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How lossy is bluetooth transmission?

juliangst

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Just a quick question about Bluetooth on iOS:
If I play lossy music with Spotify or Apple music and use a bluetooth DAC AMP, which on iOS only supports the AAC codec, will I get the same signal as with a wired connection?

Lossy music on Apple Music is AAC 256 kbit/s and the AAC bluetooth codec is 256 kbit/s too, so ideally it should not matter if using the DAC Amp wired or wireless, does it? (in this case I'm talking about the Fiio Btr5)
 

EdTice

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Just a quick question about Bluetooth on iOS:
If I play lossy music with Spotify or Apple music and use a bluetooth DAC AMP, which on iOS only supports the AAC codec, will I get the same signal as with a wired connection?

Lossy music on Apple Music is AAC 256 kbit/s and the AAC bluetooth codec is 256 kbit/s too, so ideally it should not matter if using the DAC Amp wired or wireless, does it? (in this case I'm talking about the Fiio Btr5)
Assuming that your playback device just sends the AAC stream directly to the DAC, I don't think there is any further degradation from the encoding but somebody might correct me.

That being said, Bluetooth audio is subject to stuttering even when the two devices are millimeters apart. BT uses non-licensed spectrum and it uses a non-reliable protocol. If you use the DAC wired (USB), the music will be delivered across the Isochronous protocol which has all of the same problems as Bluetooth except that wires are more reliable than unlicensed spectrum My own experience is that I hear stuttering in Bluetooth even in my car and never hear it in USB audio but that doesn't mean that it isn't a real problem

The *ideal* playback mechanism would be for music to be delivered via media-transfer-protocol (based on the USB bulk transfer protocol), buffered at the DAC, and then played back with no risk of stutter or jitter the same way that they would be played if you were to put them on a USB stick and plug that into the playback device. Some AVRs support this but I'm not aware of any standalone DACS that have this ability. I'll be delighted if somebody comes along and corrects me!
 

gfx_1

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soundexpert.org has some articles on bluetooth it is lossy but it varies by used codec:
older article http://soundexpert.org/articles/-/blogs/audio-quality-of-bluetooth-aptx

A mobile (android) phone with Qualcomm chip uses aptX which is better than SBC which seems the default of windows laptops
I'm playing a with a linux desktop with cheap bt dongle and pulseaudio 15 supports more bluetooth codecs like aptX and sbc_xq.
With version 5 of pavucontrol (a extended volumeknop thingy in linux) you can pick some codecs like multiple versions of sbc
including sbc_xq552 but I can´t find aptx.
 

RickSanchez

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Unfortunately it's not as simple as saying one codec is better than the other. It depends on what implementation of that codec exists on a device. Many people talk about how bad SBC is, but in reality it allows for some high quality encoding; it's just that very few manufacturers choose to implement SBC at those levels on their devices.

The best article I've seen regarding this here: https://habr.com/en/post/456182/ So in regards to the AAC codec it'll depending on which devices are talking to each other. From the article:

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EdTice

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I'm so old that it's amazing to me that mobile devices have enough CPU power to decode and reencode "on the fly." As I haven't written mobile apps in a long time, I am happy to be corrected here. However, iOS devices do not have full mixers. And there's also no reason they can't send the AAC via Bluetooth without reencoding (and saving a lot of battery power in the process). What they actually do is really up to Apple's implementation. However, AAC is a "fixed block" format so it would be trivial to *not* re-encode by default and only re-encode blocks where other audio (such as notifications) need to be layered on top. Again I don't know that iOS does this. Nor do I know that it does not do this. I can't find anything on this in Apple's docs either way. Probably because they don't want people opening support cases when transcoding does happen and especially since it's not audible on most playback devices.
 

EdTice

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A simple question, that with all the possible answers gets easy over the complexity of a moon landing. I will follow with interest.
And which we are unlikely to answer definitively since iOS is not open source and we have limited ability to measure such things.
 
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