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pipewire/spa/plugins/bluez5/README-SBC-XQ.md
JP Guillemin b17db2cebc bluez5: add SBC XQ readme
2021-01-07 09:54:16 +01:00

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SBC XQ

SBC XQ is standard SBC codec operating at high bitrates and thus reaching the transparent audio transport quality of AptX (HD) or other proprietary codecs.

A2DP specification (A2DP SPEC) defines SBC parameters. These parameters are negotiated between the source (SRC) and the receiver (SNK) at connection time :

  • Audio channel mode : Joint Stereo, Stereo, Dual Channel, Mono : all modes are MANDATORY for the SNK according to A2DP specification
  • Number of subbands: 4 or 8 - both MANDATORY for the SNK implementation
  • Blocks Length: 4, 8, 12, 16 - all MANDATORY for the SNK implementation
  • Allocation Method: Loudness, SNR - both MANDATORY for the SNK implementation
  • Maximum and minimum bit pool : between 2 to 250, expressed in 8 bit uint (Unsigned integer, Most significant bit first) :
    • A2DP spec v1.2 states that requires all SNK implementation shall handle bitrates of up to 512 kbps (which correspond to bitpool = 76).
    • A2DP spec v1.3 doesn't specify any bitrate limit, and some high-end SNK devices announce bitpool between 62 and 94 (bitpool 94 = 551kbps bitrate).

Bluetooth standard radio capabilities are as follow :

Bluetooth speed EDR EDR 2Mbps EDR 3Mbps
Speed (b/s) 2097152 3145728
Radio slot length (s) 0.000625 0.000625
Radio slots / s 1600 1600
Slot size (B) 163.84 245.76
Max payload/5 slots (B) 676.2 1085.8
max bitrate (Kb/s) 1408.75 2262.08

The A2DP specification V1.3 provides RECOMMENDATIONS for bitpool implementation for the encoder of the SRC : it is required to support AT LEAST the following settings :

  • STEREO MODE : 53
  • MONO MODE : 31
  • DUAL CHANNEL : unspecified, so let's assume that the MONO value can be used : 3

According to http://soundexpert.org/articles/-/blogs/audio-quality-of-sbc-xq-bluetooth-audio-codec , AptX quality can be reached either :

  • in STEREO MODE, with bitpool ~ 76
  • in DUAL CHANNEL MODE, with bitpool ~ 38 per channel
sampling Freq (Hz) 44100 48000
bitpool / channel 38 35
Frame length DUAL (B) 164 152
Frame length JST (B) 165 153
Frame length ST (B) 164 152
bitrate DUAL CH (kb/s) 452 456
bitrate JOINT ST (kb/s) 454 459
bitrate STEREO (kb/s) 452 456