The measurements were taken using FRA4PicoScope 64-bit software with a PicoScope 2204A oscilloscope and its synchronized signal generator, a 0dBu (2.19Vp-p) input signal positioned between instrument-level dynamics and near pro line level, and a 9VDC power supply. The CompIQ Mini Pro Compressor MK2 controls were set to balance input and output levels across the Dry and Wet lines. The output was connected to a high-impedance load during measurement.

The MINI Pro maintains a linear frequency response from 30Hz to 30kHz. When the Side-Chain Filter is engaged, it introduces noticeable variations in the compressor’s frequency response at the output, enabling selective emphasis or reduction of specific frequencies. We will explain how the side-chain filter works in our Mini, using real measurements, though this applies to any compressor with side-chain filtering.

Frequency response without compression

The frequency response at the compressor output for both Wet and Dry lines is shown below, with no compression applied. Hard Knee and Slower Auto Timing were pre-selected.

How does the Side-Chain filter work?

Side-chain filter action is often misunderstood in audio compression, mainly because it can resemble dual or multi-band processing. Our CompIQ Mini, along with the Stella and Twain models, add layers of complexity with variable filters that can cut or boost frequencies in the side-chain or keep the control signal unchanged.

A side-chain filter (SCF) in audio compression modifies how the compressor responds to specific frequencies in the input signal by adjusting the frequency content that the compressor “listens” to. Here’s a basic breakdown of how it works:

• Side-Chain Signal: The compressor has an internal path, the “side-chain”, which receives a copy of the input signal and drives the compressor’s response. In this path, the signal isn’t heard in the output but guides how the compressor acts. If no frequency changes are applied to this signal, the side-chain may be considered “Normal”.
• Filtering for Frequency Response: By filtering (cutting or boosting) certain frequencies in this side-chain signal, you can shape the compressor’s sensitivity to parts of the audio spectrum:
  • Low Cut (High-Pass Filter): Cutting low frequencies in the side-chain means the compressor will respond less to bass content, resulting in fewer low-frequency dips or swells in the output.
  • Low Boost (Low-Pass Filter): Boosting lows makes the compressor more sensitive to bass, which increases compression, useful for creating “tight” low end or make the compressor be triggered by smaller amplitude (weaker) signals.

Adjusting the side-chain filter can create a response similar to dual-band compression by tailoring how compression is applied across frequencies, even though the audio signal isn’t split into separate bands. Let’s see how this mechanism works within the compression circuit while plotting the frequency response at the output of our CompIQ Mini Pro compressor which further helps visualize the compressor reaction potential.

All measurements below were taken at the compressor’s output using the Wet line, with a 4:1 ratio, a low threshold for approximately -12dB compression, Hard Knee, and Slower Auto Timing. Only the side-chain processing mode (Feed-Forward / Feed-Back) and the adjustable Side-Chain Filter were varied.

The “Normal” Side-Chain

The two plots below show the compressor’s output frequency response during compression with the SCF knob at noon in “Normal” mode. The side-chain signal mirrors the input without extra filtering. With output level matching the input, the CompIQ Mini maintains a nearly perfect flat frequency response from 30Hz to 30kHz, even under this high compression.

The slight subsonic adjustment at the start reflects the RMS sensor responding to the automatic frequency sweep at 0dBu (2.19Vp-p) amplitude. The starting level is set by the Make-Up Gain for the chosen compression amount. Once compression settles in, the circuit’s frequency response at the output is linear. The plots show responses for both Feed-Forward and Feedback compression types.

Cutting Side-Chain frequencies boosts them at the compressor output

When the Mini’s SCF knob is turned fully clockwise (CW), the lows in the side-chain are progressively attenuated with a first-order variable High-Pass Filter (-12dB@90Hz, 12dB/oct), which has the effect of reducing the compressor’s response to these frequencies at the output. As a result, less compression can be applied on these lows, making them louder at the output via Make-Up Gain, as shown in the plots below. Let’s break down how this works.

This effect resembles dual-band processing, but here, the side-chain filter shapes how compression is applied across the frequency spectrum. A common misconception is that compression acts equally across the entire audio range, even when certain lows no longer trigger it. So, how can lows be less affected by compression while the rest of the frequencies are? Technically, the same compression parameters apply across the spectrum, but the amount of compression of a dynamic signal depends on the frequencies passing through the side-chain filter and their amplitudes. Only waveforms with amplitudes that trigger the compression in the side-chain (by passing above the threshold) instruct the reduction element (optical, VCA, FET transistor, etc.) to apply compression on the corresponding waveforms in the working signal; and only for as long as the control waveform in the side-chain is above the threshold. For a steady signal, this may appear to apply compression equally at all times. However, music is dynamic, and each note has a different triggering potential. On compressors with gain reduction meters, you’ll notice that higher-pitched notes tend to trigger less compression. This happens because their amplitudes decrease as pitch increases, such as when playing higher notes on a bass neck. This principle applies to any instrument or sound source. This approach has a clear advantage: if the lows no longer trigger compression, the highs remain less compressed and retain more of their natural dynamics.

