September 15, 2009 at 9:36 am #15197Ted FletcherKeymaster
I have had a number of questions about sidechain access and sidechain EQ….. here’s an overview:
The design of a compressor sidechain is not a simple matter….. A classic ‘feedback’ compressor works by the audio signal passing through a gain-changing stage, then the audio level is sensed, the sensed value is converted to a control voltage which is applied back to the gain-changing stage, thus controlling the gain in a carefully controlled way.
The amount of control (ignoring the time constants) depends on the characteristics of the sensing and the conversion to control voltage. Simplistically, the audio ‘voltage’ is converted into a DC control voltage, and this is where lies the soul of the compressor!
If the audio voltage, peak or average, is directly converted to a control voltage, the compressor will sound very unnatural, with over-compression happening at low frequencies because of the higher voltages in the audio at low frequencies.
To overcome this problem, a shaped bandpass filter is needed to even out the compression for different frequencies.
In the P38 and P38EX, the sidechain has a filter that approximates to a 6dB per octave high-pass filter for frequencies below 400Hz. This provides a fairly neutral compression characteristic, which, in combination with the built-in attack and release times, produces the added ‘depth’ and excitement that good compression can bring.
In the P38EX, there is the added function of being able to access the sidechain and alter that filter curve…. This is in addition to the existing filter….. I have kept the existing filter in circuit as it is essential to avoid the serious and very unpleasant over-compression effects with a ‘flat’ sidechain.
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