TL Audio 5051 - Manual (Page 15)

4.12 Ratio.
Once the input signal has crossed the threshold, the degree of gain reduction
is determined by the Ratio control. The Ratio control is calibrated in decibels
and is simply the change in output level that results from a given change in
input level. An uncompressed signal will have a 1:1 compression ratio - every
1dB change in input level results in the same 1dB change in output level. A
compression ratio of 1:3, for instance, means that a 3dB change in input level
will only give a 1dB change in output level. For more severe compression,
simply turn up the Ratio control.
The 5051 offers a wide range of ratios from 1:1.5 (gentle compression)
through to 1:30 (limiting). Limiting effectively clamps the input signal at the
threshold level no matter how much the signal is increased: this can be useful
when trying to ensure that the signal doesn’t exceed a certain level - for
instance to prevent a digital recorder distorting through overload.
4.13 Attack and Release.
The Attack time sets how quickly the compression is applied once the
threshold has been exceeded, and the Release time sets how quickly the
compression is released (and the signal returns to normal) once the signal
drops back below the threshold. The 5051 Attack and Release controls each
allow a choice of four switched positions between ‘Slow’ and ‘Fast’. For Attack
this covers the range 0.5mS to 40mS, while the Release control spans 40mS
to 4S. There is an element of automatic operation of the Attack and Release
on the 5051: for instance, should a very short transient occur the time
constants tend to become shorter, to prevent a slow release leaving a “hole” in
the signal after the transient. Also, a fast release setting will be extended by a
slow attack setting. Due to this automatic nature of the time constants, the
controls are simply labelled ‘Slow’ to ‘Fast’.
The speed of the Attack and Release should in general be able to work with
the tempo of the signal. For example if the signal is a snare drum, by
monitoring the gain reduction it is possible to set the Release to allow the
compression to fully recover (i.e. the gain reduction needle will settle back to
0dB) before the next snare beat. This prevents the second snare beat being
reduced in level in comparison to the first. One side-effect of having an
incorrect release setting is distortion on low frequency signals, which can
particularly occur when using a fast release setting on bass heavy signals - the
compressor is forced in and out of compression during one cycle of the
waveform, and distortion results. The 5051 has a built-in “Hold” facility which
delays the onset of release for approximately 10mS after the input signal falls
below the threshold. If distortion is still experienced, a slower release time
should be used.
4.14 Knee.