Attack
In fact, almost any compressor has an Attack control; however, the importance of this control is greatly underestimated in many cases. Using a medium setting for threshold and ratio and setting the release time short enough to allow proper regulation, the attack time modifies the characteristics of the compression decisively. The reason for this effect is easy to understand. The longer the attack time is, the less is the influence of the compression on the peaks. With most real-world signals, the peak level is exisiting during the transient effect and human ear recognizes sounds from the combination of the harmonics during this period. Therefore, the attack time setting directly affects naturality of the sound and the subjective impression of dynamic.

Using relatively short attack time in the range of several milliseconds results in a modification of the transients that are attendend to relevant modification of the original sound. A punching kick drum as well as natural density is primarily determined by the attack time.

The attack control range of the U796's Compressor section allows variation from 0.2 ms to 50 ms. Even though 50 ms are not a realistic value for any remaining compression, this range allows to start the setup with a value far above any influence of the compression on the transients. Reducing the attack time gives a clear impression what the compression does in relation to the original and makes it easy to find a setting that preserves natural transient response but prevents the envelope of the signal from breaking down right after the peak. Such a setting depends a lot on the structure of the signal and can be in the range between about 8 ms, up to a little below 20 ms. Convential compressor setting that are based on regulating the peaks down, require attack times between about 2 ms and 10 ms. Anything below 2 ms just drives down the level without noteworthy influency on the sound character.

Release
The Release time setting determines the density of the signal essentially. The faster the compressor drives up the gain after a peak, the higher is the density of the output signal. Since density and the impression of loudness are the same, achieving high loudness gain requires to keep the release time as short as possible. It is obvious that an appropriate low threshold setting is necessary to achieve such a regulation. Long release times in the range of several seconds do not result in higher loudness; such settings have the effect of an automatic gain control that sets the gain depending on the last peak and fades up very slow.

Long Time Integration
An additional control circuit in the timing section of the compressor generates a control voltage that is determined by the average of the level and not by the envelope. This signal behaves quite similar to a VU meter; however, with a release time that is a lot longer. The INT control adds this control signal to the regulation, which result in a different behaviour of the release time and the release law. After finding an appropriate setting for the characteristic of the mix, this release modulation can loosen a tight compression and restore a more natural impression. Adding INT carefully very often results in a much better result without a noteworthy reduction of the loudness, which occurs, when the influence of INT is too high. Another positive result of using INT appropriately is that after adding INT, the main release time can be reduced without adding distortion in the low frequency range.

Ratio and GR Limit
The Ratio control range reaches from 1:1 (Compressor off) via oo (limiter operation) up to the so called over easy range. With settings above oo, the maximum output level is the threshold level. Levels above threshold are attenuated below the value of the threshold level. This range of the ratio control can be used to modify the envelope of a signal in a way that forces a specific momentary level and alters the sound impression. We don't consider this range as very important for mastering applications; however, the U796 is a univeral dynamics processor that is supposed to offer uncommon functions as well.

The GR LIMIT control can set a maximum value for the gain reduction. The effect is similar to the over easy range, but more important also for mastering. When using GR LIMIT with a setting of 10 dB for instance, the compressor remains at 10 dB gain reduction independent of all other settings and also indepentent of a further increase of the level above threshold. Considering the three sections of the dynamics processor, the compressor can be used just to compress the medium level range while the higher levels are processed by the limiter section with another ratio setting. The range of the control can limit the gain reduction down to 3 dB. Please make sure to set this control all to the right during the setup phase, or you'll get the impression that the compressor is defective.

Static and Dynamic Color
Almost all dynamic processors based on traditional conceptions add considerable distortion in addition to the physically based low frequency distortion with short release time settings. FET, tube and diode compressors add 2nd harmonics in an audible range, even when the level is below threshold and no compression takes place. The higher the gain reduction becomes, the higher become the harmonics distortion. Only so called opto compressor are almost free from these effects; these compressor use a light depending resistor (LDR) to control the gain of the audio path. Due to the slow reaction of LDRs, only very long attack times are possible with these compressors. The U796 is free from these effects. The only distortion results from short release time settings at low frequencies that are based on the physical principle of the regulation and not on the design.

