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55 lines
2.5 KiB
Markdown
55 lines
2.5 KiB
Markdown
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## SBC XQ
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SBC XQ is standard SBC codec operating at high bitrates and thus reaching the
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transparent audio transport quality of AptX (HD) or other proprietary codecs.
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A2DP specification (A2DP SPEC) defines SBC parameters. These parameters are
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negotiated between the source (SRC) and the receiver (SNK) at connection time :
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- Audio channel mode : Joint Stereo, Stereo, Dual Channel, Mono : all modes
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are MANDATORY for the SNK according to A2DP specification
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- Number of subbands: 4 or 8 - both MANDATORY for the SNK implementation
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- Blocks Length: 4, 8, 12, 16 - all MANDATORY for the SNK implementation
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- Allocation Method: Loudness, SNR - both MANDATORY for the SNK implementation
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- Maximum and minimum bit pool : between 2 to 250, expressed in 8 bit uint
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(Unsigned integer, Most significant bit first) :
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- A2DP spec v1.2 states that requires all SNK implementation shall handle
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bitrates of up to 512 kbps (which correspond to bitpool = 76).
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- A2DP spec v1.3 doesn't specify any bitrate limit, and some high-end SNK
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devices announce bitpool between 62 and 94 (bitpool 94 = 551kbps bitrate).
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Bluetooth standard radio capabilities are as follow :
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| Bluetooth speed EDR | EDR 2Mbps | | EDR 3Mbps |
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|-------------------------|-----------|-------|-----------|
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| Speed (b/s) | 2097152 | | 3145728 |
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| Radio slot length (s) | 0.000625 | | 0.000625 |
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| Radio slots / s | 1600 | | 1600 |
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| Slot size (B) | 163.84 | | 245.76 |
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| Max payload/5 slots (B) | 676.2 | | 1085.8 |
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| max bitrate (Kb/s) | 1408.75 | | 2262.08 |
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The A2DP specification V1.3 provides RECOMMENDATIONS for bitpool implementation
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for the encoder of the SRC : it is required to support AT LEAST the following
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settings :
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- STEREO MODE : 53
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- MONO MODE : 31
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- DUAL CHANNEL : unspecified, so let's assume that the MONO value can be used : 3
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According to http://soundexpert.org/articles/-/blogs/audio-quality-of-sbc-xq-bluetooth-audio-codec ,
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AptX quality can be reached either :
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- in STEREO MODE, with bitpool ~ 76
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- in DUAL CHANNEL MODE, with bitpool ~ 38 per channel
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| sampling Freq (Hz) | 44100 | 48000 |
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|-------------------------|-----------|-------|
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| bitpool / channel | 38 | 35 |
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| Frame length DUAL (B) | 164 | 152 |
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| Frame length JST (B) | 165 | 153 |
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| Frame length ST (B) | 164 | 152 |
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| bitrate DUAL CH (kb/s) | 452 | 456 |
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| bitrate JOINT ST (kb/s) | 454 | 459 |
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| bitrate STEREO (kb/s) | 452 | 456 |
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