AUDIOCLINIC (Jun. 1991)

Another View of Sampling

Q. I have been trying to learn more about sampling: The examples live seen are taken from a single waveform.

But isn't music a combination of many waveforms? How is a sample taken that represents all of the waveforms present at specific times? I may be missing the boat, so I hope you can clarify the "A to D" process.

-Robert L. Keller, Gainesville, Fla.

A. Music can be represented by a single waveform, but a complex one, which can be analyzed into component sine waves of many frequencies and amplitudes. For example, a whole orchestral timbre might include a large bass wave on which are superimposed high-frequency jigs and jags representing the tones and overtones of the strings and winds.

But even complex waveforms can be represented by a sequence of points, if they're closely spaced enough. At the CD sampling rate of 44,100 Hz, a 20-Hz bass fundamental would be sampled 2,205 times, and a 20-kHz overtone would be sampled just a bit more than twice-but that's sufficient for the re-creation of the 20-kHz tone's positive and negative half-cycles. This is why a digital system's sampling rate determines the system's upper frequency limit.

Four-Second Sampling

Q. I know that CDs are produced by sampling the signal 44,100 times per second. But what is 4-second sampling?

- Eric Wong, New York, N.Y.

A. Much of the rock/pop music we hear nowadays is played on synthesizers. Some of these instruments come remarkably close to sounding like other instruments, when this is the intent of the performer. Various methods are used to bring about this result, but perhaps the best approach is to "sample" the sound of the instrument to be imitated. A rather short digital recording is made of the instrument-not on tape but in computer memory. Because such recordings require a considerable amount of random access memory (RAM), it is customary to record as short a sample of the instrument as practical. Rather than recording a chord or part of a song, all that is recorded is one note. In the case of a violin or a drum tap, a very short recording can suffice, perhaps no longer than a half a second. The piano, because of the long time needed for bass tones to die away, requires a much longer recorded sample to provide a true representation of the character of the note. It is not just a matter of the time needed for the tone to decay and its rate of decay; it is also a matter of the note's harmonic structure changing as it dies away. Therefore, to make a good piano recording of a low bass string, the recording time should be even longer than 4 S.

Because of the characteristics of the piano, each note has a different decay time and harmonic structure. The harmonic content and percussive attack vary in accordance with how hard the keys are struck. For this reason, makers who desire to produce the best synthesizers will sample each note of the piano (or other instrument) and do so at various degrees of loudness.

These wave shapes are analyzed and built into the sounds used in the synthesizer. As each sound is called for by the performer, a different set of wave tables is installed in memory, leading to the production of the correct wave shapes associated with that particular sound.

This subject is very involved, but I have tried to give you the flavor of the process.

Using Switched Outlets

Q. About 30 years ago, when I started buying audio equipment, I was told in no uncertain terms never to connect a mechanical device such as a tape recorder or turntable to a switched outlet [that loses its a.c. power when the device containing the outlet is switched off]. Not using the device's own switch to turn it off could lead to flat spots on the idlers, stretched belts, etc. live followed this advice ever since, but frankly, it's a pain! Does this old rule still hold with today's mechanical equipment?

Also, is it a mistake to use switched outlets to power components that have memory, such as programmable tuners? I know many such components have memory backup power, but some backup systems have only enough power for two weeks or so. If I were to go away for three weeks, should I set a timer to turn the stuff on for 10 minutes a day, just to keep the memories alive?

-Ira Solomon, Lynnfield, Mass.

A. You will be glad to learn that most equipment today can work just fine when connected to switched outlets.

You are correct that much of the older mechanical equipment contained parts which could be ruined if they were left engaged. If you turn these components off by their own switches, automatically disengaging these parts, it won't matter if the equipment is plugged into a switched outlet. However, such outlets could tempt you to just turn the system off while the turntable or tape decks are still playing.

This is still true of many low-priced tape decks, whose transports are operated by keys or levers that require a strong push. But even moderately priced decks now often have electronic control systems that engage and disengage their idlers and capstans when you press a light-touch switch; such decks disengage these parts when the power goes off. Turntables with direct or belt drive, and CD players, can also be switched off at the outlet without problems.

Devices which hold programmed information in memory are something else again. Many VCRs, for instance, need power at all times to keep their memories refreshed and their clocks accurate. Memories in other components may be nonvolatile types that keep their contents indefinitely without power backup, or the memories may be ordinary RAM, with backup batteries that can operate for long periods without a.c. power but which eventually will go dead or need to be refreshed.

(Lithium batteries give exceptionally long backup protection.) Consult your equipment manuals to determine backup requirements.

If you have equipment which, for whatever reason, needs its memory to be refreshed within some time period-such as two weeks--you will either have to connect it to unswitched outlets or, as you suggest, supply all such equipment with power via a timer.

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(Source: Audio magazine, Jun. 1991, JOSEPH GIOVANELLI)

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