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Representing encoder delay explicitly

Review an example that represents the temporal position of 5 seconds of 48kHz PCM audio encoded in a 48kHz AAC sound track.

Overview

Consider the following example of a typical case of PCM source sound data to be encoded as AAC:

The goal is to represent the temporal position of 5 seconds of 48kHz PCM audio encoded in a 48kHz AAC sound track. Assume a media timescale of 48000 and an encoder delay of 2112. For convenience, assume a movie timescale of 48000 as well.

Audio data

Source PCM audio data prior to encoding:

  • Sample rate: 48000 per second

  • Sample count: 240000 PCM samples (5 seconds)

  • Duration in timescale ticks: 240000 (with media timescale the same as sample rate)

AAC in the encoded sound track:

  • Encoder delay: 2112 audio samples

  • Samples per AAC packet: 1024

  • Decoder delay: 1024 samples (or 1 AAC packet)

  • Number of AAC packets: 237 (=((2112+240000) / 1024) rounded up to an integer value)

  • Sample count: 242688 (= 237 * 1024)

  • Remainder samples: 576 (= 242688 - (2112 + 240000))

The transformation from source PCM to encoded AAC results in a sound track with 237 AAC media samples corresponding to 242688 PCM audio samples if decoded and presented in its entirety. Of that total, only 240000 audio samples of source starting at sample offset 2112 (skipping the first 2111 samples) are to be presented. From this, the edit list atom and sample group atom described in the Track structures section below are used to represent the encoder delay.

Track structures

Based on the Audio data section above, the following Edit list atom ('elst') and Sample group description atom ('sgpd') are used to represent the encoder delay.

The edit list atom contains this data:

The edit list atom contains this Edit list table data:

  • Track duration: 240000 (source duration)

  • Media time: 2112 (accounts for the encoder delay)

  • Media rate: 1.0

The Sample group description atom ('sgpd') contains this data:

The sample group description atom contains this Payload data:

  • payload data: -1

And the Sample-to-group atom ('sbgp') contains this data:

The sample-to-group atom contains this Table data:

  • Sample count: 237 (number of AAC packets representing the encoder delay, original audio samples, and remainder audio samples)

  • Group description index: 1 (corresponds to first and only entry in the sample group description atom’s payload data table for these sample units)

Other things to note from this example:

  • You cannot use the edit list by itself to determine the encoder delay or remainder sample count. The sample group atoms provide the encoder delay. The placement of the end of the edit in a compressed audio packet allows calculation of the remainder samples.

  • If the encoder delay was the theoretical minimum for AAC of 1024, then the media time field value in the edit list table data shown in this example would be 1024, not 2112.

See Also

Handling encoder delay