Symetrix 601, Owner'S Manual

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D-3
Rev 2.2, 10/31/94
combined because they have opposite polarity. If the signals are of equal strength, the cancellation is
perfect and the notches are infinitely deep.(TAD)
See also: flanging , phasing .
Compressor An audio device which reduces the dynamic range of a signal. The compressor is the first
part of a compander (the combination of a compressor and expander ).
The effect of the compressor is to make the loud parts of a signal softer and to make the very soft parts
louder. Compressors are frequently used in recording popular music and in radio broadcasting, where
very soft passages may be lost in the background noise of the listening environment. For instance, when
music is playing on the radio in a car, the car's noise level will easily mask the quieter musical passages.
The limiter acts something like a compressor but operates only at the top end of the dynamic range. The
subjective audibility of a compressor depends strongly on its time constants (attack and release times)
and they are selected with care to minimize obvious "pumping" of the volume. To restore the original
dynamics to a compressed signal, a volume expander can be used, but great care must be taken that the
time constants, slopes, and thresholds match those of the compressor. (TAD)
Condenser (capacitor) microphone One of the earliest types of microphones to be invented after Dr.
Lee DeForest invented the Audion amplifier in 1906 was the condenser microphone. Thomas Edison is
sometimes credited with its invention, but this seems to be in doubt. At any rate, Wente, of Bell
Telephone Labs, designed a condenser microphone in 1917 and introduced it commercially in 1931.
The condenser microphone is a very simple mechanical system, with almost no moving parts compared
to other microphone types. It is simply a thin stretched diaphragm held very close to a metal disc called a
backplate. This arrangement is an electrical capacitor, and it is given an electric charge by an external
voltage source (polarizing voltage). When sound acts on the diaphragm, the pressure variations cause it
to move slightly in response to the sound waveform. This causes the capacitance to vary in like manner,
and because the charge is fixed, the voltage on the backplate will vary according to the laws governing
the capacitor. This voltage variation is the signal output of the microphone. The condenser microphone
has extremely high output impedance, and must be placed very near a preamplifier to avoid loss of the
signal.
It is possible by special treatment of the backplate and by combining several microphone elements to
attain various directional patterns, including bi-directional (figure 8), cardioid, and super-cardioid. (TAD)
See also:
phantom power
.
dB, Decibel Literally, one tenth of a bel. The bel is named after Alexander Graham Bell (which is why
the 'B' in dB is capitalized), and the number of bels is defined as the common logarithm of the ratio of
two powers. Thus, two powers, one of which is ten times the other, will differ by 1 bel; 10 watts are 1 bel
higher in level than 1 watt. A 360-horsepower car is 1 bel more powerful than a 36-horsepower
motorcycle. Any power ratio may be expressed in bels, and it is important to note that only power ratios
are allowed. a bel is a pure number with no dimensions.
Decibel reference quantities
Unit of Measurement Reference Remarks
dB none Only useful in a relative sense, i.e.
"3 dB hotter."
dBm 1 mw, 600 ohms 1 mw, 600 ohms = 0.775V RMS
dBv 0.775V, open circuit Note : open circuit, small V
dBV 1V, open circuit Note : capital V
dBu 0.775V RMS, open circuit Same as dBv. Becoming more
common because of confusion
between 'v' and 'V'.
The bel had its origin in the bell Telephone Labs, where workers needed a convenient way to express
power losses in telephone lines as power ratios. Because the bel is a power ratio of 10, and this is a
rather large ratio, it is convenient to divide it into tenths of bels, or decibels (abbr. dB). Ten dB is 1 bel;
thus the decibel is ten times the common log of the ratio of two powers. The decibel was originally called
the "transmission unit," or TU, by the Bell Labs people. (TAD, severely abridged)
The decibel is commonly used as a means of expressing audio signal levels. In dynamic-range processors,
like compressors and limiters, their input to output relationship, or compression ratio, is a plot of the
unit's input signal, in dB, to the unit's output signal, also in dB. Since the decibel represents a ratio of