by RALPH HODGES
GLOSSARY OF TECHNICAL TERMS--24
Omnidirectional (or, just as properly, "nondirectional") usually refers to a receptor or propagator of energy that does not discriminate against any direction in performing its function. For example, an omnidirectional microphone picks up-or should pick up-any and all sounds reaching it equally well, whether they arrive from in front, behind, below, above, or from the sides. An omnidirectional speaker, if there truly were such a thing, would be equally nondirectional as a radiator: it would sound-and mea sure-the same to listeners in front, be hind, below, etc. With our present technology, true omnidirectionality (as measured in a nonreflective environment) can only be approximated with either of these devices, but many believe that it would be ideal behavior for a high-fidelity speaker system, although there is still much controversy on the point.
A microphone that is deliberately designed to be directional in some way goes by several names, according to type: bi-directional, unidirectional, etc.
Oscillator is the general term for a frequency-generating circuit, but it also refers to test instruments such as a sine-wave generator. Among the places where oscillators are commonly found in audio equipment are: tape machines (bias oscillator and Dolby test-tone generators), tuners (a "local" oscillator helps produce the i.f. signal) and phase-locked-loop multiplex sections, and some turntable and tape-deck motors that are servo-controlled and have an internally generated reference frequency.
Oscilloscope, one of the most basic and versatile electronic test instruments, uses a cathode-ray tube to provide a visual display of an electrical signal. A basic oscilloscope can show the waveform of a single input signal, spread out over an adjustable time increment, or the phase and amplitude relationships be tween two input signals.
A few deluxe component tuners feature built-in oscilloscope displays with special circuits that indicate signal strength and multipath reception. These tend to be considerably more useful than meters in guiding the listener to optimum antenna orientation. There are also several separate oscilloscopes available, resembling audio components in appearance, that perform these functions and, in many cases, provide a four-channel display when used with a quadraphonic sound system.
Output stage, while referring generally to the circuits delivering the signal to the output jacks on any electronic device, is usually taken by audiophiles to mean the power-output stage of an amplifier or amplifier section of a receiver. This is the part of the amplifier that is typically endowed with large heat sinks and heavy-duty transistors, and which develops the high current levels necessary to drive loudspeakers.
Overload, or overdrive, is what hap pens when any device is called on to handle a higher level of energy than it can manage. In audio, the term is most frequently used in connection with amplifiers and recording tape, both of which have fairly well defined overload points.
When the overload point of an amplifier is exceeded it goes into "clipping," which is the abrupt limiting of current or voltage output because the amplifier simply cannot deliver any more, however strong the "drive" signal. The overload point of magnetic tape generally corresponds to the point of magnetic saturation, where the tape has become magnetized as much as it can and therefore won't respond to any further magnetizing force. The result of both types of overload is, of course, distortion.
Other devices such as loudspeakers and even some FM tuners can be over loaded, but terms such as "doubling" and "bottoming" (speakers) and "cross-modulation" (tuners) are more frequently used with these types of overload.
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Also see:
TAPE HORIZONS--Phasing the Music
AUDIO NEWS--Views and comment on recent developments
Source: Stereo Review (USA magazine)