TAPE GUIDE (Feb. 1983)

"Mis-Matched" VU Levels

Q. I have a problem with the response of the VU meters of my cassette deck. Before I record, I set the level in the record mode with pause engaged. When I have recorded the tape and play it back, the playback level shown by the VU meters is totally different from the level shown when recording. What is wrong?

-William Sigrist, Dumont, N.J.

A. If a cassette deck is designed to read playback as well as record levels, this is done on the basis of a specific tape chosen by the deck manufacturer. However, tape sensitivity--which refers to the amount of signal output for a given amount of signal input--differs somewhat among tape types and brands. Therefore, if you are using a different tape than intended by the deck manufacturer, playback level may not agree with record level.

On some decks, the meter shows the deck's output level, not the tape's, in playback. Such readings will vary with the setting of the deck's output level control.

If the difference between recording and playback readings is only due to a difference in tape sensitivity, that difference should not be very great-per haps no more than about 3 dB in typical cases, and probably no more than 6 dB at the outside. If your difference exceeds 6 dB, either the record-level adjustment or the playback-level adjustment may be incorrect. Many decks contain internal potentiometers for adjusting these levels. You can find out from the deck manufacturer or from the owner's manual what the situation is in your deck. If there are adjustment potentiometers, it is a simple matter for a qualified service technician to correct the reading of your VU meters. Such adjustment should be made when using the cassette of your choice. If you turn to a technician, you also might want him to optimize bias for the tape of your choice.

New vs. Old Level Meters

Q. After using a $300 cassette deck for some time, I recently purchased a $900 model. The recordings made by this machine are fine indeed, but decidedly noisier than those made by the old unit. I discovered this is due to the fact that I am now recording at a definitely lower level than before. When playing a tape made on the old deck on the new deck, or when playing a prerecorded tape, the level indicators on the new deck show very high peaks. Does this suggest the meters are inaccurate? Or is it more likely that the new deck is revealing the draw backs of the inexpensive, peak LED indicators of my old deck?

-Christopher Handy, Rockville, Md.

A. It seems to me that the meters on your new deck are calibrated differently, perhaps showing a higher level than they properly should. If the tapes made on your old deck, and prerecorded tapes as well, do not sound distorted when played on your new deck, this is confirmed.

Peak indicators-whether expensive or inexpensive, be they LEDs, meters, fluorescent bars, or other devices are superior to average-reading de vices (such as true VU meters) for optimum setting of record level by most home recordists. So don't put down the level indicators of your old deck, even though they are "inexpensive, peak LED indicators." When using your new deck, in crease the recording level until notice able distortion is heard in playback then back down a bit on level. If this causes your meters to pin (move all the way to the right and hold there for a brief moment or more), you will have to get the meters recalibrated by a competent technician for the type of tape you intend to generally use when recording.

Re-Dubbing Losses

Q. How much signal loss occurs when you record from phono disc onto cassette and re-record from the first cassette to a second one?

-Philip Leak, Roseville, Cal.

A. On each dubbing there tends to be about 3 dB of deterioration in signal-to-noise ratio. When using high-quality cassette decks with S/N or 65 dB or better, such loss tends to be minimally noticeable for a single dubbing. However, if you make, say, three or more successive dubbings, the in crease in noise tends to become appreciable.

In this connection it may be noted that prerecorded tapes tend to go through three, four, or even five "generations" before the final version reaches the consumer. That is why prerecorded tapes have had a problem in achieving high S/N unless they were recorded at levels so high as to produce audible distortion. With the advent of digital recording and duplication, which entails virtually no loss in dubbing, prerecorded tapes can pro vide very good S/N.

Doubling in NR

Q. I plan to purchase a cassette deck that incorporates Dolby B. I also plan to purchase a dbx unit. Can I use the Dolby in conjunction with the dbx to get extra noise reduction? If my cassette deck has Dolby HX, can I use only the HX feature in conjunction with a dbx in order to increase the tape's high-frequency headroom?

-Michael Deutsch, Sherman Oaks, Cal.

A. The compressed signal presented by the dbx encoding will probably give the Dolby encoding circuit incorrect information on which to work, so I doubt the two will work satisfactorily together. Perhaps your audio dealer will let you try the combination.

As things stand, you cannot use only the HX (headroom extension) feature in conjunction with a dbx unit to increase high-frequency headroom. It might be possible to go inside the deck and disable the Dolby encoding so that only the HX processing remains, but this would require much technical expertise. (The Bang & Olufsen Beocord 9000's HX Professional system can be used independently, if you have that deck.)

Using HX also appears unnecessary if you are using dbx. The latter pro vides about 30 dB of noise reduction, bringing you into the area of about 85 to 90 dB signal-to-noise ratio. If you fear there is danger of tape saturation at high frequencies when recording at high levels, you can reduce the recording level substantially and still maintain an excellent S/N ratio. In other words, headroom extension is virtually unnecessary with the great amount of noise reduction provided by dbx.

(Source: Audio magazine, Feb. 1983, HERMAN BURSTEIN)

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