Because a limiter
is actually a specialized form of audio compressor, the circuitry of audio limiters can
vary tremendously in complexity from a pair of diodes to multi-stage voltage controlled
amplifiers with split frequency bands. Diode-based limiters are instanteous, simpler in
design and have a more accurate loudness response, but suffer from distortion in the
clipping region. VCA-type limiters have low distortion, but can exhibit breathing or
pumping effects from poor gain control. This article takes especial note of design
principles for diode-based limiters and the wide range of clipping characteristics.
A diode has infinite resistance until the voltage across it is high enough to forwardbias
it (typically 0.7V for a silicon diode) at which point, current flows. The diode's voltage
drop remains fairly constant throughout the operating range, and it is this property that
is exploited in diode-based limiter circuits. Since audio signals are AC, two diodes
together can symmetrically clip both the positive and negative halves of the waveform.
The output of a hard limiter has a maximum
voltage that is the forward bias voltage of the diodes. Figure 1 shows the most basic of diode
limiter circuits, a hard limiter. The design assumes that the input will exceed
1V. The trimmer pot P is adjusted for maximum volume. Because the clipping can be abrupt and drastic, the
distortion of a hard limiter in the clipping range is harsh (crackly) and is sometimes used for
a fuzzbox-like effect. By scaling the resistors down, the hard limiter (and the soft limiter in
The hard limiter can achieve an output with
a slightly more dynamic characteristic with the addition of
a resistor to form a voltage divider with
R1. The soft limiter
shown in figure 2 still clips a waveform above
voltage, but also lets through an amount (Vin - 0.7V)/10 by taking the output
above R2. The graph in figure 3 compares
the response ofboth types of limiters. The soft
limiter is a type of compressor
with acompression ratio determined by R1 and R2. Increasing R2 will give a more
dynamic sound at the expense of
harsh characteristic of a hard limiter is partially due to the odd-order distortion
harmonics that are generated during clipping. Germanium diodes
(such as the 1N34A) have a smoother, tube-like clipping quality because they distort with
more even-order harmonics, which are more pleasant the human ear. Germanium diodes have a
forward bias voltage of about 0.3V and are usually to deployed 3 per side.
A pair of LEDs
improves on silicon or germanium diodes as they clip with stronger even-order harmonics.A pair of one
red and one green LED will sound better still, because the mismatching will highlight the
even-order distortion. The limiter in figure 4 is an active design with the LED clippers
in parallel with the feedback resistor of an opamp. The clipping threshold is about 1.9V,
which is the forward bias voltage of an LED.
5 shows two limiter configurations based on zener diodes. The circuit in
5a clips when the input exceeds the
breakdown voltage of one zener and the forward bias voltage of the other. The second
limiter in figure 5b requires only one zener diode and clips when the input is greater
than the breakdown voltage of the zener combined with the forward bias voltages of each
diode pair. Zeners have the advantage of being easier to match for symmetric
clipping than other types of diodes over a wide range of voltages.
limiter circuits shown so far have fixed clipping levels (and are adapted to different
headphone amplifiers by way of attenuators). One way to vary clipping levels is to apply
abias voltage to the normally grounded side of the diode array. The circuit in figure 6a
permits manual adjustment of the positive and negative portions of the waveform
separately. The variable clipper in figure 6b puts the audio signal through an inverting
amplifier, the output of which connects to the bottom of the zener array. The limiter
clips when the audio signal exceeds (Vf
+ Vz) / (1 + (R2 * R5 / R3 * Rx)), where Vf
is the forward bias voltage of a zener and
Vz is the breakdown voltage. R5 adjusts the feedback of the inverting amp and
thus the clipping level.
limiters work across the entire frequency range. There are times when clipping should be
frequency dependent. For example, if the audio signal is laced with noise, then clipping
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 clipping
level of the diode array is modulated by the output of the low pass filter, so that only
the high frequencies are affected.
Another way to
configure a frequency dependent limiter is pre-emphasize the audio signal before the
clipping stage. The analog tape compressor by LXH2 in figure 8 simulates the high
frequency clipping distortion in analog tape recorders (useful in digital recording) by
applying a 70 microsecond pre-emphasis (high frequencies above 2.4kHz) in the first stage
network. After the clipping stage, the signal is de-emphasized in another network with the
same corner frequencies. The diode array is a mix of silicon and germanium diodes to
achieve the desired clipping characteristic.
the audio signal is clipped, a limiter will generate harmonic and intermodulation
distortion above and below the frequency of clipping.
A multiband limiter (figure 9)
minimizes these distortion components by splitting the audio signal into pre-determined
frequency bands, each with their own clipping stage. The output of each clipping stage is
then filtered for distortion outside the band before all bands are recombined into a
single waveform. In practice, multiband limiters are difficult to implement. Because the
various filters of different orders will phase-shift the signals in each band, the overall
response of a multiband limiter may not be flat and can sound unnatural. Several
techniques can improve the performance of multiband limiters. First, the bands can be
chosen according to psychoacoustic masking curves that help conceal distortion. Second,
compressors can be introduced before the clipping stage to control the degree of clipping.
Distortion detectors can control the amount of compression. Third, all-pass filters Can be
sprinkled about to collimate the phase responses of the bands.
average, the bass frequencies in an audio signal usually have the highest amplitudes. When
a low frequency signal of sufficient amplitude trips a limiter, high frequencies are also
"pinched." The design in figure 10 is a multiband clipper that specializes in
minimizing the harmonic and intermodulation distortions due to low frequency clipping
only. The high and low pass filters split the audio signal with the low frequencies going
through a variable clipper that is controlled by an intermodulation distortion detector.
The second low pass filter after the variable clipper removes any harmonics caused by the
clipping before summing the output with the high frequency band. The IM detector estimates
the amount of high frequency clipping in the fixed clipper resulting from low frequency
clipping, and correspondingly decreases the threshold in the variable clipper to decrease
the low frequency amplitude. The difference recovers high frequency peaks, which determine
the clipping level in the variable limiter. The peak detector enables the IM detector only
when there are peak bass frequencies.
any limiter, if the audio signal is conditioned with high frequency pre-emphasis (see the
analog tape compressor above), then de-emphasis will reduce the high frequency distortion
components due to clipping. The circuit in figure 11 cancels low frequency distortion. The
audio signal pre- and post-clipping is compared in a differential amplifier and the output
is put through a low pass filter to extract the low frequency distortion. Since the output
of the low-pass filter is phase shifted, the clipped signal is delayed through an all-pass
filter (assuming a minimum phase low-pass) to compensate and then differenced with the
output of the low-pass filter. In a multiband limiter, the output of each band may have
its own distortion cancellation circuit.
Burger, John Robert,
"A Voice-Balancing Audio Peak
Clipper," QST, July 1998, p. 45.
Chandler Jr., James, "Simple
Projects," Electronic Musician, January 1990, p. 24.
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No. 4,256,975, March 17, 1981.
Orban, Robert, "Apparatus and Method for
Peak-Limiting Audio Frequency Signals,"
No. 4,208,548, June 17, 1980.
Orban, Robert, "Multiband Signal
Processor," U.S. Patent
No. 4,412,100, Oct. 25, 1983.
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No. 4,138,612, Feb. 6, 1979.
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Limiter," U.S. Patent
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