TAPE GUIDE (Mar. 1990)

Gilding the Lily

Q. Can a dbx 224X noise-reduction system be used with the audio portion of a Hi-Fi VCR when recording LPs or CDs? I'm speaking, of course, of the high-fidelity recording mode, not the standard longitudinal tracks.

-Richard Dean; Hamilton, Mich.

A. Yes, a noise-reduction unit can be used with a Hi-Fi VCR, but I see no good reason for doing so. The Hi-Fi VCR's audio signal has an S/N ratio that's already so high--about 80 dB--that use of a noise-reduction device would be gilding the lily. There is a wise principle that says you should not introduce more components into the audio chain than strictly necessary.

While each component offers the opportunity to improve the sound in some way, it also presents the possibility of somehow adversely affecting the sound. In other words, we have a cost/ benefit situation. When the benefit is very little, why risk the cost?

More Gilding

Q. I have a dbx NR system that allows me to record tapes with both dbx and Dolby NR, either Dolby B or C. Is this a good idea? if so, which would be better, Dolby B or C NR?

-Don Paulsen; Des Plaines, Ill.

A. Generally, if you want to avoid possible aberrations in frequency response and other respects, it is not a good idea to use two noise-reduction systems in tandem. However, you really can't know unless you try, so you might as well experiment. The cost is zero, except for time.

I suspect that better results would be obtained by coupling dbx with Dolby C NR, rather than with Dolby B NR, inasmuch as Dolby C NR does a better job of maintaining good response in the upper treble when recording at high amplitudes. Noise-reduction systems tend to exaggerate departures from flat response. Since dbx NR already does a superlative job of reducing noise, it seems you have little to gain by coupling dbx with Dolby B or C NR. With dbx NR, the achievable S/N is usually better than 80 dB.

Automatic Level Control, Part II

The May 1989 column contained an item about the use of automatic level control (ALC) in some tape decks, generally lower priced ones. As I have some experience with these circuits, I would like to share my impressions with readers.

When my friend and I were heavily involved in recording music from radio, we noticed, when using a deck with ALC, that there was a lag time before the source material was recorded at normal level. For example, if a song began with a loud passage, the initial milliseconds would be recorded softly and then grow progressively louder. It was as if the ALC circuit was unsure of the level of the incoming signal and set itself low at the start.

A nice thing about manual control of volume, as opposed to ALC, is that one can fade the program material in or out if desired. For example, if there isn't enough tape at the end of a cassette to finish dubbing a selection, a fade-out is preferable to an abrupt end. Similarly, fade-in is desirable at the outset of a tape if one begins in the middle of a selection.

-Ki Suk Hahn; West Covina, Cal.

Still More on Hi-Fi VCR Recording

In the January and November 1989 columns, readers Brent Jessee and John H. Markell, respectively, discussed the problem of dropouts when using Hi-Fi VCRs for audio recording.

While considering results to be very good overall, Jessee stated that dropouts are a serious problem, even with the best videotapes; that the number of dropouts increases with tape use; that the problem is aggravated in slow speed, and that, unlike PCM processors, Hi-Fi VCRs provide no method of dropout compensation. Markell agreed that high speed is necessary to minimize dropouts, but felt that the problem resides more in the sync circuits of given VCRs than in the tape or the nature of Hi-Fi VCR recording. Very different views are expressed by reader Leo Backman of Helsinki, Finland: Dropouts tend to become fewer [emphasis added] as the tape is used.

I have observed a reduction factor of about 5 to 50 with various brands of videotape. The reduction is due to the polishing action of the heads as they scan the tape's surface. This removes the excess impurities and magnetic oxide flakes, although permanent defects--such scratches--remain.

However, in the first and last 10 turns of the tape, dropouts invariably increase. This phenomenon occurs in every tape system, including DAT. The loose oxide flakes are squeezed between the first turns of the tape as it is wound onto the hard reel hub. Being of a stone-hard substance, the particles cause irrevocable surface damage.

Hence it would be desirable for manufacturers to coat the reel hubs with something soft, like chamois. I've done just that and gotten consistent service from a VHS videotape with over 250 passes; the first 10 meters have experienced no increase in dropouts.

It is erroneous to claim that tape dropouts pass the heads more slowly at slow speed (and therefore are more likely to have a significant audible effect). Quite the opposite is true. The slower the tape speed, the higher the relative speed between the heads and the tape because the heads and the tape are moving in substantially the same direction. It is fairly accurate to state that the relative head-to-tape speed is the scanning velocity of the heads minus the tape speed. So, halving the tape speed increases head-to tape speed. (This increment raises the output voltage of the Hi-Fi heads, which is why Hi-Fi VCRs often have slightly better S/N figures at LP rather than SP speed.) I'm not convinced that sync loss is the reason for increased dropout rates.

My experience is that the problem is due to misalignment of the tape path or the guide post.

The problem of incompatibility between recording on one VCR and playback on another is often the result of a slight offset of the audio FM carriers.

The NTSC standard puts these at 1.3 and 1.7 MHz for the left and right channels, respectively. A steady FM carrier requires a well-optimized adjustment in recording current, which isn't always the case with factory-adjusted VCRs.

Thus, FM audio retrieval is somewhat prone to dropouts and sudden carrier loss. Still, we've come a long way from the dark ages of early Hi-Fi VCRs.

(Audio magazine, Mar. 1990, HERMAN BURSTEIN)

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