3.3.5.3.1 Sending a Format Change PDU

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The structure and format of the Format Change PDU are specified in section 2.2.4.1.

When the server determines that a new format is necessary, it MUST choose one from the list of audio formats negotiated in the initialization sequence. The server MUST set the NewFormat field of the Format Change PDU to the index into the list of the new audio formats.

The server MUST continue to decode any audio data that arrives using the old audio format until it receives the confirmation Format Change PDU from the client.

If the originally selected format was an AAC format, then the version advertised by the server and client in the Version PDU (section 2.2.2.1) affects the usage of the Format Change PDU:

  • If the client advertised support for version 0x00000001 in the Version PDU (section 2.2.2.1), the server SHOULD NOT send a Format Change PDU.

  • If both the client and server advertised support for version 0x00000002 or greater, the server SHOULD send the Format Change PDU to request that the client use a different AAC format if changes in network conditions require using an AAC codec with different parameters.