PYE 588 Stereo Feed Forward Compressor
Analogue compressors are particularly good at controlling volume levels and work as useful tools for individual instruments in a recording.
An ongoing problem with all types of compressor whether new or old, is to get a fast smooth attack without compression overshoot, that is, when there is an audible ‘ripple’ immediately following a sudden transient. It is an effect that even shows itself on BBC TV in some classical music feeds.
Analogue compressors are generally ‘feedback’ types, they operate by sensing audio levels at their output and then applying correction to a gain reduction stage near the input. This arrangement works well, but it is liable to suffer from ‘overshoot’ where when faster attack times are demanded, the control system fails to react efficiently causing momentary peak overloads and dipping effects.
Digital compressors are normally ‘feed-forward’ types where the incoming signal is sensed and used to control the gain of the audio path. Most are modelled on historic analogue compressors with varying accuracy.
The PYE 588 operates as a ‘feed forward’ compressor sensing the incoming signal and immediately sending a control signal to a gain-setting circuit near the output. The control signal circuitry sets the momentary compression ratio and the audio signal is instantly controlled by the control signal and the switchable attack time.
The ‘sound’ of this compressor is consistently superior because of tonal effects created during the compression process. A digital compressor handles audio totally symmetrically; any minor distortion produces symmetrical (odd order) harmonics. These extremely small errors influence the sound in an unnatural way, they produce a slightly flat clinical effect.
The PYE 588 compressor operates more naturally; the harmonics generated by the compression process are asymmetric (even order) similar to effects common in nature and pleasant to the ear.
This sounds technical and complicated but the effect is clear and simple; The PYE 588 brings audio into focus; it is like having a variable contrast control, particularly effective for clarity and for bringing a solo voice forward and more present in the mix.
In order to retain perfect symmetry between the left and right channels of the stereo equaliser the audio signal is processed into ‘sum and difference’ mode (that is L+R = ‘sum’, L-R = ’difference’), the two audio signals are then processed by the EQ controls before being re-processed back into left and right format. By going through this process the left and right channels remain perfectly symmetrical with no image shift, any minor difference between the processed sum and difference signals show up only as small deviations in width of the stereo image. A further useful advantage of the sum and difference processing is that the width of the stereo image is controlled by the volume level of the ‘difference’ channel so by adding a gain control the stereo width can be accurately controlled. With the width control at minimum the audio output is mono. When the control is half way, the processing accurately recombines the audio into left and right. If the control is increased further, the image is extended beyond the original stereo.