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strange audio distortion, any ideas about what could cause it?
AndyH-ha
post Jul 10 2012, 09:34
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I'm guessing this is probably a hardware or system problem rather than an mp3 or an application software problem, but the symptoms are unlike anything I've encountered in the digital realm before. I have no idea of what kind of computer problem could produce this kind of distortion. I've put in a support request to the application software company but I have little hope they will find the source of the problem. Maybe someone has a useful idea.

A sample is attached as the last entry in
http://www.hydrogenaudio.org/forums/index....mp;#entry801778
(this restriction for attachments that Hydrogen Audio has, unlike any other forum I've ever used, is rather a pain).

The process starts with a perceptually encoded audiobook file. One program plays it in real time. Another program inputs that playback and encodes it as mp3. The second program has installed a "virtual soundcard" that the first program uses for output; no audio hardware is involved.

There is no actual hardware soundcard on the computer (drivers for the MB soundchip have not been installed and said soundchip is turned off in the BIOS). The computer is not connected to any network. It has no wireless hardware. It is running no anti-virus or automatic updates. The OS is 64 bit Win7, so it does whatever Windows always does in the background, but nothing has been added to that.

This process worked without flaws for at least 20 audio books. The audio quality was always good. The distortion in this eight hour file started a little more than three hours into it. It is not continuous, there are some breaks back into normal, but most of it is distorted. The source file has no distortion.

I did not listen to more than about 15 minutes of distortion, so I don't know how far it extends, or if there is more of it later. I did sample a few snippets further on, here and there, without finding any problem, but this covered only a tiny part of the total audio. I also sampled a little bit of another audio book processed weeks later and found nothing wrong, but again the sampling is too limited to really know anything for sure except that the distortion is not continuous. (Both books are 30+ hours long, each.)
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2Bdecided
post Jul 10 2012, 10:09
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Just a guess: it's possible this specific type of distortion is linked to the original file format - is the file/codec "audible", or some other speech-optimised codec?

If that's the case, the reason you haven't heard it before is because you've never heard the decoding of this type of file "go wrong" before - it's the sound of the codec trying to decode data that's been corrupted in some way.

Why the decoder is getting wrong data is another question - but if you can prove that this kind of distortion is caused by sending corrupt data to that codec, then you at least know at what part in the audio chain things are going wrong.

If corrupt data in this format doesn't cause those kinds of distortions, then it doesn't help at all. It could be something entirely different, like a buffering problem.

Cheers,
David.

This post has been edited by 2Bdecided: Jul 10 2012, 10:10
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AndyH-ha
post Jul 11 2012, 02:13
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So, you suggest that maybe the first program, decoding the perceptually encoded file that is its input, made an error that is simply carried forward into the final product of the second program? Corrupted data might be a good explanation: the distortion is not the result of a process, it started out that way so it ended up that way, but the source is not corrupted. It can be played and sounds quite normal. Therefore we need an explanation as to why it did not play normally in this case. What kind of computer jerk off behavior can give this result?

I reran the process and there was no distortion in the output, at least none where it occurred in the first preparation, and going forward for 30 minutes or more. Since the file is 8 hours long, I can't think of any way to be certain about such an intermittent problem other than to listen to the entire 8 hours. There are also probably more than 200 hours of other audio books already prepared but not yet heard, so that process of elimination is not a satisfactory approach.

If it helps anyone's thinking, the source is Audible .aa files. They can only be played in iTunes (or maybe Windows Medial Player?) on a computer activated for them specifically by Audible. Audible could also activate certain personal players to play them.

Capture of the iTunes output in the digital domain can be accomplished by any internal soundcard that has a mixer chip set up for it, such as my Audiophile 2496, or many models of Creative cards. In this case I am using a dedicated program sold for just this purpose. It provides the virtual sound card and the output encoding all in one. Such programs have been around for a long time.

The reason for the entire activity is that my daughter wanted to buy from Audible but they do not support either of her otherwise quite satisfactory personal players. We choose this way rather than a much larger purchase for another mp3 player. She buys one large book per month on her contract with Audible, and has purchased quite a few other, shorter, books from the "specials" they offer every time she signs on, so she has ended up with quite a backlog she hasn't yet had time to hear.

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2Bdecided
post Jul 11 2012, 11:04
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Sansa Clip says it supports audible - don't know about the protection. I can see the reticence to buy another player, but the time+hassle+PC use to do it this way may cost more.
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