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diff --git a/spa/plugins/bluez5/README-SBC-XQ.md b/spa/plugins/bluez5/README-SBC-XQ.md new file mode 100644 index 0000000..3b393aa --- /dev/null +++ b/spa/plugins/bluez5/README-SBC-XQ.md @@ -0,0 +1,54 @@ +## 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 | |