The Stereo Mastering Peak Limiter TM215 has been optimized for the use as output limiter of a stereo mastering processing chain. The attack time of this feedforward VCA limiter with a special peak detector circuitry is extremely fast and reacts within the first period of a 20 kHz sine wave. This property is the most important qualification of a limiter that drives the A/D converter at the end of a mastering chain. In combination with a compressor that is optimized for high loudness gain, which requires a long attack time, the stereo limiter takes care of an effective control of the signal peaks that will pass the compressor without causing any regulation. This setup is necessary to increase the A/D converter input level as close as possible to 0 dB_Fs without distortion. With an appropriate setting, peak regulation of 3 to 6 dB will not cause audible side effects with an appropriate setting of the parameters but increase the loudness by suppression of the peaks.

The TM215 stereo limiter does not alter the sound or the frequency response and produces no high frequency loss or similar effects. These are very important qualities of a stereo mastering device.

The possible gain reduction of the stereo limiter is more than 20 dB; however, the possible gain reduction is a lot lower and limited by the short release time setting that is necessary to prevent pumping. When optimizing the release time in a way that pumping barely disapears the physically depending effect of low frequency distortion due to regulation within the cycle duration of the audio signal determines the maximum gain reduction. Depending on the portion of low frequency components at high levels in the output signal, a gain reduction of 3 to 6 dB is realistic und possible without negative side effects.

The Threshold can be adjusted in the range from -10 dB to + 20 dB. The 0 dB position is internally calibrated.

In order to allow precise optimization of the Release Time, the control range is extended down to a value far below the optimum value of approximately 0.3 sec for pump free operation. Although a release time setting of less than 0.3 sec is not possible with low frequency signals, the range below can be used with audio signals with no considerable low frequency components. Such short release times result in a very high density of the audio signal. The minimum release time is 50 ms; the longest release time is 3 sec. A long release time will cause pumping, but offers the choice to adjust a wide range of conventional settings, as they are used with broadcasting.

Only the shortest possible Attack Time of 20 µs is important when using the TM215 as peak limiter with stereo mastering applications. Longer attack times will cause short peaks to pass thru the limiter. Such peaks can overload the input of the A/D converter that is usually driven from the limiter output The attack time control range reaches up to 7 ms to offer the choice of using the TM215 for other applications than stereo mastering as well. A longer attack time is well suited to use the device as additional compressor that adds some density above a threshold level that is higher than the threshold of the main compressor.

An additional Output Gain Control with +/- 10 dB range and precisely calibrated 0 dB position at the center detent affects only the output level but not the settings of the limiter. After adjusting the limiter, the gain control is used to adapt the output level to the A/D converter connected downstream of the limiter.

The Gain Reduction Display is a LED bar graph with 5 LEDs and 10 dB display range.

The TM215 Stereo Mastering Peak Limiter is a 2U ToolMod module that can be used in all ToolMod frames. As usual, horizontal and vertical versions are available.