# AAC encoding background AAC 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. ## Overview AAC 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. AAC encoding uses a transform over consecutive sets of 2048 audio samples, applied every 1024 audio samples (overlapped). For correct audio to be decoded, both transforms for any period of 1024 audio samples are needed. For this reason, encoders add at least 1024 samples of silence before the first ‘true’ audio sample, and often add more. This is called variously “priming”, “priming samples”, or “encoder delay”. A couple of definitions for use in this discussion: - *Encoder delay* is the delay incurred during encoding to produce properly formed, encoded audio packets. It typically refers to the number of silent media samples (priming samples) added to the front of an AAC encoded bitstream. - *Decoder delay* is the number of “pre-roll” audio samples required to reproduce an encoded source audio signal for a given time index. For AAC this number is typically 1024 and is algorithmically based. This is in contrast to encoder delay which is determined by the encoder and encoding configuration used. However, decoder delay establishes the minimum encoder delay possible (that is, 1024 for AAC). The common practice is to propagate the encoder delay in the AAC 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. 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. The following figure gives an example of a typical encoded AAC audio bitstream. The upper portion of the illustration represents the AAC encoded domain with equal-sized AAC packets, the lower portion represents the PCM sample domain: ![A diagram with two vertically-stacked horizontal sequences illustrating an audio bistream. The top sequence contains square boxes with rounded corners lined up next to each other. Above the sequence is a caption that highlights one box and says “Access unit 1024 samples”. The first two boxes in the sequence are filled with a light color. The third box is filled vertically with the light color for a small part of the box, and the rest is filled with a dark color. The fourth through sixth boxes are filled with the dark color. The last box in the sequence is partially filled with the dark color, then filled with the lighter color. The second sequence starts with a light blue background, and a solid blue horizontal line in the middle. It extends from the beginning of the sequence, and ends where the third box in the sequence above changes color. The next part of the sequence shows a red line, starting from the end of the blue line, then proceeding a wave pattern. The red line ends where the last box in the sequence above changes color. The end of the sequence has a light blue background with a solid blue line in the middle.](images/com.apple.qtff/encoded-aac-audio@2x.png) The source audio to be encoded, shown as the red waveform, is 5389 samples long. 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. This data will be represented in 8 AAC packets, where each packet represents 1024 audio samples. 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). The result breaks down into the following values: - 2112 priming samples at the start—Required to correctly encode the start of the audio. - 5389 samples of actual audio. - 691 remainder samples—Required to pad out to the AAC packet size. Therefore, to correctly extract the original 5389 samples of source audio, the first 2112 samples of priming and the last 691 samples of the remainder must be removed. - 8192 - 2112 - 691 = 5389 original source samples. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)