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Topic: The wierdest hing ... (Read 2478 times) previous topic - next topic
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The wierdest hing ...

I posted this on r3mix but it seems pretty quiet ...

I was doing some mp3 tests just out of interest for myself. I ripped the track Paul Simon - Graceland, encoded it to mp3 using lame --r3mix as well as --alt-preset extreme. I then decompressed them to wav using winamp with the MAD plugin, and burnt then to cdr along with the original ripped wav.

I played them on my system for a little while, couldn't hear any difference so i got out my headphones. Now these are a really ancient pioneer set. The jack was pretty corroded but i did nothing about that and just plugged them in and started listening. I couldn't understand what was wrong because the first track to start playing was the --r3mix one, but I thought i had got the input mixed up and was listening off my pc to a really low bitrate mp3.

All the bass was cut off, only the high frequencies were audible. There was a really apparent echo, or some sort of phase delay. And the most noticable thing was the twinkling in the really high frequencies that i would normally associate with low bitrate mp3s. I didn't even have to listen hard, it was very obviuos.

I tried the next track (extreme) and all the echo and lack of bass was still there. The twinkling was greatly diminished but still audible. On the original track, same thing ragarding the echo and bass but no artifacts at all.

I couldn't understand it, I fiddled with the system a bit but nothing changed so I unplugged the phones and the sound comming out of the speakers was fine, nothing odd. So I plugged the headphones in again and now they were fine too. Then I tried wiggling the headphone jack and found that the sound crackled, must be due to the corrosion. If I wiggled carefully, suddenly the bass disappeared, the echo started and the twinkling became apparent.

What's going on? It must be something to do with the dirty jack but how is this affecting the sound so profoundly?

This is what I am thinking : What cuts out low frequencies? A capacitor. The corrosion insulates the jack but the insulating layer is so thin that it makes a capacitor. But how is this causing the echo or making the mp3 artifacts so apparent?

Any Ideas?

The wierdest hing ...

Reply #1 –
What probably happened is that the ground connection of your headphones did not make decent contact. The headphones then only plays the difference between left and right channel. Because bass is mostly placed in the center of the soundstage, it will disapperar almost completely.
 

The wierdest hing ...

Reply #2 –
As for the twinkling artifacts, it could well be that the louder low frequencies would have masked the distortion (particularly securely in the case of APS, less securely with r3mix - the r3mix forum is dead, I believe, and much of the site is out of date).

Because you've so seriously messed up the frequency response, the masking frequency is almost totally absent and you can now discern the distortion in the high end. The same would likely be true if you decoded to WAV and used software (e.g. CoolEdit) to highpass filter pretty severely.

If you were to apply the filter to the WAV first then encode to a VBR mp3, the psychoacoustics ought to render the high frequencies correctly with distortion that would be inaudible, and it should sound the same as the filtered WAV.

BTW, the filter effect does sound like capacitance. It might even be a break in the wire, which can be made to touch by changing the bend on it. It's plausible that the capacitative reactance (impedence) at high frequencies is a few ohms and still provides sufficient matching to allow audible power transfer between the output impedance of your amp and the input impedance of the headphones via the wire, while at bass frequencies the reactance, X = 1/(2*pi*f*C) is much much higher due to the lower value of f.

Regards,

Dick Darlington.