The TM212b is a feedforward VCA-based Stereo Compressor with variable AC/DC conversion. The Crest switch modifies the characteristic of the converter that determines the basic character of the compression. The Peak position selects operation as an extremely fast peak rectifier; in the RMS position the RMS value of the input signal is calculated by an analog computation circuit and used as control signal for the vca. With the peak setting, the compressor regulation is controlled by the peak levels in the signal while with the RMS setting the loudness of the signal controls the compressor. In position M the control signal combines the best of the two worlds; neither the hard response of the peak position nor the slow reaction of the RMS setting are used but a well designed balance in between.

Threshold, Ratio, Attack, Release are controlled independently. The Threshold control that determines the begin of the regulation has a range from -32 dB to + 15 dB. The Ratio control covers the range from 1:1 to 20:1. At 20:1 the compressor operates as limiter. The control range of the Attack Time reaches from 0.1 to 30 ms; the Release can be adjusted in the range from 50 ms to 3 sec.

The Envelope control makes it possible to compress low frequency signals with short release times that avoid pumping. This is achieved by a special filter in the side chain combined with dynamic modulation of the release time. Envelope reduces the problem of low frequency distortion with short, pump-free release time settings that limit the maximum compression.

Both features are controlled with a single pot. The center position OFF disables both functions. The Soft Knee range of the pot determines the range of the transition around the threshold. Without Soft Knee, the compressor applies the same regulation law to all levels above threshold while with a setting all to the left the regulation starts with a small ratio at the threshold level and increases the ratio to the setting of the ratio pot over a range of 12 dB. While the effect of soft knee is low with small ratio values, soft knee can achieve higher compression with less side effects when using high ratio settings.

The SMOOTH range of the pot adds the uncompressed input signal to the compressed output. Using a high compression, the transients get more and more distorted which can be covered very well by adding the uncompressed, undistorted input signal. However, too much input signal will reduce the loudness gain. All to right the pot adds - 4 dB input level to the output. In most cases, a range from -20 to -8 dB is appropriate.

Autogain is an analog computation circuit that determines the gain reduction caused by the actual settings of threshold, ratio and attack and adjusts the output gain to achieve constant output levels, independent of the compressor's setting. The compensation is active if the theshold is below the autogain zero level that is adjusted internally to a factory standard of + 6 dBu / + 4 dBV.

Autogain operates with less than 1 dB accuracy with fast attack times. With longer attack times, the output level depends on the structure of the signal. Since the peak levels pass thru the compressor with long attack time settings, the peaks determine the output level, which cannot be calculated without establishing another dynamic regulation parameter. The attack compensation is adjusted to an experimentally ascertained value that results in a very good compensation with almost all signals. Autogain can be switched off by the push-pull switch of the ratio control. In addition, there is a manual gain control pot with +/-

The gain reduction display is a LED chain with 5 LEDs and a display range for 20 dB. The first LED shows a gain reduction of 1 dB, followed by 3 dB, 5 dB and 10 dB.