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off topic: bluetooth MP3 and AAC

post #1 of 6
Thread Starter 
Hi

now, I know this sounds like its about speakers, but that just happens to be where I started with this. I wonder if there are any tec savvy folks out there who can perhaps answer this.

I play MP3 (256Kb/s sometimes as VBR with floor limit of 160Kbs) on my Nokia E72 and was listening to speakers (Pioneer BTS5 ones) and noticed that the highs were a little 'wrong' (Vangelis track, Alpha off Albedo 0.39) ... so I whacked on my headphones (socket on the phone) and noted it was much clearer.

A little digging revealed that Bluetooth A2DP actually seems to support sending data in a few formats, like MP3 and AAC format.

Hmm, so I started thinking (too late I guess) and thought that given the spec is AAC that the phone could be doing some sort of cross encoding from what my native MP3 is.

I notice however that when playing stuff at 128Kb/s that the phone has a greater 'range' from the speakers than playing 256. This implies to me that greater bandwidth is needed and so the Bluetooth connection requires a better 'signal to noise' on the analog Tx/Rx side of the data transfer. Meaning that it may be sending the data at the higher rate.

If it is at the higher rate, then that either means that the Pioneer speakers have an inferior DAC in them to my Nokia or that the analog audio bits on the Pioneer speakers are inferior to my Bose headphones ... equally likely.

Does anyone know?

Thanks :-)
post #2 of 6

There are a lot of moving parts here, so it would be hard to answer with any certainty.  

 

The fact that some of your 128 kbps files sound better on the speaker (over the bluetooth hop) than the 256 kbps is probably not due to SNR.  I assume that you can put your phone essentially on the bluetooth receiver, and you should have plenty of bandwidth.

 

My guess is that your bluetooth receiver, for some reason, is not accepting your 256 kbps files in their native format, so your phone is forced to use some inferior, or lower bitrate, codec, e.g. SBC.  Your Nokia is a few years old by now, so that may be part of the problem.  Can you retry playing the same files from a newer device?

 

When I was trying out different bluetooth receivers for my stereo, I was surprised by the large variation in quality while playing the same track from the same iPhone 5.  I ended up choosing the Avantree Roxa, which does a pretty good job.

post #3 of 6
Thread Starter 
I think you misread my post. I didn't say the 128 sounded better or worse, I said at they got better range (distance at is) than the higher baandwidth ones. Probably I should not have quoted range.

The signal to noise that I was referring to was that of the RF transmission of the data. Data is still transmitted over RF in an analog domain (there is no other way) and the bit rate of a channel is determined by that channels S/N ratio...

I do have a late model tablet here (android ICS) and this exhibits similar although does not get as good a distance from the speakers as the phone. Interestingly we are talking only a few meters and line of sight too.

I expect from my readings at the DAC of different devices will be a big part as you seem to have experienced. Personally I rate the actual hardware of my Nokia higher than many other more modern phones, of course it is limitedin other areas such as browser support....
Thanks anyway for your answer.

:-)
Edited by obakesan - 10/1/13 at 8:52am
post #4 of 6
Quote:
Originally Posted by obakesan View Post
The signal to noise that I was referring to was that of the RF transmission of the data. Data is still transmitted over RF in an analog domain (there is no other way) and the bit rate of a channel is determined by that channels S/N ratio...
 

 

I do not think the bitrate of the bluetooth codec varies in real time with the SNR (which determines the maximum theoretical bitrate).  I understand your observation, but I don't think it is correct.  If you think A2DP codecs dynamically change their bitrate based on RF conditions, please point me to your reference.

 

Cheers!

post #5 of 6
Thread Starter 
Hi

again I think you've misunderstood me
Quote:
Originally Posted by klaberte View Post

I do not think the bitrate of the bluetooth codec varies in real time with the SNR (which determines

I have not said this either. I said that the channel gets less range. As I walk away the music breaks up. I believe this is because it is unable to get all the packets through because the channel no longer supports the required bandwidth.

So while I know that I didn't write in perfectly comprehensible unambiguous terms you still seem to have misunderstood my initial post and are reading things into it that I did not say.
post #6 of 6

You wrote that your phone might be  "doing some sort of cross encoding" and that "the bit rate of a channel is determined by that channels S/N ratio".  Regarding the second statement, no, the SNR provides a theoretical upper bound in capacity (measured in th b/s).  

 

I was responding to these theories you were making.  I think I understood just fine what you wrote, but perhaps not what you meant.  :-)

 

Welcome to head-fi!

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