Symetrix 601, Руководство пользователя

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D-2
Rev 2.2, 10/31/94
In order to affect the audible signal as little as possible, an anti-aliasing filter is designed to be very
steep, having an extremely rapid fall-off above the upper frequency limit. (TAD, abridged)
See also:
brickwall filter, analog to digital converter, digital to analog converter .
Anti-Imaging Filter In a digital audio system, in order to recover the signal from the digital words, a
D/A converter is used. The output of this is a stair-step type of waveform which contains a great deal of
high-frequency energy called "images." To reconstruct a smooth replica of the original signal, the stair-
step is passed through a steep low-pass filter called an anti-imaging filter. It is similar, or even identical,
to the anti-aliasing filter found at the input of the A/D converter, but its purpose is quite different. (TAD)
See also: brickwall filter, analog to digital converter, digital to analog converter .
Attack Time
Attack time is the time it takes for a
compressor
or
limiter
to reduce its gain after a
strong signal is applied to it.
Transient signals which are shorter than the attack time of the device will not be affected by the gain
reduction, so it is important that the attack time be as short as possible. (TAD)
See also: release time .
Bandpass Filter A bandpass filter is a filter which has a bandwidth . Bandpass filters can be "broad,"
having a wide bandwidth, or "narrow," having a narrow bandwidth. They may be fixed in frequency and
bandwidth, or variable in either frequency and/or bandwidth. (TAD)
Bandwidth The bandwidth of a bandpass filter is the upper cutoff frequency minus the lower cutoff
frequency. It is thus the extent, in Hertz, of the frequency range, or band, passed by the filter.
Bandwidth is literally a frequency span, and is not necessarily connected to the specification of a filter.
For instance, the human voice can be transmitted with good intelligibility if the
frequency response of the
transmission chain extends from about 100 Hz to about 3000 Hz. Thus, a 2900-Hz bandwidth is needed
to transmit voice. This is about what a standard telephone system attains. The audio bandwidth,
however is generally considered to be about 20 kilohertz. (TAD)
Brickwall Filter Some lowpass filters have such a steep cutoff slope that the graph of the slope resembles
a brick wall (the slope of the sides, being vertical, is infinitely steep). Brickwall filters are commonly used
for
anti-aliasing and anti-imaging . (TAD)
Chorus An electronic music effect that modifies the sound of a single instrument to simulate a large
group of the same instruments, for example, a vocal chorus or a string section. The subjective effect of a
real chorus is caused by the fact that the many sound sources being mixed together all have slightly
different frequencies and also do not have precisely steady frequencies. The mixture because extremely
complex as the relative phases of the signals cause partial cancellation and reinforcement over a broad
frequency spectrum.
The synthetic chorus effect was first attained by subjecting the input sound to a series of very short time
delays and mixing the delayed sounds. The delays were then randomly varied, or modulated, to increase
the uncertainty of the combined pitch. This could be called the "time domain" chorus synthesis and can
be quite expensive if enough delay times are used to ensure a satisfactory result. A new type of chorus
device operates in the frequency domain and is somewhat simpler and at the same time more convincing.
The signal is split into many frequency bands by a series of
bandpass filters , and each band is randomly
varied in phase and amplitude, after which they are recombined. (TAD)
Clipping If a signal waveform is passed through an amplifier or other device which cannot
accommodate its maximum voltage or current requirements, the waveform is sometimes said to be
clipped, because it looks like it has had its peaks clipped by a pair of scissors. A clipped waveform
contains a great deal of harmonic
distortion and sounds very rough and harsh. Clipping is what typically
happens when an audio amplifier output is overloaded or its input overdriven. The clipping point of an
amplifier is defined as the maximum sine-wave signal level which, when viewed on an oscilloscope,
shows no signs of flat-topping to the trained observer. (TAD)
Comb Filter A comb filter is a filter which has a series of very deep notches, or dips in its frequency
response .; The spacing of the notches along the frequency axis is at multiples of the lowest frequency
notch, so they look evenly spaced along a graph plotted on a linear frequency scale. On the more
common logarithmic frequency scale, the notches become closer together on the paper as frequency
increases.
A comb filter is produced when a signal is time-delayed and added to itself. Frequencies where the time
delay is one-half the period and multiples of these frequencies are concealed when the signals are