bluez5: add SBC XQ readme

spa/plugins/bluez5/README-SBC-XQ.md

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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 |