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Bluetooth SBC and AptX

I'm doing some research into A2DP streaming and the codecs used.  I'm reading that SBC is required as part of the SBC spec.  So, here's my list of questions for which a search of this forum and Google did not provide the ELI5 answers I am looking for.

1.  Since SBC is required, is it applied to all A2DP streams, even those that already have a lossy codec?  So, if I have compressed a FLAC file to MP3 and am streaming it to my car stereo, is the MP3 again being compressed by SBC.  Or is the protcol smart enough to know I am not saturating the full 761 kbps and there is no need to apply SBC?  I know stacking lossy codecs is not really a good idea and I wonder what kind of degradation this causes.  If I can figure out a good way to ABX it, I will try it.

2. I was reading about the Apt-X series of codecs.  If you're using Apt-X Bluetooth, does it replace SBC, or is SBC applied to the Apt-X stream?  If Apt-X does replace SBC, I am going to assume that you are no longer A2DP compliant and will need something proprietary that can talk Apt-X on both ends.

3. Is there any way to stream audio over bluetooth that is not compressed by the Bluetooth protocol?

Bluetooth SBC and AptX

Reply #1
For audio the A2DP protocol is needed.
This protocol allows for various codecs.
SBC is mandatory but other are possible like
MPEG-1,2 Audio
MPEG-2,4 AAC
ATRAC (Sony)

If both sender and receiver support it, other Codecs like APT-X are possible.
So it is A2DP + some common Codec

You can’t do uncompressed audio over Bluetooth, it’s bandwidth is to limited.
There are protocols like KLEER or SKAA but that is not Bluetooth
TheWellTemperedComputer.com

Bluetooth SBC and AptX

Reply #2
For audio the A2DP protocol is needed.
This protocol allows for various codecs.
SBC is mandatory but other are possible like
MPEG-1,2 Audio
MPEG-2,4 AAC
ATRAC (Sony)

If both sender and receiver support it, other Codecs like APT-X are possible.
So it is A2DP + some common Codec

You can’t do uncompressed audio over Bluetooth, it’s bandwidth is to limited.
There are protocols like KLEER or SKAA but that is not Bluetooth


Is SBC support mandatory or SBC implementation mandatory?  Are my lossy compressed music files (MP3, AAC, Ogg) being compressed with SBC again, or is SBC only being used when the file you're trying to push will exceed the available bandwidth?

Bluetooth SBC and AptX

Reply #3
Is SBC support mandatory or SBC implementation mandatory?

I'm not sure I see the distinction you're trying to draw.

The point is that all certified devices MUST support the least common denominator.  This is how certification ensures compatibility between all certified devices, regardless of what optional codecs either might also support.
Creature of habit.

Bluetooth SBC and AptX

Reply #4
Is SBC support mandatory or SBC implementation mandatory?  Are my lossy compressed music files (MP3, AAC, Ogg) being compressed with SBC again, or is SBC only being used when the file you're trying to push will exceed the available bandwidth?


It will be recompressed every time. I had the same problem with a portable speaker using Bluetooth. SBC can use up to 372 kbps if you are lucky.
So if you don't want to have lossy to lossy recompression you will have to use FLAC or not Bluetooth. AptX will recompress too.
Only if your listening device supports AAC or MP3 directly then there won't be recompression to SBC, with your AAC or MP3 files.

Hope this clears it up. Thats why i am wating for the Chromecast Audio.

On the other side i couldn't complain even with SBC and using Spotify. Quality was good for a portable speaker in the woods.
If you are using car radio maybe just use ABX if possible.

Quote
In “Middle Quality” mode (@229kbit/s) it is roughly comparable with mp3@128kbit/s or aac@96kbit/s. At max possible bitrate 372kbit/s SBC codec is comparable with aac@192kbit/s and most artifacts it produces are beyond human perception.

Bluetooth SBC and AptX

Reply #5
Is SBC support mandatory or SBC implementation mandatory?  Are my lossy compressed music files (MP3, AAC, Ogg) being compressed with SBC again, or is SBC only being used when the file you're trying to push will exceed the available bandwidth?


It will be recompressed every time. I had the same problem with a portable speaker using Bluetooth. SBC can use up to 372 kbps if you are lucky.
So if you don't want to have lossy to lossy recompression you will have to use FLAC or not Bluetooth. AptX will recompress too.
Only if your listening device supports AAC or MP3 directly then there won't be recompression to SBC, with your AAC or MP3 files.

Hope this clears it up. Thats why i am wating for the Chromecast Audio.

On the other side i couldn't complain even with SBC and using Spotify. Quality was good for a portable speaker in the woods.
If you are using car radio maybe just use ABX if possible.

Quote
In “Middle Quality” mode (@229kbit/s) it is roughly comparable with mp3@128kbit/s or aac@96kbit/s. At max possible bitrate 372kbit/s SBC codec is comparable with aac@192kbit/s and most artifacts it produces are beyond human perception.



I have never had any issues listening to music over Bluetooth.  I'm sure if I did some critical listening, I MIGHT be able to find something after hours of listening.