Harmonics distortion must not necessarily be a disadvantage. Usually, the designer will put a lot of effort into the reduction of any kind of distortion; however, a small amount of 'clean' distortion can add nice colors to the sound and increasing 2nd harmonics with higher levels or higher gain reduction matches our expectation that results from the experience of many decades of listening to records that all contain this kind of distortion. As soon as a mix is entirely free from these effects we are missing something. Harmonics distortion can be of advantage or not - just depending on the style of the music, on the sound of similar productions and also just by change. Even though adding harmonics can ruin a mix, it is of advantage to have this features available. The compressor section of the U796 has two 'Color' controls. Both controls add 2nd harmonics to the output signal in a similar way vintage gear does. The big difference is that you are able to determine if and how much distortion you want to have. Setting the controls all to the left switches the distortion off.

The STATIC COLOR control adds a constant distortion that is not altered with more or less gain reduction. However, it will increase with higher levels, just like the original vintage gear does. You can add up to 5 % of distortion at nominal level with this control.

The DYNAMIC COLOR control adds distortion depending on the gain reduction. If the signal if below threshold, no distortion is added. The higher the gain reduction becomes, the more distortion is added. The control allows to add up to 5 % of distortion at nominal level and with 10 dB gain reduction.

The law of both controls is double logarithmic; precise adjustment of very small amounts of the distortion is easy.

Regulation Reverse
The compressor of the U796 mastering dynamics processor is a feedforward controlled circuit. This principle offers many advantages; precise adjustment of the threshold, the timing and the ratio as well as fast reaction, since the control signal is the unregulated input signal and not the output signal that is already processed. Feedforward compressors require precise and fast logarithmic ac/dc converters and precise vca's in the audio chain as well. All other construction principles do not offer the necessary precision for feedforward regulation. So most of the traditional compressors, and all vintage gear are feedback circuits, where the output signal controls the regulation. The behavior of a feedback compressor is different from a feedforward regulated system. Therefor the U796 is supposed to offer this regulation principle alternatively. The REG REV push button switches over from the default feedforward regulation to feedback regulation. With feedback regulation there are some differences in the behaviour of some controls. First of all, the manual gain setting and the autogain, now alters the threshold setting. Adding gain causes a lower threshold value and vice versa. It is possible to set the gain control to the output only by an internal jumper. In addition, the ratio control has a different scaling. Setting the control all to the right results in a ratio of 4 to 1; the center position results in 1.8 to 1.

Side Chain Insert
The electronically balanced side chain insert input and output allows access to the side chain of the compressor section. External EQ's, like the W799 Side Chain Equalizer can be integrated in the control loop to modifiy the frequency response and sound performance under regulation. The output signal is always available on the D-Sub connector of the frame. The input signal is switched with the S-C INS push button.

Autogain and Gain
The Autogain circuitry determines the actual attenuation by a calculation based on the postiions of the Threshold, Ratio and Attack controls and adds the calculated gain automatically. Autogain refers to an internal zero level and raises lower levels to this nominal level. Higher levels remain unchanged. The reference level is internally adjusted to the standard level of + 6 dBu or to any custom value that you prefer. The autogain circuit avoids the annoying and confusing need to compensate the gate manually after changing the setting of a control. Since judging of different settings is blurred if the levels are not precisely matched, autogain is a huge advantage. The analog computation circuitry calculates the necessary gain very precisely as long as the attack time is set all to the left. With short attack times the structure of the signal is not important for the output level. With longer attack times, the peaks are not controlled anymore and the output level cannot be calculated precisely anymore, since the structure of the signal has a huge influence on the level. The autogain circuitry uses a correction that has been determined empirically and results in a good compensation with an accuracy of 3 dB with common program material. Considering the small changes of the setting during the optimisation process, this tolerance that refers to the full range is sufficient. Autogain is not a perfect system but a little helper. Side Chain Equalizer settings, extensive use of the envelope or the fill control is not compensated. The gain control is always active and makes possible to correct the gain manually. Sometimes, in the beginning of a new setup, autogain can be confusing, since the threshold point is blurred by the compensation. Therefore, autogain can be disabled by the OFF switch.

Display
The gain reduction display is a LED chain with 20 leds and a display range of 30 dB.

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