#Aac audiobook creator quicktime iso

The result breaks down into the following values:Ģ112 priming samples at the start-Required to correctly encode the start of the audio.Ħ91 remainder samples-Required to pad out to the AAC packet size.

The total duration represented by these 8 AAC packets is 8192 audio samples (note that this is longer than the duration of the source audio). This data will be represented in 8 AAC packets, where each packet represents 1024 audio samples. You can see how it is represented with the blue priming and remainder samples against the fixed size access units–AAC packets–drawn above it. The source audio to be encoded, shown as the red waveform, is 5389 samples long. The upper portion of the illustration represents the AAC encoded domain with equal-sized AAC packets, the lower portion represents the PCM sample domain: Figure G-1 AAC encoded audio Since encoded audio packets hold a fixed number of audio samples (for instance, 1024 samples) additional trailing or ‘remainder’ silent samples following the last source sample are required so as to pad the final audio packet to the required length.įigure G-1 gives an example of a typical encoded AAC audio bitstream. When these audio packets are then decoded back to the PCM domain, the source waveform represented will be offset in its entirety by this encoder delay amount. The common practice is to propagate the encoder delay in the AAC bitstream. However, decoder delay establishes the minimum encoder delay possible (that is, 1024 for AAC). This is in contrast to encoder delay which is determined by the encoder and encoding configuration used. For AAC this number is typically 1024 and is algorithmically based. It typically refers to the number of silent media samples (priming samples) added to the front of an AAC encoded bitstream.ĭecoder delay is the number of “pre-roll” audio samples required to reproduce an encoded source audio signal for a given time index. A couple of definitions for use in this discussion:Įncoder delay is the delay incurred during encoding to produce properly formed, encoded audio packets. This is called variously “priming”, “priming samples”, or “encoder delay”. For this reason, encoders add at least 1024 samples of silence before the first ‘true’ audio sample, and often add more.

For correct audio to be decoded, both transforms for any period of 1024 audio samples are needed. AAC encoding uses a transform over consecutive sets of 2048 audio samples, applied every 1024 audio samples (overlapped). For the encoded audio data, the terms “AAC packet” and QuickTime “media sample” are used.ĪAC requires data beyond the source PCM audio samples in order to correctly encode and decode audio samples due to the nature of the encoding algorithm. The terms “sample” and “audio sample” in this appendix are used to refer to PCM samples.

#Aac audiobook creator quicktime iso

AAC implementations typically represent 1024 PCM audio samples in one AAC packet (synonymous in this context with a QuickTime media sample, and also referred to in ISO documents as an “access unit”).