Loudness gain is the most important issue of stereo mastering compression. Conventional compression principals often cause poor results as far as the loudness is concerned. Audible side effects like pumping and/or distortion in the low frequency range, limit the possible compressions rate. Very often, the mix looses its originality and is only loud in an unpleasant way.

The Stereo Mastering Compressor TM222 has been optimized for massive loudness gain. A set of additional functions can compensate the negative side effects of high compression rates that are caused by physical and physiological limitations to some extend. Using these features appropriately and adapted to the audio signal can result in massive loudness gain of 10 dB or even more. A mix processed in this way sounds still natural and simulates a subjective impression of dynamic by maintaining the original transients.

This principle of master compression has been used in the extensive master sections of our large format consoles for more than 25 years and has been improved in many little steps.

The control ranges of all parameters are more than sufficient to cover common settings very well. This results is a very high variability of the characteristic that can simulate all standard compressor settings.

The Standard Parameters Threshold, Ratio, Attack, and Release have independent controls and more than sufficient control ranges. The Threshold ranges from + 18 dB down to - 24 dB, the Ratio has a control range from 1:1, which actually disables the compressor, to 20:1, which is limiter operation. The minimum Attack Time is 50 µsec; a maximum of 30 ms is possible. The variation of the Release Time is possible from 50 ms to 3 sec.

The additional uncommon features of the TM222 stereo mastering compressor allow higher compression rates by the compensation of the negative side effects.

The Crest switch allows adapting the character of the AC/DC converter to the audio signal. The 6 steps cover the range from very fast peak detection to RMS conversion with long integration time. The steps 2 to 5 offer different combinations of peak and RMS. With RMS settings, the minimum attack time is also determined by the AC/DC converter and not by the attack control alone.

The 'Envelope' (ENV) function can reduce low frequency distortion with fast, pump free release time settings that is caused by regulation during the period of the low signal frequency. Envelope uses a combination of a special filter design with the dynamic modulation of the release time. The control determines the upper limit frequency of the compensation. Using envelope with well adapted settings will result in higher compression rates without pumping.

The Soft-Knee function changes the transition range from levels below regulation that cause no regulation to the regulated range that is controlled by the ratio setting. A setting of 0 dB selects hard knee operation; any other setting results in a successive approximation to the characteric over a transition range of up to 12 dB. Appropriate setting of this control will also result in a higher compression rate and/or lower side effects.

The Fill control mixes the un-compressed input signal to the compressed output. High compression rates will damage the signal structure considerably. Deformed low level components cause the impression that the signal is falling apart. This also limits the possible compression rates. Adding the original signal appropriately can 'fill' these 'holes' and conceal the damaged signal very well. Natural transients and less audible regulation in spite of high compression rates are possible using this feature.

LT-Int operates as a second regulation circuit that can overlay a long time integration component. The attack time of this time element is equal to the attack time of a conventional VU meter. The LT-Int control works like a second release control with a characteristic that is different from the main release control. All other settings than OFF can modify the behavior in a way that the tight characteristic of high compression rates becomes 'aerated' and the mix sounds more natural. Using an appropriate setting makes it possible to reduce the main release time without additional low frequency distortion, which results in a higher density that is modulated by the second timing circuit. Some conventional compressors use this principle as 'autorelease' with a fixed setting. LT-Int makes possible to adapt 'autorelease' to the signal in the best possible way.

The Autogain circuit of the TM222 calculates the gain reduction depending on the settings of the threshold, ratio and attack controls. Autogain eliminates the annoying need to correct the output gain manually during the setup process. Any threshold settings above 0 dB disables the autogain circuit. Below 0 dB, the gain reduction is fully compensated. An analog computation circuit calculates the necessary output gain. The circuit has a very high accuracy with fast attack time settings. With slower attack times, the output level depends on the peaks that pass the compressor without causing regulation. Since the peaks depend on the structure of the signal, autogain cannot calculate exact compensation values without causing another regulation parameter. Empirical values are used for the attack related compensation; with almost all audio signals, the overall tolerance is less than 3 dB. Autogain can be disabled by a switch; an additional manual gain control with +/- 20 dB range is installed as well.

Using the Side-Chain Insert makes it possible to integrate external filters and equalizers in the regulation loop. Inputs and outputs of the side chain insert are electronically balanced and can handle levels of + 30 dBu. The output signal is always available; the insert input is switch on by the Side Chain Insert switch. The additional switch S-C TO O/P routs the side chain insert input to the output of the compressor. External equalization can be controlled easily in this way.

The TM222 Stereo Mastering Compressor is a 4U ToolMod device. It fits into two adjacent slots of a 1U high frame and any slot of a 4U high frame. As usual, horizontal and vertical versions are available.