complexity

• Acceptable source

  • 44.1 kHz or 48 kHz. It is recommended that when encoding audio, your source should be an even multiple of those numbers.

  • Mono or stereo

• Output

  • Mono: 16 to 256 kbps

  • Stereo: 16 to 256 kbps

  • The sample rate is automatically scaled to the bitrate.

  Important:  The output sample rate is linked directly to the output bitrate and number of channels. Note that the listed sample rates are input sample rates. Using QuickTime, the encoder can take any sample rate that QuickTime can play. But no matter what sample rate you happen to provide, that rate will be converted to the selected sample rate before it is fed to the encoder. Thus, it is best to provide a sample rate that divides evenly into the selected sample rate. For example, if you have a 22.050 kHz source, select 44.1; if you have a 16 kHz source, select 48.

Table 1-1 is a mapping of the input sample rate + output bitrate + output number of channels to output sample rate.

You are best advised to provide content in these sample rates, regardless of the target bitrate. If you already have content in a different sample rate, however, it is not a problem. QuickTime will perform the necessary Sample Rate conversion.

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