Arthur Harrison, editor
Note: Neither author nor copyright declaration is indicated in the primary source document.
The limiter is a specialized
form of audio compressor used in applications such as commercial radio broadcasting, sound recording, and, in some forms, electric guitar "distortion" and "fuzz"
effects that are used to emulate the sonic characteristics of vacuum-tube
amplifiers. This article considers the design principles for diode-based limiters
and their wide range of characteristics.
Limiter designs can vary in complexity from
a pair of diodes to multi-stage voltage controlled amplifiers (VCAs)
with split frequency bands. Diode-based limiters are simpler in design, instantaneous in
response, and have a more natural-sounding loudness response than VCA topologies. However, diode-based limiters also introduce a greater degree of distortion in the limiting
region than VCA types. On the other hand, VCA-type limiters may exhibit
"breathing" or "pumping" effects.
A diode has extremely high resistance until the voltage across it is
large enough to forward bias it,
which point, current flows. A diode's voltage drop remains fairly
constant while forward-biased and that property
is exploited in diode-based limiter circuits. Since audio signals are
AC, two diodes together can symmetrically limit both the positive and
negative halves of a waveform.
Two general categories of limiters may
be described as those with "soft" and "hard" responses, although there
is a continuous spectrum of response between those two extremes. The
output voltage of a hard limiter, shown
below in Figure 1, has maximum peak and valley values that are equal to
the forward bias
voltages of the diodes. Since (for silicon diodes) that value is about
0.7 volts, the design assumes that the voltage values at the slider of
the left potentiometer will
exceed +/-0.7 for limiting to occur. In practical designs, the voltage
at the potentiometer slider would be in the region of +/-1 volt with
respect to ground, with the voltage difference between the slider and
the diodes dropping across resistor R1.
This type of limiter in Figure 1 can achieve an output with a more
dynamic characteristic with the addition of a second
resistor to form a voltage divider with R1, as illustrated below in
Figure 2. When configured with two resistors, this limiter becomes
a simple type of compressor with a compression factor determined
by the ratio of R1 and R2. Figure 2's configuration may be
characterized as a "soft" limiter. Increasing R2 will give a
sound at the expense of level control. The circuit will still
waveform above the diode forward bias voltages, but also lets
through an unmodified portion of the signal by taking the
the top of R2. The graph in Figure 3 compares the response of both
types of limiters.
The harsh characteristic of audio signals processed by hard
limiters is partially due to the
odd-order distortion harmonics that are generated during
limiting. Germanium diodes (such as the ubiquitous type 1N34A)
have a smoother,
vacuum tube-like limiting quality because they distort with more
harmonics, which are pleasant to the human ear. Germanium diodes
have a forward bias voltage of about 0.3 volts and are sometimes used
series-groups for both the positive and negative-conducting
paths. Parallel pairs of light emitting diodes (LEDs) may provide
stronger even-order harmonics than paralleled-pairs of silicon or
germanium diodes. A paralleled-pair of one red and
green LED may produce yet more pleasant harmonics, because their
forward-voltage differences will increase even-order distortion.
The limiter in Figure 4 is an "active" design with the LED limiters in
parallel with the feedback resistor of an operational amplifier. LED1
provides limiting for negative input excursions, and LED2 for positive
input excursions. The
negative and positive limiting
thresholds are values between 1.4 and 2.6 volts, which is the range of
forward bias voltages for a majority of red, green, and yellow LEDs.
For this configuration, the response will transition from hard to soft
as the value of resistor R2 goes from infinity to a finite value. Since
the operational amplifier is configured in an inverting mode, the
output will be a voltage inversion of the input. If R1 and R2 are made
equal in value, then the response of the circuit will have unity gain,
and be linear, for input voltages below the diodes' forward bias
thresholds. When that voltage value is exceeded, the gain will diminish
in accordance with the resistance presented by the forward-biased
diode. An important distinction between this active configuration and
the previous configurations is that the output, when limited, will have
an amplitude that correlates to the input amplitude, as opposed to
being limited exclusively by the diodes' threshold values.
Figure 5, below, shows two limiter configurations based on zener
diodes. Circuit A limits when the input exceeds the
breakdown voltage plus the forward voltage of a zener. Since both
zeners are in inverse-series, the design works for both polarities of
the signal. Circuit B requires only
one zener diode and limits when the input is greater than the breakdown
voltage of the zener combined with the forward voltages of two of the
bridge-configured diodes. Zeners have the advantage of being easier to
match for symmetric limiting than other types of diodes, and they are available over a wide
range of voltages.
The circuits shown so far have fixed limiting levels. One way
to vary levels is to apply a bias voltage to the normally-grounded side of the diodes. The circuit in Figure 6A permits
manual adjustment of the positive and negative portions of the waveform
separately. Circuit 6B puts the audio signal
through an inverting amplifier, the output of which connects to the
bottom of the zener array. Limiting occurs when the audio signal
exceeds (Vf + Vz ) / [1 + (R3 x RP1 / R2 x RP1x)], where Vf is the
forward bias voltage of a zener and Vz is the breakdown voltage. P1
adjusts the feedback of the inverting amplifier and thus the limiting level.
Broadband limiters work across the entire frequency range. There are
times when limiting should be frequency dependent. For example, if the
audio signal is noisy, then limiting the high frequencies
can result in smoother, more intelligible sound. The circuit in Figure
7 is one example of a bandwidth restricted limiter. The low frequency
limiting level of the diode array is modulated by the output of the low
pass filter, so that only the high frequencies are affected.
Analog tape recorders may be used in a signal processing chain to archive compression. They provide certain desirable
sonic characteristics including a subjective "warmth." Part of
these characteristics results from the analog tape media's limiting of
high-frequency signal content. The circuit in Figure 8 emulates this process. A high-frequency (above 2.4 kHz) pre-emphasis
is applied in the first op amp section, facilitated by resistor R1 and capacitor C1 at its
negative input. After the diode limiting stage, the
signal is de-emphasized in another network with the same corner
frequency, facilitated by the R5 and C2. Although
only two diodes are shown, practical implementations may include more
diodes, both silicon and germanium, to achieve the desired limiting
characteristic. The emphasis and de-emphasis
scheme reduces the high frequency distortion components that occur due to
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