19
Using the 20/20 AD
20/20 AD User Guide
___| threshold |___
| threshold| – gain
ratio =
Note that the 6dB GAIN setting is given only as an example. Feel free to
experiment with any level in this mode. (GAIN settings above 12dB are not
available because at that point practically all of the dynamic range of the
converter will fit onto 16-bit media. )
Set THRESHOLD to -24dB. Now, when the signal level exceeds -24dB, the gain
is gradually reduced until, at 0dB signal level, the compression gain is also at 0dB
(unity).
The graph below shows a plot of gain vs. input level with compressor GAIN at
6dB and THRESHOLD set to -24dB. Note that, unlike some compressors, the
gain curve is not linear, but changes smoothly. This reduces high-frequency
artifacts which can result from aggressive compression.
"0" Level
Unity Gain
Output
Threshold
(-24dB)
Input
"0" Level
-110dB
-104dB
The settings of the GAIN and THRESHOLD knobs determine the compression
ratio, according the formula:
In our example, the compression ratio is 24
÷
(24 – 6) = 1.33:1. This is not a very
intense compression by most standards but, remember, the point is to pack as
much music onto the recording media as possible without introducing audible
artifacts. As the following table of compression ratios shows, most of the ratios
are below 2:1. In order to minimize the audibility of compression artifacts, use
ratios from the unshaded portion of the table. Setting the threshold at any level
higher than a 4:1 ratio will force the ratio to 4:1.
