Weird AV problem, is it the audio file or something else?

[Tricky]

Hi all,

 

We had a weird problem yesterday, when playing certain mp3's in our hall we had lot of distortion.

 

Here is an overview of the setup;

 

HP PC with onboard audio connecting to;

- Stereo Amp & Mixing desk

or

- Mono Amp with multiple inputs

then to the large wall speakers obviously.

 

Testing other audio sources on either Amp worked fine.

 

 

The issue;

 

With a few specific mp3's there would be significant distortion on the Stereo Amp, but when switching over to the mono amp it was fine. While the hardware is probably a factor my assumption was an issue with the stereo separation in the mp3 file itself. I later found the source of the mp3 was ripped from Youtube, so while 320bps it's not exactly professional grade!

 

What I did next was open the mp3 in Audacity, the problem remained when hitting play. However, I then panned the channel in Audacity to Left and Right and found the further off center you moved it the better it sounded. Essentially, 100% left or right and it sounded perfect.

 

I'm not savvy enough in audio technologies to know what specifically could cause this. I tried converting to mono within Audacity but that left me with no sound at all, again unless I panned left or right.

 

While I can to some extent just blame the mp3 and move on, I've love to know what is going on here!

 

I've stuck the mp3 on the link below;

http://www.filedropper.com/begodsbydaniellerosewlyrics

 

 

Cheers

 

PS. If anyone knows a better place to post this please let me know.

[Marci]

Nothing wrong with the MP3. Plays fine on everything here with no issues / distortion, been compressed at PPM-6 by the looks of things which is standard for broadcast. First place I'd turn to would be simply turning the volume down on whatever is playing the MP3. Panning left or right decreases overall output level. How much by depends on what panning law is selected within your software.

 

I'd also check whether your wiring is in phase from source to mixer, mixer to amps, and amps to speakers (ie: the consistency of hot to hot, ground to ground, neutral to neutral). If I invert the phase of either left or right channel on it's own, the vocal leaps out of the mix ridiculously loudly (which would result in unexpected overdrive if the system was set close to gain limits). If I convert to mono with the phase inverted on either side, the overall level drops to almost none existent (as the waveform is inverted, and one side cancels out the other, which would result in almost no perceivable sound). Sounds as tho somewhere within your setup, the phase is being inverted by dodgy wiring.

Edited May 8, 2015 by Marci

[Chuckster]

I've seen with a few HP laptops that a combined audio and speaker input jack causes this issue no matter what audio file you play.

 

A separate audio and speaker jack produces none of the symptoms.

[Tricky]

Nothing wrong with the MP3. Plays fine on everything here with no issues / distortion, been compressed at PPM-6 by the looks of things which is standard for broadcast. First place I'd turn to would be simply turning the volume down on whatever is playing the MP3. Panning left or right decreases overall output level. How much by depends on what panning law is selected within your software.

 

I'd also check whether your wiring is in phase from source to mixer, mixer to amps, and amps to speakers (ie: the consistency of hot to hot, ground to ground, neutral to neutral). If I invert the phase of either left or right channel on it's own, the vocal leaps out of the mix ridiculously loudly (which would result in unexpected overdrive if the system was set close to gain limits). If I convert to mono with the phase inverted on either side, the overall level drops to almost none existent (as the waveform is inverted, and one side cancels out the other, which would result in almost no perceivable sound). Sounds as tho somewhere within your setup, the phase is being inverted by dodgy wiring.

 

Thanks Marci, what you are saying rings true. The reason I even mentioned the mp3 itself is that other mp3's and (audio sources) played without distortion.

 

Will investigate the wiring as you suggest.

Edited May 8, 2015 by Tricky

[Marci]

Depends how each individual track's been mixed as to the level of effect phase inversion has, and what effect that has on the system. With this particular tune, inversion makes the vocals leap out massively. I've got tracks where doingso makes the vocals vanish entirely, others where phase inversion simply removes the kick drum, bass & snare (items typically summed to mono in the LFE band if the track was mastered with multiband compression). Chuckster's point is valid within the same context - a dicky jack socket which is bridging across the L & R poles or shorting L to Grnd or R to Grnd will also produce phase issues (or a dicky driver which isn't digitally detecting & switching the socket from one role to the other correctly - usually it's looking at impedance to determine what mode it needs to be in).