D-6
Rev 2.2, 10/31/94
Digital filters operate on signals which have been digitized. They are purely mathematical, performing a
series of arithmetic operations on the digital words. In a sense, digital filters are synthesized filters;
digital techniques being used to emulate or simulate analog filters. Digital filters have the advantage of
being drift-free. They always do their job in exactly the same way. They can be designed for nearly any
desired characteristics in the frequency domain and in phase response. (TAD, abridged)
Flanging
A special effect made popular in the 1960's where a delayed version of a signal is mixed
with the signal, creating a "swooshing" sound.
Flanging was first done by recording a signal on two similar tape recorders, playing them back
simultaneously, and mixing them together. The record-playback sequence on the tape recorders results
in a small time delay of perhaps a tenth of a second. Both output signals are delayed by the same
amount if the tape recorders are similar and they add together in the mixer, and the sound heard is
essentially the same as the signal at the input to the tape recorders.
To achieve the effect of flanging, one recorder is slowed down a little, increasing its time delay. This is
done by pressing one's thumb against the flange of the tape recorder supply reel, hence the name
"flanging." When the time delay is different for the two combined signals, there will be frequencies where
the phase shift is 180 degrees, and the signals will cancel, causing deep dips or holes in the frequency
response curve. This is called the comb filter effect. As the speed is varied, the frequency of the dips is
swept across the frequency range, giving the swooshing sound. Attaining the most desirable effect
requires an educated thumb. The best effect is obtained when the signal being flanged contains
frequencies over a wide range. (TAD)
See also:
phasing
.
Frequency Response
Also known as magnitude response, is the graph of the variation in output level
of a device over frequency, with a constant amplitude input signal.
Full Scale
When audio signals are converted to their digital equivalents using an
analog-to-digital
converter,
the signal level that causes the output of the converter to reach its digital maximum is referred
to as full scale.
Gate
A circuit which performs like a switch, allowing a signal either to pass or not, is called a gate.
The position of the gate (open or closed) is controlled by an applied voltage which can come from a
number of different places. If the level of the signal determines the gate opening, it is a noise gate, closing
when the signal level is so low that the noise would be audible. (TAD) A noise gate is an extreme example
of a downward
expander
.
Group Delay
The slope of the phase-versus-frequency curve of a frequency response function, that is,
the rate of change of phase of the response as a function of frequency. Group delay is a property of a
device or a system.
A pure time delay, equal at all frequencies, gives a constant slope of phase versus frequency. If in an
audio component this slope is not constant, but varies with frequency, the component is said to produce
group delay distortion. This is equivalent to a time delay that varies with frequency. For instance, an
anti-aliasing filter will typically have a phase response curve which slopes sharply down at high
frequencies, which means that the high-frequency components will be delayed longer in their passage
through the filter. The audible result is a loss of precision in musical transients; they are spread out, or
"smeared," in time, and a more diffuse stereo image results. (TAD)
Highpass Filter
A highpass
filter
uniformly passes signals above a certain frequency, called the cutoff
frequency. The cutoff frequency is where the filter response is 3 decibels below the nominal response. The
response rolloff in the stopband may be gradual or sharp.
The "rumble" filter found in many record player systems is a highpass filter. (TAD)
See also:
lowpass
filter
.
Impedance
In an electric circuit containing direct current, the magnitude of the current is
determined by the voltage across the circuit divided by the resistance of the circuit. This is known as
Ohm's law.
In a circuit containing alternating current, the situation is more complex; the "resistance" presented to
the current is a function of frequency. This "AC resistance" is called impedance and is also measured in
ohms. Impedance is the vector sum of resistance, capacitive reactance, and inductive reactance.
Alternating currents are affected by resistance the same way as direct currents, and Ohm's law can be
used for AC if the reactances are zero, that is, if there are no capacitors or inductors in the circuit. (TAD,
abridged)
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