TAPE GUIDE (Jul. 1988)

PCM Error Correction

Q. I own a VCR and may purchase a PCM unit for recording audio with it.

How seriously do tape dropouts affect recording? Does the PCM processor have error correction and concealment facilities?

-Charles Roberts; Springfield, Mo.

A. According to Sony, its PCM units contain error-detection and error-correction circuits. Dropouts can be a problem, more so at the slower videocassette speeds. However, I understand that when high-quality tape is employed, the problem occurs so infrequently as to be a minimal hazard to good recording.

Avoiding Saturation

Q. Please tell me the best way to copy a CD onto a cassette so as to minimize saturation of the tape during musical peaks.

-James G. Bennett; Cincinnati, Ohio

A. First establish the maximum safe recording level, as indicated by your deck's record level meter, for the tape you will be using to copy CDs. (This is relatively easy if your meters are peak reading, as is usually the case. If they are average-reading, as is the case for true VU meters, the task is more difficult, because the meters may understate transients by anywhere from roughly 10 to 20 dB.) Also take into account the deck manufacturer's recommendations. For example, the owner's manual might say that you can safely record Type II tapes up to a level of +3 dB: your own experience with a particular brand of tape might show that this is just about right, or that you can safely go a couple of dB or so higher, or that it is best to stay close to the 0-dB mark.

Next, play the loudest passages of your CD, with the deck in record mode; observe and adjust the record level so that the meter doesn't exceed the safe point, which is typically about 3 to 5 dB above 0 VU. If this is too tedious, sampling the CD on a random basis by using the fast-forward control may tell you what you need to know.

If you employ dbx noise reduction, the signal-to-noise ratio is so great--typically 80 dB or more, and occasionally over 90 dB-that you can afford to sacrifice a few dB of S/N by deliberately under-recording by a few dB for safety's sake. Even with Dolby C NR, which ordinarily achieves S/N of 70 dB or more, you can follow pretty much the same course to avoid saturation.

One more thought: Maximum output level of a CD player is very close to 2V. If you have access to an audio generator that can deliver about 300 to 400 Hz at 2V, feed this signal into the CD input jack of your receiver (or preamplifier or integrated amplifier). Put the deck into record mode, and adjust the record level control so that the record level meter doesn't exceed the maximum safe point. Put the record level control at this setting when recording CDs.

Chide Remarks

On a subject that very much bears repeating, reader Bob Katz of New York City writes:

I read your comments in the October 1987 issue regarding a reader's cleaning misadventure with isopropyl alcohol. While I doubt that the 70% isopropyl solution damaged the individual's heads, there is a very good reason why the sound quality deteriorated.

The reason lies in the other 30% of the solution. All 70% isopropyl solutions available in drugstores contain glycerine or other lubricants to aid their intended use as rubbing alcohols. To prove this, put a small amount of 70% alcohol on a counter; after it dries, there will be a smooth, soapy residue.

It is this deposit which gets onto the head and guide surfaces, causing the capstan to slip, and it can even get into the head gap. If the 30% of the solution were pure water, there would be no problem in using 70% isopropyl. That is why 91% (or 99%) isopropyl is always recommended for cleaning the components contacted by the tape.

By the way, grain alcohol (190 proof) leaves no deposits and is perfectly acceptable for head cleaning. However, vodka is not good for cleaning because the flavorants and charcoal that are frequently added for taste can leave a deposit. Prosit.

(Source: Audio magazine, Jul. 1988, HERMAN BURSTEIN)

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