adt-audio
ToolMod® Pro-Audio Module System
TM222 - Stereo Mastering Compressor
6
re that you don't have different levels with and without compressor for this test. The
autogain should take care about this; however, you may need to fine trim the output
level with the gain pot.
3. optimizing Threshold and Ratio with the new Attack Time
After adjusting the attack time, optimize the setting of threshold and ratio for best
loudness gain and/or to your liking.
4. optimizing Crest
Try to get an improvement by changing the Crest setting. Choose the setting that is
best suited for the mix you are working on.
5. adjusting the Release Time
After optimizing Crest, we need to fine tune the release time. Release time setting is
always a compromise between audible pumping (= you can hear the compressor fade
up when the level drops) and low frequency distortion, which is a physical effect that
appears if the release time is too short.
'Pumping' becomes audible if the release time is longer than the release time of the hu-
man ear of approx. 0.3 sec/10 dB with individual and program depending differences.
If the release time of the compressor is shorter than this approximate value of 0.3 sec,
the audible pumping disappears. Such a setting is absolutely necessary to increase the
density - and therefore the subjective impression of higher loudness.
However, with a release time setting in that range another problem appears that limits
the minimum release time. With short release time settings and low frequencies, the
control voltage, which is actually the rectified audio input signal, is no dc voltage but
a dc voltage with overlayed remains of the audio signal. The shorter the release time
and the lower the frequency, the lower is the filter effect of the compressor's time
constant circuit. The ripple on the dc control voltage causes additional regulation and
the additional regulation causes the distortion. This is a physical problem that is not
limited to certain compressors.
Set the release pot from the start value of 0.3 sec for a moment to about 1 sec - now
you will hear the pumping - and turn slowly to the left to find the point where the pum-
ping just disappears. Ignore any lf distortion for the moment.
6. compensating LF Distortion with the Envelope Circuit
The Envelope circuit (ENV) can reduce the low frequency distortion considerably. The
circuit combines a special filter that reduces the bass compression with a short and in-
audible increased release time for the time a low frequency signal is present. The fil-
ter characteristic compensates the increasing distortion toward lower frequencies in
