The compressor section of the U786 is almost identical to the compressor section of the Stereo Mastering Dynamics Processor U796. Under consideration of the special usage as Bass Compressor and M/S Compressor respectively, some details like parallel compression and dynamic release time modulation have been added and/or adapted. Like all V700 mastering compressors, the U786 allows massive loudness gain while it preserves natural sound performance.

The Standard Parameters Threshold, Ratio, Attack and Release have huge control ranges to adapt the settings to the mix in the best possible way. The range of threshold control allows variation from - 32 dB to + 15 dB. The Ratio controls covers the range from 1, equal to compressor off, via oo (limiter operation), up into the so called 'over easy' range. This range reduces the level above theshold below the value of the threshold and can be used to alter the envelope of the audio signal in a special way. The Attack control range from 0.2 msec to 25 ms allows anything between hard, limiter-like regulation and very soft regulation that does not affect any peaks. The Release control reaches from 30 msec up to 4 sec, which also covers all meaningful applications.

The MAX control can limit the maximum gain reduction to a value down to 3 dB. In combination with a suited threshold setting, the level range where compression takes place can be limited. With higher input levels the gain reduction will not increase anymore.

Since one of the most important operation modes of this compressor is the operation as Bass Compressor the problem of LF distortion with fast release time settings and audible pumping are very important. The U786 has two additional features that allow to achieve a better compomise of both effects. The ENV (Envelope) control adapts the characteristic of a special filter in the side chain of the bass compressor that is optimized to reducing low frequency distortion under regulation. The control shifts the upper limit frequency up to 200 Hz when set all to the right. When set all to the left, the filter is not active.

The dynamic Release Time Modulation increases the release time with low frequency signals at high levels that can cause distortion. In simple words, this sections turns the release control a little to the right if there is any signal that could be distorted and returns to the original setting right after the bass signal. Appropriate adaption of this section to the spectrum of the mix reduces low frequency distortion considerably, without that pumping becomes audible. The dynamic release time modulation section has an additional ac/dc converter that operates totally independent of the main ac/dc converter. A special filter in combination with an analog computation circuitry generates a control signal that only affects the release time setting, depending on the settings of the Release Ratio control and the Release Max control. The Release Ratio control setting determines the steepness of the release time regulation. The more this control is to the right, the higher is the difference between the steady release time and the release time with LF signals. This pot is used to compensate any distortion with bass signals. If the setting is too high, a short pumping becomes audible right after the LF signal. If the mix has huge variations in the level of distortion-criticle LF components, it is hard to find an appropriate setting for the entire title. The second control, RELEASE MAX, allows limiting the release time to a maximum value. This control should be used in such cases to limit the release time to a pump free maximum value. The SET switch makes the setup of this control easy. It applies a test signal to the release modulation circuit that is equal to the maximum control signal that the section would generate with a 100 % LF signal. After the setting, this switch must be released to allow normal regulation.

The additional long time Integration Overlay adds another component to the timing section of the regulation. Another control circuit computes a signal by integrating the envelope of the audio signal over several seconds with an attack time that can be altered by the ATT control between 100 ms and 3 seconds. This signal can loosen a tight compression and restore the natural character; however, high settings of this control result in loss of loudness. The control range is more than appropriate to find a good compromise between natural behavior and loudness. When using this circuit you must consider that changing settings will take some seconds before the result is stable. The long time contants cannot react to a setting at once, of course.

The CREST switch changes the basic characteristic of the compressor. The 6 position stepper switch modifies the reaction of the ac/dc converter from 'fast peak' to 'RMS' with 4 steps in between. The steps are well adjusted and make possible to adapt the characteristic to the mix.

The Soft Knee control affects the transition from the range below threshold to the level range that causes the compression that is determined by the setting of the ratio control. If this control is all to the left, the transition is hard while any other setting approximates the ratio over a range that matches the scaling of the control. The setting of the ratio control always determines the maximum value; therefore the influence of the soft knee control is high with high settings of the ratio but of minor importance with ratio settings below 2.

Two additional controls, Static Color and Dynamic Color make possible to add harmonics to the signal that can simulate the behavior of vintage compressors. Apart from the physically based low frequency distortion with short release time settings that can be compensated by the envelope control in combination with the dynamic release time modulation, the U786 is free from distortion with or without regulation. Static color adds 2nd harmonics independent of the actual gain reduction. The Dynamic Color control adds 2nd harmonics that increases with the gain reduction of the compressor. The percentage of the harmonics that produce colors similar to a triode, depends on the input level and increases at higher level. With nominal level, sensitive adjustment of the colorization up to 5 % harmonics is possible.

Gain and Autogain
The Autogain circuitry calculates the static attenuation from the settings of the Threshold, Ratio, and Attack controls and adds an appropriate gain to keep the output level constant, independent of the settings. The annoying adjustment of the gain after changing settings is not necessary anymore. Since the bass compressor can be used for a particular frequency band or as part of a multiband compressor, it is not possible to provide a constant reference level for all applications. The ZERO control allows the variation of the autogain reference level in a range of +/- 10 dB. With short attack time settings, autogain is very presice and compensates the loss of any setting with less than 0.5 dB accuracy over the entire range. Longer attack times increase the influence of the peak levels to the output level, since peaks cause no regulation and will pass the compressor without the original level. This effect cannot be compensated without causing another regulation; which is acutally not what the autogain circuit is supposed to do. The position of the attack control is compensated by an empirically determined value that results in a good accuracy with almost all kinds of signals. The remaining accuracy for the compensation is better than 3 dB over the entire range. The small changes during the setup process will not result in changes of the output level. When adjusting the threshold, autogain can be confusing, since the effect on the output level when changing the threshold is blurred by the compensation. The AUTOGAIN OFF switch can disable the circuit in such cases. The manual gain control is always active and operated independent of the autogain. The control range depends on the switch 10 dB that selects between the default 20 dB control range and +/- 10 dB.

Display
A LED display with 20 LED's and 30 dB range displays the actual gain reduction.

The U786 Bass & M/S Compressor is a double wide V700 module that uses two slots of a V700 standard frame.

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