Stereo Sub Group Compressor with many additional Functions, Side Chain Insert, and Expander
The Stereo Compressor U798b includes the important features for the compensation of negative side effects of high compression rates. The side chain insert makes possible to modify the behavior by a the side chain equalizer W799. The U798b is designed as a sub group compressor for stem mixing and can be used as well as pre or post compressor for complex dynamics treatment with stereo mastering with side chain eq to treat a particular frequency band.
The compressor section of the device uses high precision ac/dc converters that can operate in different modes, depending on the settings of the crest switch and the envelope control.
The CREST switch determines the reaction of the ac/dc converter on peaks and therefore the basic characteric of the compressor. The 3-position lever switch selects the characteric of a peak rectifier (1), RMS conversion (4) or a medium value in between (2). The medium value is suited for most applications very well, since it has neither the aggressive characteristic of the peak detector nor the slow reaction of the RMS setting.
The ENV switch modifies the reaction of the ac/dc converter to low frequencies and makes possible to find a better compromise between pumping and distortion in the low frequency range. The 3-position lever switch alters the influence of this effect and is switched off in position 1.
The attack setting determines the reaction of the compressor on peaks and has a very important influence on the tonal behavior. Longer attack times result in a more natural sounding compression, since the peak can pass thru without causing regulation. The range from 0.1 msec to 30 msec covers all meaningfull values.
Der Ratio control determines the compression rate for all levels above threshold from 1:1 (no compression) to limiter operation, when set all to the right.
The Threshold control determines the input level that causes regulation in a range from - 30 dB to + 20 dB. Any level above threshold result in regulation.
The release control determines the recovery time of the compressor after a peak signal in the range from 0.1 to 5 sec. In addition to the attack setting this control determines the loudness gain essentially. Settings in the range of 0.3 sec result in best possible loudness gain; however, the final position of this control is mainly determined by 'pumping' that occurs, when the release time is in the range where human ear can hear the fade up and by the physically based distortion of low frequencies due to regulation within the period time of the signal, which happens when the release time is too short. Envelope allows finding a better compromise for the adjustment of the release time between these limits.
Any changings of the settings of threshold, ratio and attack change also the gain reduction and therefore the output level. Autogain calculates this static loss of gain and keeps the output level constant for any level below 0 dB. With short attack time settings, the influence of the attack time on the ouput level is negligible; however, with long attack times the peaks that don't cause regulation determine the output level considerably. Autogain compensates this effect by including empirically determined compensation values.
An additional manual gain control affects only the output level in a range of +/- 15 dB.
Side Chain Insert
The side chain insert affects only the compressor section. The insert output signal is always available, while the insert inputs are switched into the side chain when the SC-INS button is pressed. The insert allows using external equalizers, like the side chain equalizer W799 to modifiy the tonal behavior of the compressor.
A LED chain with 10 LED's and 30 dB range displays the actual gain reduction.
The expander is an effective tool for the oppression of noise that occurs during breaks, when the compression increases the gain considerably. An adapted setting of the expander controls can compensate this effect. The attack time is fast enough to avoid any audible fade in effects, while the release time determines the fade down speed that can be adjusted to an appropriate value.
The threshold, ratio, and release control settings determine the behavior of the expander. Regulation takes place below the threshold level. The ratio setting determines the steepness of the regulation. With a ratio of 2, a signal 5 dB below threshold is expanded to 10 dB; with ratio 4, 5 dB are expanded to 20 dB, and so forth. With a setting of 8, the expander behaves similar to a noise gate. The attack time defaults to a fixed, very fast value of 0.1 ms and can be switched to slow mode. The control range of the release control allows variation from 0.1 sec to 5 sec for 10 dB fade down. A second LED chain with 5 LED's and 20 dB range indicates the actual gain reduction of the expander.
The U798b stereo compressor is a single slot V700 standard module that can be used in all V700 standard frames. The module is available with electronically balanced inputs and outputs or transformer balanced inputs and outputs. All inputs and outputs can handle levels of more than + 30 dBu; the dynamic range is more than 120 dB RMS. The hard bypass connects inputs and outputs and floats the internal inputs to avoid loading the source singals. The audio path uses high precision VCA's in a balanced circuitry. The side chain insert is always electronically balanced; inputs and outputs are available on the D-Sub connector of the V700 frame.