AUDIOCLINIC (Sept. 1988)

NR in Hi-Fi VCRs

In his "Audioclinic" for January 1988, Mr. Giovanelli incorrectly stated that noise reduction is neither needed nor used in Hi-Fi VCRs. In fact, it is both needed and used to achieve the high S/N ratio of 80 to 90 dB. The VHS Hi-Fi standard mandates an NR sys tem using peak level detection.

There used to be serious incompatibility problems with Hi-Fi VCRs, a fact to which none of the reviewers paid much attention in their test reports.

Some of the first-generation models had as much as 8 dB of difference between their 0-dB levels. [Editor's Note: According to some authorities, the problem only affected some commercially recorded tapes.] Problems also arose from deviations in the vari able (level-dependent) frequency emphasis used in the NR circuit, which is very similar to Dolby B NR.

The reason for employing Dolby B NR on the VCR's longitudinal audio tracks is most likely the infamous "breathing" effect. The inherently low S/N ratio of the longitudinal tracks creates an audible breathing effect with a fixed-frequency-emphasis NR system such as dbx. The Dolby NR system, however, uses a variable frequency emphasis which is less prone to breathing.

-Leo Backman; Helsinki, Finland

Loudspeaker Power

Q. I am powering a pair of loud speakers with a receiver capable of 10 watts per channel. The speakers are marked "25 watts capacity." I read in a dictionary that capacity means maxi mum. Does this mean that I am really limited to using 25 watts? Is there any possibility of using 30 or perhaps 35 watts?

-Richard Gumner; Washington, D.C.

A. If your loudspeaker is marked "25 watts capacity," the theoretical maximum amount of power it can safely handle is 25 watts. Driving it with more power than recommended will damage the loudspeakers. This dam age may take a little time to accrue if the overload is small. If the damage is significant, the speakers may be blown out immediately.

If the instructions indicate that the speakers require 25 watts for average listening conditions, the loudspeakers probably can handle considerably more power. It is also likely that the maximum power-handling capacity is indicated in the instructions.

Given all of what I have told you, it is still possible, and desirable, to use a receiver rated at 30 or 35 watts to drive those speakers. Because you are now using a receiver capable of 10 watts per channel and are reasonably happy with the volume of sound produced, it's unlikely that you will require any more power than 35 watts.

If you drive your 10-watt receiver to maximum, you can damage your tweeters because of amplifier clipping.

This damage could be of more consequence than what might result from driving your loudspeakers with slightly more power than is recommended. Of course, if you overdrive a more powerful receiver, amplifier clipping will occur and speaker damage is even more likely than when using the 10-watt unit.

The ideal arrangement is to use a receiver having a bit more power than your speakers can handle, but never drive the amplifier beyond the power capacity of your loudspeakers.

More About Humidifiers

I am taking the liberty of commenting on an item in "Audioclinic" (January 1988) which dealt with ultrasonic room humidifiers.

I have a suggestion as to how to reduce or eliminate the calcium-carbonate dust produced by these humidifiers. Use hot water to fill the tank.

Most home water heaters have conditioners which treat the water to prevent this lime buildup in the heaters them selves. Hence, by using the treated water, you stand a better chance of avoiding the white dust without other, more elaborate water conditioning.

I would suggest, however, that you allow the water to cool before attaching the tank to the humidifier.

- David E. Licht, Jackson Heights, N.Y.

Rain Static

Q. What is "rain static"?

-June Thomas; Wantagh, N.Y.

A. Rain static is a phenomenon I have only noticed on VHF bands, including the FM broadcast band. When it's really coming down in buckets, you may hear a kind of "chattering" as a background to the station to which you tuned. It is most noticeable when receiving weak signals.

If you are using an indoor antenna, you'll never be bothered with this kind of interference. (It's about the only ad vantage I can think of to using an in door antenna, except when it is your only recourse.) I have not been able to verify this, but it appears that the friction produced by the tremendous quantity of rain moving over the surface of the outdoor antenna produces a charge and discharge which create noise just as a car's ignition system might.

Questions, Please?

I know this question will sound stupid to you, but . . . .

Questions which start like that (I've been getting them for the 30 years I've been writing "Audioclinic") remind me of two sayings: "There are more foolish or stupid answers than foolish or stupid questions," and "If you ask a question, you may feel like a fool for five minutes; if you don't ask questions, you'll be a fool for life." Even though I've been playing around with audio since 1947 or earlier, I keep on asking questions, too.

Many of my long-time readers know that my formal electronics studies began and ended with high school, and that I cannot see (which makes it impossible to do even a fraction of the reading I should do). My best teacher is hands-on experience. I've taken CD players apart and played with the transports' mechanical alignment to see what would happen. I've also worked with a DAT recorder that had a manual written in Japanese.

So don't think I'm judging the "dumbness level" of your questions. I only make judgments about which questions are suitable for inclusion in "Audioclinic." Even those questions Audio doesn't print are answered by mail. That is true for the "Tape Guide" column as well. I might have a backlog which would keep me from answering as quickly as I would like to, but unless a letter gets lost, it will be answered.

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(Source: Audio magazine, Sept. 1988, JOSEPH GIOVANELLI)

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