using freq response to adjust speed

[Dan McDonald]

I have a number of tapes that have gone through several generations of copies from various people so the speed is off. Sometimes it is noticeable, sometimes not.

Has anyone tried adjusting the speed of a recording based on the frequency response? I would guess that there are published sources of what notes are at what frequency. I was thinking that I might be able to get notes from tunings, etc. between songs, and find their frequency, then calculate what percent over or under the recording is. Has anyone tried this?

Dan McDonald

[Craig Maier]

Re: using freq response to adjust speed

You might want to try running the signal through the spectrum analyzer in its highest resolution mode of operation. It will take some time to integrate. Look for the 60 Hz hum signal which is often present. See how far off it is from true 60 Hz. Then use ratio proportions to correct the tape. Otherwise, you will have to rely on your hearing. Only if you have perfect pitch, and the band tuned up correctly will this second method work. I do not know how to do this using frequency response as the reference. I suppose that it may be possible, but would require some research into the physics of the tape and its interaction with the tape recording and playback heads. Complicated problem when you are several generations away from the source tape.

[Beautman]

Re: using freq response to adjust speed

If you hit the library, you should not have a problem finding the frequencies of the notes of the scale, although I can't think of a particular publication to direct you to.

The A above middle C, often used as the reference for tuning by orchestras, is 440 hz in the U.S. today (so the A an octave lower is 220 hz, etc., and if you know the necessary physics you can derive other pitches yourself). There are even little devices sold for musicians with mics or instrument inputs that tell you if the pitch you're playing is above or below the fixed standard.

So what you could potentially do is identify a note that should be an A in the music (or whatever other pitch), highlight it, and bump the speed until it reads at 440 on the spectrum analyzer.

But here's the problem. In different countries and during the last century, an A has ranged from 438 hz to 446 hz. During the baroque period, they tuned an entire half-step lower. The pitch that is called an A today would have been written as an A-sharp or B-flat in Vivaldi's time.

Thus, while you could figure out how to adjust the speed of the recording in order to bring the pitch to the "correct" frequency, you'd have no way to know what frequency the musicians on any given recording tuned to at the time they recorded (except that it's a decent bet that U.S. orchestras during the past four decades and many electronic keyboards are set to a 440 A).

Even if you know the right standard, it may not have been used. If the piano happened to be tuned slightly high or low, all of the other musicians would have had to adjust to match it. Some older pianos _can't_ be tuned to correct pitch without an overhall.

You'd have to further assume that the engineers or producers didn't deliberately change the speed for artisic reasons. I have heard stories of musicians tuning a little flat, then speeding the recording up to the correct pitch to make it seem like they can play faster. Miles Davis' Kind of Blue was issued a half-step off, although that was probably an engineering screw up.

So your idea makes sense, but only rarely might you have an opportunity to apply it.

[Dan McDonald]

Re: using freq response to adjust speed

The textbook "Audio in Media" by Stanley Alten lists the keys of the piano and the frequency of each.

Just thought I'd post this reference if anyone else needs it.

I was thinking that the speed adjustment by frequency response should give a closer approximation than just listening by ear. I have some tapes where the musicians are using electronic tuners between pieces, so the tones are quite audible. I'll give it a try. Thanks for the suggestions.

Dan McDonald

[Craig Maier]

Re: using freq response to adjust speed

You can also find the frequencies of all of the musical notes in the Diamond Cut Users Manual. As was pointed out, the potential problem is that there still is no real reference here. If they tuned up to a different A or tuned up incorrectly, all bets are off.