Bluetooth A2DP
A2DP defines Bluetooth Classic media-audio distribution with source/sink roles, codec negotiation, and framed audio transport.
A2DP, the Advanced Audio Distribution Profile, defines media-audio delivery over Bluetooth Classic. The transmitting device is the source (SRC); the receiving headphones, speaker or receiver are the sink (SNK).
A2DP standardizes the supported audio configuration and delivery procedures. It does not mean that every Bluetooth device accepts the same optional codec, nor that an app can assume the codec from the soundtrack's filename.
A compatible configuration is selected
The source and sink exchange capabilities through the audio/video distribution transport framework, AVDTP, and select an agreed configuration. SBC supplies the mandatory common codec. Optional codecs, including AAC, require compatible support at both ends.
Codec support is only the first question. The chosen sampling frequency, channel mode and codec parameters must also be accepted. A device advertising an option does not prove that it was selected for the current stream.
In an ordinary Emby route, the app or platform decodes the selected movie or music track before the host's wireless subsystem produces the A2DP stream. This can add another lossy encode after a lossy source decode. A lossless source avoids the source's lossy generation, while the separately negotiated radio coding remains.
One SBC frame is not one transport packet
SBC divides the signal into subbands and encodes blocks of subband samples. For a selected configuration with 16 blocks and 8 subbands, one SBC frame represents 128 samples per channel.
At 48 kHz:
16 × 8 = 128 sample frames
128 / 48,000 = 0.0026667 s ≈ 2.667 ms
| Quantity | Example meaning |
|---|---|
| Sample frame | Simultaneous left/right samples at one instant |
| SBC frame | 128 sample frames in this configuration |
| Media payload | Can contain several complete SBC frames, or specified fragmentation |
| Output delay | Includes more than the duration of one coded frame |
Three of those SBC frames cover 384 sample frames, or 8 ms of audio. Packing several frames together changes transport overhead and delivery grouping; it does not change the waveform's sampling rate.
SBC's bitpool contributes to its coded size along with channel mode, block count and subband count. “SBC” therefore does not describe one fixed bitrate. A bitrate number also needs to distinguish codec payload from packet and radio overhead.
Controls and conversation use other roles
AVRCP supplies remote-control functions such as media commands. A2DP carries the media audio. A headset can support both while their responsibilities remain separate.
Likewise, a communications profile for microphone and call audio serves a different use case from A2DP's media distribution. Switching into that use case can change the selected audio format and processing. Successful pairing alone does not establish which route is active.
Receiver delay is part of movie playback
A sink buffers incoming coded audio before decoding and presenting it. A2DP includes delay-reporting procedures that can help a source account for the receiving side. The resulting end-to-end delay also depends on producer, operating-system and transport buffering.
For movie playback, the useful comparison is sound presentation against picture presentation. A 2.667 ms SBC frame does not promise 2.667 ms total latency. If a route changes during playback, the player may need an updated delay estimate and a new output configuration.