AAC WebCodecs Registration

1. Fully qualified codec strings

This codec has multiple possible codec strings:

• "mp4a.40.2" — MPEG-4 AAC LC

• "mp4a.40.02" — MPEG-4 AAC LC, leading 0 for Aud-OTI compatibility

• "mp4a.40.5" — MPEG-4 HE-AAC v1 (AAC LC + SBR)

• "mp4a.40.05" — MPEG-4 HE-AAC v1 (AAC LC + SBR), leading 0 for Aud-OTI compatibility

• "mp4a.40.29" — MPEG-4 HE-AAC v2 (AAC LC + SBR + PS)

• "mp4a.67" — MPEG-2 AAC LC

2. EncodedAudioChunk data

If the bitstream is in adts format, the [[internal data]] of EncodedAudioChunks are expected to be an ADTS frame, as described in section 1.A.3.2 of [iso14496-3].

If the bitstream is in aac format, the [[internal data]] of EncodedAudioChunks are expected to be a raw AAC frame (syntax element raw_data_block()), as described in section 4.4.2.1 of [iso14496-3].

3. AudioDecoderConfig description

If AudioDecoderConfig.description is present, it is assumed to a AudioSpecificConfig as defined in [iso14496-3] section 1.6.2.1, Table 1.15, and the bitstream is assumed to be in aac.

If the AudioDecoderConfig.description is not present, the bitstream is assumed to be in adts format.

The AudioDecoderConfig.sampleRate and AudioDecoderConfig.numberOfChannels members are ignored.

4. EncodedAudioChunk type

The [[type]] for an EncodedAudioChunk containing AAC is always "key".

NOTE: Once the initialization has succeeded, any AAC packet can be decoded at any time without error, but this might not result in the expected audio output.

5. AudioEncoderConfig extensions

partial dictionary AudioEncoderConfig {
  AacEncoderConfig aac;
};

aac, of type AacEncoderConfig

Contains codec specific configuration options for the AAC codec.

5.1. AacEncoderConfig

dictionary AacEncoderConfig {
  AacBitstreamFormat format = "aac";
};

format, of type AacBitstreamFormat, defaulting to "aac"

Configures the format of output EncodedAudioChunks. See AacBitstreamFormat.

5.2. AacBitstreamFormat

enum AacBitstreamFormat {
  "aac",
  "adts",
};

The AacBitstreamFormat determines the location of the metadata necessary to decode the encoded audio stream.

aac

The metadata of the encoded audio stream are provided at configuration via AudioDecoderConfig.description.

adts

The metadata of the encoded audio stream are provided in each ADTS frame, and therefore no AudioDecoderConfig.description is necessary.

6. Privacy Considerations

Please refer to the section Privacy Considerations in [WEBCODECS].

7. Security Considerations

Please refer to the section Security Considerations in [WEBCODECS].