Where I do have problems is with spoken word.  I'll be driving in my car listening to an audiobook or podcast (usually sped up to 1.7x with pitch correction) and I have trouble understanding what's being said.  I'll turn up the volume and it doesn't help my comprehension.  I pull over, turn off bluetooth and plug in an AUX cable and suddenly I can understand every word withou issue.

I'm not one of those guys that claims to have golden ears or that I can hear a difference between a FLAC and a good MP3.  I know the limits of human hearing.  And I would be happy to try and ABX this in some way, if I could find a good way to do it.  I'm sure the fact that I am speeding up the spoken word is a contributing factor in all this.  But the fact that I am straining to listen to a 128kbps MP3 file over Bluetooth, but can understand it without issue over a cable tells me there is something going on with the frequencies that contain human speech patterns.

Having said that, I think I am going to rip my next audiobook to FLAC and see if that makes any difference whatsoever.

As for the Chromecast Audio...  It's on Christmas list.  Hopefully, my wife will help me geek out this Christmas.  Once you can synchronize audio across Chromecasts, I'll be able to build some cheap speaks, buy a few amps and have a low costs Sonos like system in my house.

Bluetooth SBC and AptX

Reply #6
I have never had any issues listening to music over Bluetooth.  I'm sure if I did some critical listening, I MIGHT be able to find something after hours of listening.

Where I do have problems is with spoken word.  I'll be driving in my car listening to an audiobook or podcast (usually sped up to 1.7x with pitch correction) and I have trouble understanding what's being said.  I'll turn up the volume and it doesn't help my comprehension.  I pull over, turn off bluetooth and plug in an AUX cable and suddenly I can understand every word withou issue.

I'm not one of those guys that claims to have golden ears or that I can hear a difference between a FLAC and a good MP3.  I know the limits of human hearing.  And I would be happy to try and ABX this in some way, if I could find a good way to do it.  I'm sure the fact that I am speeding up the spoken word is a contributing factor in all this.  But the fact that I am straining to listen to a 128kbps MP3 file over Bluetooth, but can understand it without issue over a cable tells me there is something going on with the frequencies that contain human speech patterns.

Having said that, I think I am going to rip my next audiobook to FLAC and see if that makes any difference whatsoever.

As for the Chromecast Audio...  It's on Christmas list.  Hopefully, my wife will help me geek out this Christmas.  Once you can synchronize audio across Chromecasts, I'll be able to build some cheap speaks, buy a few amps and have a low costs Sonos like system in my house.


When we decided which portable speaker to buy. Just a little box to hear music on the way without cables. One model was maybe using less bitrate. It had better specifications but the sound still sounded worse than the other one. Of course i don't really know that they used less bitrate, but thats what i think it probably was. And we only used AAC CVBR 256 kbps, Spotify at Q5 and Q9 Vorbis and FLAC files. So what you describe with the voice and audio books at 128 kbps MP3 sounds plausible to me. If i have time i will probably make a test with low bitrate audio books.

If you really want to find out, you could take a lossless voice sample and compress it in different bitrates (64, 96, 128, 160, 192 kbps and so on).
Of course you could try music  and other codecs too. If it is really recompression it should go away with the higher bitrates.

I even found some devices from a company who delivered lossless music but used their own protocol. It was just not cheap and they didn't have many products.
It's a pity that because of the licensing only SBC and not MP3 is mandatory. Otherwise we wouldn't need to talk about this. In my dreams they would use FLAC, Vorbis and Opus for A2DP. But it's just a dream. :-D

PS: AptX sounds like a alternative. It has a good support and a better audio codec. I have read from some people that it doesn't produce artifacts like SBC does. My cousin liked it but i didn't test it myself. It even has an hybrid lossless mode now, where it only uses lossy encoding when the bandwidth constraint can not be reached. So a small apt-X Dongle over USB could maybe already solve your problem.

Bluetooth SBC and AptX

Reply #7
I'm reading that SBC is required as part of the SBC spec.

Wat?
Quote
So, here's my list of questions for which a search of this forum and Google did not provide the ELI5 answers I am looking for.

You suck at googling?

Quote
1.  Since SBC is required, is it applied to all A2DP streams, even those that already have a lossy codec?  So, if I have compressed a FLAC file to MP3 and am streaming it to my car stereo, is the MP3 again being compressed by SBC.

A2DP sources as well as sinks must support SBC. So, it will always be available as a common denominator for devices supporting A2DP.

Quote
2. I was reading about the Apt-X series of codecs.  If you're using Apt-X Bluetooth, does it replace SBC, or is SBC applied to the Apt-X stream?

That would be pretty crazy. A2DP devices can optionally support other codecs such as MP3, AAC, and aptX. So if both source and sink support aptX (and all goes well), streaming will use aptX *instead* of SBC.

Quote
If Apt-X does replace SBC, I am going to assume that you are no longer A2DP compliant and will need something proprietary that can talk Apt-X on both ends.

Not sure what you mean. Drunk posting? aptX is proprietary. But, once again, A2DP allows for optional support of other codecs. So, if both source and sink support aptX, you can stream it over a Bluetooth A2DP connection. No spec violation there.

Quote
3. Is there any way to stream audio over bluetooth that is not compressed by the Bluetooth protocol?

I think the more interesting question would be: Are there A2DP compatible sources and sinks out there which support a losslessly compressed codec?


 
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