DPL1 is an look-ahead digital peak limiter, the kind you would use as the final step to avoid clipping when mastering or mixing. It can be used as an effect on individual instrument tracks as well.

Latency is 1.2 ms rounded up to the nearest multiple of 8, 16 or 32 samples depending on sampling frequency. This amounts to 56 samples at 44.1 kHz, 64 samples at 48 kHz, and twice those values for 88.2 or 96 kHz.

Use zita-dpl1 -h to see all options. In most cases you only need -k number_of_channels, which can be 1 to 16. The same gain reduction is applied to all channels.

The rotary knobs can be used in two ways:

* Click on the knob with the left mouse button, keep it pressed and move either left..right or up..down.

* Using the mouse wheel. Press Shift for smaller steps.

From Left to right we have:

* Input gain. This sets the gain applied before peak detection or any other processing. Range is -10 to +30 dB in steps of 0.2 dB.

* Threshold. The maximum sample value at the output. -10 to 0 dB in steps of 0.1 dB. DPL1 will not allow a single sample above this level, if it does please report as a bug.

* Release time. This can be set from 1 ms to 1 second. Note that DPL1 allows short release times even on signals that contain high level low frequency signals. Any gain reduction caused by those will have an automatically extended hold time in order to avoid the limiter following the shape of the waveform and create excessive distortion. Short superimposed peaks will still have the release time as set by this control.

* Level and gain reduction display. The range is -10 to +20 dB, ticks every 5 dB. The meter at the bottom shows the peak signal level after input gain and relative to the threshold which corresponds to the green to yellow transition. The scrolling waveform above the meter shows the gain reduction history, with the range (minimum to maximum gain reduction) being displayed for every 50 ms segment of the signal.

Response of zita-dpl1. The input signal is a continuous 1 kHz sine wave with amplitude 0.5. To this are added a single sample, a 5 kHz burst and a 100 Hz burst, all with amplitude 2.0.

Gain = 0 dB, threshold = 0 dB, release time = 10 ms. The red line is the gain.

Zita-dpl1 limiting the sum of a 567 Hz and a 2345 Hz sine wave. Both have amplitude 1.0, so the peak level and gain reduction is 6 dB.

The same using the TAP Scaling Limiter plugin.