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, without directly acting on the actual sound (as is the case with dual-band compression). Here’s a basic breakdown of how it works:

• Side-Chain Signal: The compressor usually 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 on those frequencies, 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.

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 SCF knob is turned fully clockwise (CW), the lows in the side-chain are progresively 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 depends on the frequencies passing through the side-chain filter and their amplitudes.

To understand this better, we must see that the audio signal is actually alternating current (AC), with varying amplitudes across different frequencies. Unlike a spectrum plot, which shows all frequencies at once in a graph, we hear sound in real time, as it’s produced, waveform by waveform. Each waveform is tied to a specific frequency and has its own amplitude. When the amplitude of a waveform crosses the set threshold, compression is applied only to the part that exceeds it (this is how our CompIQ compressors with VCA design work). This means compression is applied to each waveform at a time in the signal as it moves through the circuit, with the side-chain filter’s slope and other dynamics settings (like timing and knee) determining the output frequency response.

If a waveform’s amplitude stays below the compression threshold, it bypasses compression entirely but is still amplified by the Make-Up Gain. This process happens for each frequency in the sound, as we hear it and as it’s processed by the circuit – waveform by waveform. Since not all frequency waveforms fully reach the threshold – some barely touch it, while others exceed it by varying degrees – they each receive different amounts of compression (or gain reduction). This can also be interpreted as a frequency-dependent threshold, even though the threshold setting itself stays fixed, regardless of which frequencies pass through the side-chain.

This explains why lows filtered out 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, 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 similar and can be somewhat replicated by a dual-band compressor.

The Feed Forward plot below shows 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) is proportional to a reduction at 100Hz in the side-chain (shown in Green), with the Make-Up Gain setting the overall output level. 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.

 

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.