Stereo Mastering Limiter with analog Delay Line and dynamic Release Time Modulation
The stereo mastering limiter U795 has been designed in consideration of the special demands of analog limiting with mastering. A delay line, based on a sophisticated pure analog circuitry that maintains the original sound performance and phase response can be switched into the signal chain to compensate the attack time of the analog limiter system. Limiting without overshoots is possible with this principle.
adt-audio's dynamic Release Time Modulation allows higher gain reduction by compensating the negative, audible side effects.
All details are here.
The delay line of the U795 mastering limiter is based on a special analog circuitry that results in a time shift of 35 µsec. The group delay is constant up to a frequency of 40 kHz. The phase response between both stereo channels is precisely trimmed to less than 0.5° error at 20 kHz. The delay circuit causes no changes in frequency response, dynamic range, headroom, spatial image, or sound performance. The peak detector of the limiter is driven from the input while the delay is part of the audio path. The inevitable delay of the rectifier and the control circuitry, which usually causes overshoot of peaks in the output signal, is entirely compensated by the delay, which results in an overcompensation of the attack time, if the attack control is set to the mimimum value. The quasi negativ attack time drives down the gain before the audio signal reaches the level control element. The output level of the mastering limiter can feed the a/d converter at the optimum level. The delay is activated by an additional switch, DELAY, which operates independent of the limiter.
Release and Attack
Both modulation circuits of the dynamic release time modulation alter the release time that is predetermined by the STEADY RELEASE control in the range from 20 ms to 3 sec. The release law of the limiter is optimized for fast, linear recovery from gain reduction, which reduces audible pumping, even without using the modulation circuits. Here are the details on the
dynamic release time modulation.
The attack time has a control range from 20 µsec to 6 ms, whithout considering the delay time. The longer values for the attack time are not meaningful for limter operation; however, the device can be used as a very effective, hard and tight operating compressor as well.
The threshold control range from - 6 dB to + 20 dB is expanded around the center position + 10 dB. The values refer to a nominal level of 0 dB = + 6 dBu ~ + 4 dBV.
The Hue Filter makes possible to alter the frequency response under regulation. The U795 is free from any effects like loss of high frequencies under regulation and remains neutral with and without regulation. The Hue control lets you apply a colorisation to the regulated signal that meets your taste or fits better to the overall character of the entire mix. Hue controls a special kind of shelving filter with a range of +/- 4 dB. The center frequency of the curve is determined by a 6 positition stepper switch in the range from 1 kHz to 10 kHz. This allows to simulate the behavior of convential limiter by reducing high frequencies under regulation as well as the colorisation of the limited signal by adding high frequency, which can be an improvement and cover remaining effects of the limitation. The filter is located in the side chain, so the results are reversed. Boosting results in more regulation of the boosted frequency range and less level and vice versa.
Input and Output Gain
The Stereo Mastering Limiter has separate gain controls for input and output. While the input gain control affects the operation of the limiter just like a lower threshold setting, the output gain control only changes the output level. The range of both controls is +/- 10 dB. Since input and output level can be controlled independently, all common operation principles can be used.
The gain reduction display takes place with a LED chain in dot mode with 20 LED's and a display range from 0.5 to 10 dB. The display is not integrated and displays the real control voltage and the original speed. Fast changings of the control voltage are precisely displayed.