How to get rid of mains (?) hum?

[lethargic_man]

I've had the intention for a while of transferring all the music I have for my Hybrid Music System to epicyclic, and then burn it to CD-ROM, before the hardware it runs on (1987-vintage BBC Master and 1986-vintage Music 500) complete packs up. A year ago I had an attempt, but ran into problems with what I thought was mains hum. compilerbitch suggested plugging everything into the same mains socket, so, finally having got around to moving the pedestal housing my stereo, CD player and Beeb across the room (and getting and tacking down longer speaker wire so I could connect the first of these back to the speakers on the side they had come from), I tried this out... and found the situation unimproved. My father suggested cutting the earth contacts on the Music 500 output, as I'd probably got a mains hum loop, but that made no difference either.

The funny thing is that when I simply play the Music 500 through epicyclic, it plays fine (there's a small amount of background noise, about as much as a computer fan); it's only when I try and capture the sound to record it (using Audacity) that the bad noise turns up (71k MP3). I've tried various techniques on Audacity for getting rid of it, but they always result in a degradation of the sound quality (60k MP3); better would be to eliminate the noise from the input. Anybody got any idea why I only get it when I try to capture the audio?

(Also, when I tweak the Alsa volume controls, there's a master control for playback; there's also a PCM one, and increasing that increases the noise dramatically. Increasing the line-in control also increases the noise when listening to straight play-through, but I've a feeling Audacity bypasses this.)

Comments

[compilerbitch.livejournal.com]

The other standard approach is to use an (audio) isolation transformer in line with the signal that you're having trouble with. They are not so thick on the ground these days, unfortunately, though.

[compilerbitch.livejournal.com]

*has idea*

Ah, I think I know what it probably is. The old Yamaha DX7 keyboard synthesizer used to have a similar problem.

Basically, the Music 500 has a very primitive DAC, which doesn't cut off at 20KHz. Most likely, it is giving you a spray of high harmonics that are aliasing against the sample rate of the ADC in your sound card (which itself probably doesn't have a very good low pass filter). Playing the same sound through your sound card won't cause the problem, because the audio is mixed in analogue form so the ADC doesn't play any part.

Try stringing one or more ferrite beads on the audio cable, or using a ferrite ring and winding the cable through it a few times. It's a bit of a crude solution, but it often works wonders in these cases.

(listens to mp3 to check)

Yes, that's definitely it. What you have there is definitely NOT hum, it's aliasing noise.

[compilerbitch.livejournal.com]

What you need is an audio low pass filter. It may be that the Music 500 puts out a lot of crap at just above the audio band, so you may need a steeper filter than you'd get with the ferrite approach. Do try passing it through the stereo -- it is possible that it itself has limited bandwidth, which would actually be of benefit for this purpose.

If none of the easy answers work, I could probably come up with a small circuit that you could build on a bit of Veroboard that would fix the problem.

all-embracing

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