Regular Spikes

[Dan McDonald]

OK, here's one...
I have a wave file take from a Mississippi John Hurt performance in 1963. It was recorded on open reel. I was using the spectrum analyzer and noticed one big spike at 60 Hz, which I thought was normal, but there were also spikes at 50, 100, 150, 200, 250, etc. on up the spectrum. It is very regular. Does anyone know what that is?

I have the spectrum analyzer set on averaging, 100 dB, 2048 and .67. The spikes are very clear.

Thanks

Dan

[Craig Maier]

Re: Regular Spikes

I am not certain, but I would guess that the original tape, having been recorded in the US picked up some 60 Hz line noise. Then someone transferred it in another country in which the power line freq. is 50 Hz. So the spectra that you are seeing are harmonic multiples of the country in which the transfer took place. Another possibility is that the tape deck used an AC servo motor (Akai did that on their open reel machines). The Akai motor ran closed loop against a phase locked loop circuit. It is possible that the speed of the motor servo oscillator happened to be 50 Hz fo the speed at which the tape was recorded having the multiples therof. Anyone have any other theories?

[Dan McDonald]

Re: Regular Spikes

Thanks, Craig - your second explanation may be more likely, as the tape has not been out of the U.S. to my knowledge.

Is there any sources you know of for getting some of the basics of forensic audio? I'm getting interested in studying some of the tapes I have to see what I can figure out about them. For example, is there any way of telling if a recording was made using a microphone versus through a soundboard? I have a recording that people insist is from a soundboard, but I hear a sound that sounds to me like a handheld microphone being moved slightly - sounds just like a recording I made in which I held the mic the entire concert.

Anyway, I was looking for books or articles that might deal with what you can find out about tape histories.

Thanks

Dan

[greg_m]

Re: Regular Spikes

Where was the original performance? North America, or Europe? Any chance some european film or video equipment was used at some point?

Here's a rather far-fetched theory...

Some recorders which were intended for double-system film recording (like the widely-used Nagra III and IV), had an extra head that recorded "pilot tone" on the tape, so that the tape could later be synchronized with the film. The pilot tone was an internally generated tone, the same frequency as the power line frequency; so in the US it was 60Hz, in Europe it would be 50Hz.

The pilot tone head was rotated 90 degrees relative to the audio head on the recorder! (since the signal frequency was so low, this worked). In theory, then, the regular audio head would not pick up any pilot tone when the tape was played. But if the alignment wasn't exactly 90 degrees, there could be some crosstalk. Also, these were full-track mono machines; I suspect that if you played a pilot-tone tape back on a stereo head, you might get a large amount of pilot crosstalk.

The pilot tone had to be very accurate and stable, so it was generated by an internal crystal and divider circuit; I guess in theory it might have a small amount of harmonic content that didn't get filtered out.

So, JUST MAYBE... if someone recorded an audio tape on a recorder that had a pilot tone head, and IF the recorder was set for the European pilot frequency (50Hz) and IF there were some harmonics generated by, or remaining from, the pilot tone process... then you MIGHT have some 50Hz and harmonics on the tape. (Even if the performance were in the US, if it were being filmed by a European film crew, this might have happened.) And if the tape was recorded in the US, it would have some 60Hz hum from power wiring, dimmers, whatever, also recorded on the tape.

Also, of course, VCRs use a sync track (near the edge of the videotape) to lock up the servos. It's not unheard of to get some crosstalk from the sync track (or video sync circuitry, for that matter) that bleeds over into the audio track(s). Now I'm not absolutely certain whether european machines use a 50Hz sync pulse, or a higher frequency; but if the performance was recorded on a european-standard VCR, that might be an explanation.

Well, as I said, it's far-fetched, but who knows????? Knowing the location of the performance might help narrow down the probabilities.

You could always buy a used Nagra on E-Bay (typically around $2k - $3k range), set the pilot generator for 50Hz, and make some test tapes. I wish I had one, they're beautiful machines; if I did, I'd gladly run a test for ya.

Happy trails!

[This message has been edited by greg_m (edited 09-09-2002).]