high speed mastering

[Dan McDonald]

This may be a really stupid question, but I have a decent open-reel machine that plays at 7 and 15 ips and some tapes that are at 3.75 ips. I am just wondering what I'd lose if I played back at 7, recorded onto disc and reduced the speed through DCArt. The tapes are exceedingly rare (e.g. only one in existence), so I don't want to do this unless I have a chance to retain most of whatever quality exists.

How stupid would this be?

Thanks

Dan McDonald

[Beautman]

Re: \"double-speed\" mastering

You could lose some quality.

Say the program has high frequencies in the 12,000 - 15,000 cps range. When you double the speed, these become 24,000 - 30,000 cps. They're still on the tape, but somewhere along the path to your hard drive, some components may not have such super-sonic frequency response. If your tape deck/head preamp rolls off frequency response above 20,000 cps (quite good, about the upper limit of human hearing), then when you slow the recording back down on the computer you won't have anything above 10,000 cps. (This may not be strictly true, because there are certain off-setting benefits from higher tape speed, but you get the idea.)

It's also generally a good idea to avoid unnecessary digital processing.

Of course, whether this matters depends on the source material. If you've got tapes of school plays recorded on a 1950s Webcor, it may not matter.

[Dan McDonald]

Re: \"double-speed\" mastering

Thanks Beautman.

This is a folk concert recorded in 1955 on a home open-reel machine. I doubt the frequency response of the original is particularly good. The instruments involved are male voice, banjo, acoustic guitar. Mike was set about 5 feet from the stage (my guess). In this case, as I read your post, it may work?

Dan

[Beautman]

Re: \"double-speed\" mastering

So record a few minutes at each speed and see if it matters.

[Craig Maier]

Re: "double-speed" mastering

It will work, but if you record at 44.1 KHz, the maximum theoretical frequency response (due to sampling theorem) of the end result will be only 11.025 KHz. As was stated, this may not matter, depending on the material, the quality of the original recorder, and of course, the quality of the microphone(s) used. I personally own recordings from the mid 50's that were recorded at 7.5 ips (none at 3.75 ips) that do contain freq responses all the way out to about 18 KHz. Pro recordings made at 15 ips had no trouble at all achieving a full 20 KHz response during that period of history. I am not sure what performance lower cost machines operating at the very slow 3.75 ips were capable of. So, just try what Beautman recommended. Try it.

[Dan McDonald]

Re: \"double-speed\" mastering

If I could, I'd record at both speeds and see, but I can't record at the correct speed - only twice as fast. That was why I was wondering about what I would lose - I have no comparison basis.

Dan

[EarlCat]

Re: \"double-speed\" mastering

Why not record from the 7 ips machine at 96KHz sampling rate? When the recording is slowed down to half that speed, you will end up with a recording at 48K, thus preserving frequencies (theoretically) all the way up to 24K.

[Craig Maier]

Re: \"double-speed\" mastering

The critical process that determines the highest frequency recorded by a tape machine is determined at the time of recording. During playback, running at a higher speed will not magically give you something that does not exist to begin with. As a matter of fact, you may loose a little because of the bandwidth limitation of the audio pre- amplifiers in the tape playback equipment.

[greg_m]

Re: \"double-speed\" mastering

Well, I agree with everything said above. (Perhaps a first!)

In practice, the weak link in most tape machines (and certainly in all home machines) was the width of the gap on the recording and playback heads (which may have been one dual-function head on many cheaper machines).

The width of this gap, relative to the tape speed, determined the highest frequency that could be recorded and played. It's unlikely that a tape recorded on a home machine at 3.75 IPS would have much useful info about 10 kHz anyway, so you shouldn't lose much by "double speed mastering."

Another factor to consider is that the playback de-emphasis curves were different for the two speeds, so you won't get quite the correct frequency response... but it should be close enough, based on your description of this project.

A bigger issue is: what's left on the tape? Magnetic tapes lose their magnetization over time, and there are several variables that affect how much magnetization (i.e. signal level) they lose, and how fast. Higher frequencies "self erase" faster than lower frequencies, so older tapes will tend to sound muddier than when they were newly recorded. Heat cycling, and high temperatures in general, accelerate deterioration. DROPPING the tape will tend to erase it, again affecting high frequencies more than low. (Yes, this is true... remember 5th grade science class, when you hit the magnet with the hammer, until the magnet was demagnetized... same holds true with magnetic tape recordings.)

At any rate, with all these other variables, I think "double speed mastering" is the least of your worries.

And a caveat: old tapes shed their oxide as you play them. Sometimes you get only one or two chances to play a tape before it has lost so much oxide as to be nearly useless. So turn on the computer, click "record" on your software, and then begin to play the tape for the first time! If there is a logical pause in the program material (applause between musical selections, etc.), stop the tape, check the heads and head guides. If they're starting to gum up with oxide sheddings, then take the tape off the machine, clean the machine, let it dry for 5 minutes, then re-thread and commence playback from that point onward.

Good luck!

[Dan McDonald]

Re: \"double-speed\" mastering

Thanks Greg -

I did do it, and it worked out pretty well. The artist (Pete Seeger) blew out his microphone or the P.A. system about half-way through the show, and it sounds better after that (since the mic is then picking up only Pete on stage, and not the P.A. echoing around the place). It's an interesting performance from an historical perspective and has some nice music. The double-speed didn't seem to lose much.

Dan

[greg_m]

Re: \"double-speed\" mastering

Pete Seeger! Gosh, Dan, what a find! I'd love to hear that one.

If you've seen the film "O Brother, Where Art Thou?" (and what an excellent film!) you know that there is a lot of potential value in old recordings; the soundtrack won an award for proving that point.

Listen closely to some of the restored cuts on that CD, and you'll find that they don't necessarily have a "modern" amount of HF information. But they're clean, listenable, and interesting because of their content.

One of the great things about programs like DC-Art is that it gives us a chance to try to salvage our old recordings -- family, school, church, whatever -- and make them available to other folks who will enjoy them.

Since getting my copy of DC-Art, I've started digging through the attic (a terrible place for tape storage, BTW), and I'm sad to discover that I don't have most of the old tapes that I made back in the '60's. But the few that I have are wonderful to re-discover, and there's a real sense of achievement when you clean up something like that. Long live DSP!

Now... what the HECK did I do with those Doc Watson masters... ???

[Dan McDonald]

Re: \"double-speed\" mastering

Doc Watson masters?

email me at droncit@yahoo.com and maybe we can figure out what to do. The main reason I got this software to begin with was to restore my old tapes.

Dan