AUDIOCLINIC (Apr. 1989)

Sampling Rate Explained

Q. What is sampling rate; what does it have to do with frequency?

-Tony Perkins; Los Angeles, Cal.

A. Digital recordings are made by "sampling" the signal's voltage many thousand times per second and then storing these measurements as "samples" consisting of binary numbers represented by pulses. In theory, the maximum frequency a digital system can accurately record is one-half its sampling frequency. CDs, which use a sampling frequency of 44.1 kHz, there fore have a theoretical limit of 22.05 kHz; DATs, recorded at 48 kHz, have a theoretical 24-kHz limit, and satellite and 8-mm video systems are limited to 16 kHz by their 32-kHz sampling rate.

In practice, the limits are even tighter. The actual top audio frequency allowable on CD is about 20 kHz, and for DATs, it's about 22 kHz. Higher frequencies must be filtered out of the signal before it is recorded. Otherwise, the recording will contain spurious tones, a phenomenon "aliasing."

Cleaning Up Broadcast News Recordings

Q. One of my job functions is the production of radio news releases, usually including recordings and interviews made in the field. From time to time, I need to get such recordings and interviews over the phone, which, as you know, is not a superior audio source. At other times, even reports which arrive on tape may sound indistinct or muddy. Can I improve their audio quality by using a graphic equalizer, either during the original recording or in post-production? If so, how much "cleanup" is possible?

-John T. Sulzmann; Corvallis, Ore.

A. "Cleaning up" recordings made via ordinary telephone circuits is not really possible, at least in terms of high fidelity. Sometimes, small improvements can be made at frequencies in the range from 250 Hz to 2.7 kHz, and that's about it. The higher and lower frequencies passed by the phone seldom surpass these limits.

The "muddy" Live recordings may sometimes be improved by rolling off some bass and the extreme highs perhaps completely. You then boost frequencies in the range of 2.7 to 5 kHz. Although this technique will not produce a superior recording, it will sometimes produce one which is at least understandable to most listeners.

Where a news event or on-the-scene interview is impossible to recapture, this may well be better than an unusable recording.

Transparent Phonograph Records

Q. Some time ago, I received a mailing about a record collection of the type they sell on TV through "800" numbers. These records, the mailing claimed, were of high quality because the vinyl was so pure that you could see right through it. Does this transparency really have anything to do with sound quality?

-Ki Suk Hahn; West Covina, Cal.

A. Phonograph records are black because lampblack or similar material is added to the vinyl mix. This material is added mainly for appearance's sake. I recall that some record manufacturers attempted to produce transparent records, only to meet with consumer resistance to those products. I have to believe that record pressers would love to get rid of the lampblack because of the mess it can make.

Some authorities say that lampblack is also included in the mix because of its lubricating properties, though I've always thought it somewhat abrasive.

If my observations are correct, then eliminating lampblack should reduce friction of the stylus against the groove, thus reducing both stylus and record wear. Further, if lampblack is not properly dispersed within the vinyl, it can increase the record's noise level. Leaving out the lampblack would probably result in a slight improvement in back ground noise.

Automotive Engine Noise

Q. My new car stereo system (which includes a cassette-player/radio combination, a car amplifier, and a car equalizer) picks up engine noise. A stereo shop advised me to hook up a noise filter for both the amp and the equalizer, but this did not help. Next, I was told to install resistive spark plugs and resistive spark-plug wires. I changed only the plugs; do you think I should also change the wires?

-Name withheld

A. The noise you are hearing basically comes from a sparking some where in your car's engine, probably with the ignition wiring acting as a broadcast antenna and the input of your cassette-player/radio combination picking it up.

You definitely should use resistive spark-plug wires, and it is also possible that filtering the power input to the system will help. Perhaps you can accomplish this with the same filter for both the radio and the equalizer. Special automotive noise-suppressor kits will sometimes help. These can be found in auto supply shops, though probably not in a stereo dealership.

Because of the manner in which the accessory contacts of your automobile are set up, it is sometimes necessary to wire audio equipment directly to the battery, using suitably heavy-gauge wire. In this case, the filters should still be used, but the ignition will not turn off the equipment.

I would also write to the maker of your car stereo equipment, detailing both the noise and the automobile involved. Ordinarily, these firms have a specialist on staff whose job it is to help with such questions. Often, their experience is such that they can pin point the difficulty simply because of the particular combination of car and components.

You should also be aware that there are literally thousands of possible sources of this noise, and a similarly large number of ways to suppress it. Unfortunately, it is beyond the scope of this column to attempt an exhaustive answer to your question.

Defining AM Suppression

Q. What is "AM suppression" as applied to FM reception?

-Robert Beiswinger, Townbank, N.J.

A. In FM (frequency modulation) broadcasting, the strength, or amplitude, of the transmitted signal never changes, but its frequency shifts up and down in accordance with the pro gram signal. In AM (amplitude modulation), the frequency of the signal never changes, but its amplitude varies with the program signal.

While the amplitude of the signal leaving the FM transmitter never changes, the amplitude of the signal reaching your antenna can change, due to multipath, signals bouncing off airplanes, and noise bursts from passing cars' ignition systems. If the signal's overall strength is not great, or if an FM set cannot reject these forms of inadvertent "AM," the program will be disturbed by effects which range from flutter to noise bursts. The better the AM suppression, the less the tuner or receiver will reproduce these unwanted sounds.

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(Source: Audio magazine, Apr. 1989, JOSEPH GIOVANELLI)

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