Rupert Neve Designs 543 | VCA Compressor / Limiter

THRESHOLD
Sets the level at which the compressor starts to react, from -30 dB to +20 dB. When the control is fully turned up, compression is minimal or non-existent. As the knob is turned counterclockwise, it becomes more sensitive and tends to reduce gain more (which may not be intuitive to some).

RATIO
Determines the “gradient” of the compression from 1.1:1 (minimum) to 40:1 (maximum). For example, if 3:1 is set and the input signal increases by 3 dB, the output signal only increases by 1 dB. In general, low ratios may not affect the music as much as high ratios, but high ratios may be more useful to minimize clipping and OL lights in the recorder.

COMP INPUT
Switches on the compressor section of the device.

F/F + F/B
These are two very different compressor modes, FEED FORWARD and FEED BACK. The F/B mode (key in) usually sounds softer and more natural and is quicker to set up. The F/F mode can be more useful when it comes to shaping the envelope of the sound and creating more bounce and pump in time with the song, if that is the goal.

GAIN
Continuously adjustable from -6 dB to +20 dB.

ATTACK
This sets how quickly the compressor reacts and starts attenuating. Infinitely adjustable from 20mS to 75mS.

RELEASE
This sets how quickly the compressor returns to zero after attenuation. Engineers generally use slower release times if the intention is to minimize an obvious compressor effect or gain change. Infinitely adjustable from 100mS to 2.5 seconds.

RMS/PEAK
With this setting, the compressor reacts not only to the RMS level of the audio signal, but also to the PEAK level. RMS (Root Mean Squared) circuits are considered better suited to mimic the way the ears perceive apparent loudness, while Peak circuits tend to respond directly to the waveform voltage, which is more suitable for avoiding clipping and maximizing levels.

SIDECHAIN HPF
Here, a high-pass filter set to 250 Hz is fed into the circuit that the compressor uses to determine the level. Note that the rest of the circuit and output will not filter out the low tones – this only determines the part of the signal that is affected by the compressor. This is very useful if you want the low frequencies to remain unaffected while the mids and highs continue to be compressed.

LINK
If the 500 series rack has a link bus, this can be used to connect the reduction of two 543 units with each other.