 P38 - THE COMPLETE COMPRESSOR / LIMITER LIMITING AND COMPRESSION Why is the TFPRO compressor different to a modern high tech. compressor limiter? It was designed as a stereo effects compressor rather than a 'levelling amplifier' as the early compressors were called. These are the basic definitions:
A LIMITER is a device which stops the output of a signal path going above a predetermined level.
A COMPRESSOR is a device which reduces the dynamic range of programme material.
A 'perfect' compressor is an amplifier where the input/output ratio is constant: So using a 2:1 compressor, increasing the input by 2dB gives a corresponding 1dB increase in the output.
Early compressors which used variable mu thermionic tubes or photoelectric devices only approximated true compression over a limited range. They had a soft 'threshold' where compression started and held to a predictable ratio up to a certain level, then they returned to a more linear amplification allowing transients through. This is in stark contrast to modern VCA compressor/limiters where designers thought it 'sensible' to combine the functions of compressor and limiter and to 'stonewall' any and all signals above a certain level. The musical effect is that (I know I'm over simplifying) VCA compressors sound muddy and flat, while old compressors sound
lively and retain sparkle.
IT'S HORSES FOR COURSES, a good VCA compressor limiter will do a good job making medium wave radio sound a bit louder and protecting a radio transmitter from exploding, but it is a poor tool for making a voice track stand out. TECHNICAL DESCRIPTION Input stages are 20Kohm impedance floating balanced line via balancing transformers operating in ‘current mode; this means that the inputs are very resistant to noise and interference, and may be operated balanced or unbalanced. The grounding system used follows latest balanced studio practice where chassis metalwork is grounded through the power plug.
Outputs are nominally 50 ohm balanced and may be operated balanced or unbalanced.
Operating level is 0dBu with a maximum balanced output level of +23dBu.
Gain at the input stage is variable from off (mute) up to +16dB.
Once past the input stage, the stereo signal is processed into 'sum and difference' signals and these are compressed through 2 identical compression channels. The sum signal is used to drive the compression sidechain where precision rectifiers create an instantaneous DC voltage proportional to the audio signal.
RATIO and MODEL switch settings alter the timings and shape of both the attack and decay; that is, the characteristics of the integral of the DC voltage; for example, in 1 (VCA) mode, the attack is fast and linear, the release is a simple linear capacitor discharge. In 4 (opto compressor) mode, the attack is fast for the first 500 microseconds, then slower. The release is highly non-linear with the first 20% fast, then a linear portion, then the last 25% speeding up again.
'TRANSIENT RELEASE' control (push button) alters the dynamics of the sharp transients in the signal; it can change the sound and effect of a track in a dramatic way.
There are further variations for the other 'MODEL’ settings. The heart of the compressor is the gain-cell; this is a pair of cadmium sulphide light dependent resistors illuminated by an LED array. The LEDs get their drive from a servo amplifier capable of driving the high instantaneous current necessary to achieve the faster attack times.
The RATIO control selects a huge range of gain reduction (compression) ratios, from a relaxed 1.5 to 1 up to a ‘brick-wall’ 100 to 1.
Prior to the output stage there is a control that varies the sum/difference ratio; this alters the stereo width, which is variable from full 'mono' through normal stereo (where the control detent is) to 150% stereo.
Further control is given by the ‘BALANCE’ control. This allows the engineer to correct for slight errors in L/R balance without going back to the mixer.
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TFPRO - P38
