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Topic: Relationship between Mono/Stereo MP3 and Bluetooth?!? (Read 628 times) previous topic - next topic
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Relationship between Mono/Stereo MP3 and Bluetooth?!?

I have a technical question relating to an unusual problem I am having, and I thought this would be the best place to ask.

I've determined that many of my MP3 files that play fine when listened to via direct headphone connection are playing in a consistently distorted way (a sort of "swishy" sound) when listened to through Bluetooth headphones (BOSE A2), earphones (B&O H5) or speakers (UE Boom 2). The good MP3's always play fine, but the bad ones are always distorted.

These "bad" files are encoded with low bitrate, but as  I say, sound fine as long as it's not through Bluetooth. Interestingly, of three players I have, these files sound bad on Bluetooth through an AK70 Mark I, an AK70 Mark II, but sound fine on Bluetooth from an AK JR.

I looked at all my files in Similarity, and found an obvious differentiating factor. The "bad" ones that won't play well through Bluetooth are all mono, and the ones that are stereo always play fine.

This seems strange. How would one account for this? Thanks.

I've attached a sample recorded by my iphone of the swishy file playing from an AK70 through a UE Boom2 speaker.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #1
Probably a bug in your player or a BT codec. Have you tried inserting a filter that converts from mono to stereo? (that is, 2x identical channels). foobar2000 can do it, for example.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #2
Mmmm, that would probably explain the problem. As in the codec they implemented only supports stereo, and when feed with a mono  entry, it interlaces invalid data. It might also be a problem of the samplerate, but I can't be sure about that either. (usually mono mp3 are also 22Khz instead of 44Khz).

You should judge if those files are important to keep, and might opt to transcode them (reencode) to stereo mp3 or another codec, and recommend labeling the file name as transcodes.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #3
The vendor never bothered to test mono files, and never noticed they don't work with Bluetooth. They probably don't properly format the pcm stream if it's mono.

Like I said in your last thread, those obscure mp3 players tend to be extremely buggy. They're usually a reference design with outsourced firmware. No one is paying for testing or QA.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #4
Mmmm, that would probably explain the problem. As in the codec they implemented only supports stereo, and when feed with a mono  entry, it interlaces invalid data. It might also be a problem of the samplerate, but I can't be sure about that either. (usually mono mp3 are also 22Khz instead of 44Khz).

You should judge if those files are important to keep, and might opt to transcode them (reencode) to stereo mp3 or another codec, and recommend labeling the file name as transcodes.

Thanks.

The bad ones are all encoded at 44.

Bitrate varies from 90's to 160.


Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #5
Mmmm, that would probably explain the problem. As in the codec they implemented only supports stereo, and when feed with a mono  entry, it interlaces invalid data. It might also be a problem of the samplerate, but I can't be sure about that either. (usually mono mp3 are also 22Khz instead of 44Khz).

You should judge if those files are important to keep, and might opt to transcode them (reencode) to stereo mp3 or another codec, and recommend labeling the file name as transcodes.

So, I might transcode them with Foobar to, say, 192 AAC Stereo, and that should do the trick? I should have thought of that.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #6
The vendor never bothered to test mono files, and never noticed they don't work with Bluetooth. They probably don't properly format the pcm stream if it's mono.

Like I said in your last thread, those obscure mp3 players tend to be extremely buggy. They're usually a reference design with outsourced firmware. No one is paying for testing or QA.
And it's kind of funny that these players also cost a lot of money. https://us.astellnkern.com/products/ak70-mkii costs 700 bucks and could be used to play an audiophile bingo:
"Dual DAC separates the left and right audio channels from input to output"
"refined sound signature"
"4-pole balanced output" (directly to headphones!)
"when used as a USB DAC, will bring out more detail and provide a better listening experience than your computer is capable of delivering" (oh we all must have terrible computers I guess)
Yet they suffer from such basic, in-your-face obvious bugs.

Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #7
So, I might transcode them with Foobar to, say, 192 AAC Stereo, and that should do the trick? I should have thought of that.
Yes they should play normally after that.


Re: Relationship between Mono/Stereo MP3 and Bluetooth?!?

Reply #9
So, I might transcode them with Foobar to, say, 192 AAC Stereo, and that should do the trick? I should have thought of that.
Yes they should play normally after that.

Albeit, just about twice the size.
As sad as it may sound, I guess this is the least of things to worry about in this situation.

 
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