When the side-chain filter is active, the reduction element does not function as a simple “volume control” that lowers the entire signal at all times. Instead, the filter alters how different frequencies trigger compression, allowing certain frequencies – such as lows, if they are filtered out in the side-chain – to pass with less compression, making them appear louder at the output. While compression technically applies across the entire spectrum, the amount of compression affecting the filtered lows is much smaller than that applied to the mids and highs. If one were to compute the compression ratio for each frequency at any given time, enabling the side-chain filter would result in a higher apparent ratio applied to mids and highs compared to the lows, all in proportion to the frequency amplitudes in the original signal. And this occurs even with unfiltered side-chains, where some highs can end up overly compressed due to compression being triggered by stronger low frequencies, even if those highs never exceed the threshold themselves; that’s why we need side-chain filters in the first place. At perception level, the effect is further influenced by Make-Up Gain, which boosts the entire spectrum evenly (though it may not be sufficient to optimally restore the highs). This behavior can also be interpreted as a frequency-dependent threshold, even though the actual threshold setting remains fixed in the circuit regardless of which frequencies pass through the side-chain.

This explains why lows filtered out with a high-pass filter in the side-chain receive progressively less compression toward the start of the audio spectrum, or none at all, depending on the filter’s configuration (set it high enough in the spectrum and it may free the frequencies below completely), making those frequencies louder through the Make-Up Gain at the output. While this isn’t true dual-band compression (since the signal isn’t split into separate bands for different processing), the result is somewhat similar and can be replicated by a dual-band compressor.

The Feed Forward plot below illustrates the side-chain filter signal, which mirrors the output signal below the 0dB line. For example, the +10dB boost at 100Hz in the output (shown in blue) results from a proportional reduction at 100Hz in the side-chain (shown in green), with Make-Up Gain determining the overall output level. While this plot represents the conditions set in the test measurement, where all test frequencies have the same amplitude to pass above the threshold, its primary purpose is to clearly show the slope of the side-chain filter (something that’s usually not depicted anywhere) and how its characteristics are reflected in the output signal. This demonstrates how the side-chain filter influences compression and, consequently, the dynamic response of the output signal. For additional side-chain frequency response plots, including examples of both boosting and cutting frequencies, refer to the CompIQ Stella Pro Compressor – Frequency Response Analysis.

The compression feed topology notably shows the differences between Feed-Forward and Feed-Back processing modes, with all other parameters unchanged. In Feed-Back mode, there’s a smoother compression slope across the spectrum, indicating a lower compression ratio than in Feed-Forward mode. This results in a gentler, more “airy” compression that feels less intrusive.

VCA output, 4:1 Ratio, 12dB of Compression, Feed Forward, with maximum cut on SCF

Output (Blue), Side-Chain (Green). See more SCF plots for CompIQ Stella.

VCA output, 4:1 Ratio, 12dB of Compression, Feed-Back, with maximum cut on SCF

Boosting Side-Chain frequencies lowers them at the compressor output

The CompIQ Mini (as well as the Stella and Twain) includes a boosting function in the side-chain filter. When the SCF knob is turned fully counterclockwise (CCW), the side-chain amplifies the lows progressively (+12dB@90Hz, 12dB/oct), increasing the compressor’s sensitivity to lower and lower frequencies. This boosting in the side-chain can be interpreted as a “frequency-dependent threshold,” as waveforms in the boosted range cross the fixed threshold more easily.

The plots below show how this filtering in the side-chain affects the compressor’s output, with a clear difference between Feed-Forward and Feed-Back side-chain processing: Feed-Back produces a smoother response. The level difference between lows and highs in both plots comes from the fixed Make-Up Gain (which, notably, is also part of the side-chain’s feedback loop in our CompIQ compressors design). As more compression is progressively applied to the lows toward the start of the audio spectrum, based on the side-chain boosting slope, these frequencies receive less of the fixed gain initially dialed in. If we would want to fully balance the lows to the input level, additional gain would be needed, which would also raise the highs proportionally – thus, the filter-induced imbalance in the side-chain is preserved regardless of Make-Up Gain adjustments. For better understanding, fully balancing the output lows to match the input level, as could be done with a dual-band compressor, would effectively cancel out the intended side-chain filtering effect.

To conclude, introducing a frequency filter (cut or boost) in the side-chain causes an intentional imbalance in compressed frequencies at the output.

Note that SCF measurements were taken after balancing input and output levels with the SCF in the neutral position for the initial measurement. The filter was then adjusted to extremes (fully CW and CCW) to illustrate its effect. In practical use, Make-Up Gain and other parameter adjustments are needed to maintain an average or perceived input-output balance, taking the SCF setup into account.