Wiki

This version is outdated by a newer approved version.DiffThis version (25 Jun 2012 17:27) was approved by William Jahn.The Previously approved version (25 Jun 2012 17:26) is available.Diff

This is an old revision of the document!


PeakRMS Combo (no gain)

Click here to return to the Dynamics Processors page


The PeakRMS combo compressor is a dual detection path compressor. Both Peak and RMS detection are performed on the detection input signal and then a combination of the two detection methods is used for the final gain application of the compressor. Depending on the time constants, Peak or RMS can dominate the effect on either the attack or release. The idea behind this combination detection is to provide the best of both worlds between the two detection methods, allowing fast reaction to undesired peaks, while still having a natural auditory quality to audio compression.
The image of the algorithm shown is the Mono version of the algorithm. There is also a stereo version of the algorithm, shown in the example below.

Input Pins

NameFormat [int/dec] - [control/audio]Function Description
Pin 0: Audio Inputdecimal - audioInput signal to be compressed
Pin 1: External Detectdecimal - audioSignal Detection input. This is the signal that is run through both the Peak and RMS detection.

Output Pins

NameFormat [int/dec] - [control/audio]Function Description
Pin 0: Audio Outputdecimal - audioCompressed output signal

GUI Controls

GUI Control NameDefault ValueRangeFunction Description
PeakHold (ms)20-1000Controls the hold time constant used in the Peak detection path
PeakDecay (dB/s)90000 -10000Controls the decay time constant used in the Peak detection path
RMS TC (dB/s)10001-10000Controls the RMS attack time constant used in the RMS detection path
Hold (ms)20 -2000Controls the hold time constant used in the RMS detection path
Decay (dB/s)5000 - RMS TCControls the decay time constant used in the RMS detection path. The max value of the decay for the RMS path is limited by the RMS time constant value
Soft KneeOffOn/OffEnables a soft knee calculation for the gain table threshold points. Clicking this button will toggle between having soft knee enabled or disabled, and new compressor curve points will be calculated.
Show Graphn/an/aClicking on the “Show Graph” button, opens the compressor curve editor. In this window, 33 points can be dragged or edited to a precise compression curve.

DSP Parameter Information

GUI Control NameCompiler NameFunction Description
PeakHold (ms)PeakRMSCombo1holdAllThe peak detector hold time in (ms) is written to the DSP according to this formula: Fs *Hold_Time/1000
PeakDecay (dB/s)PeakRMSCombo1decayAllThe peak detector decay time in (dB/s) is written to the DSP according to this formula: Decay / ( 96 *Fs)
RMS TC (dB/s)PeakRMSCombo1RMSThe RMS detector time constant affecting mainly the RMS attack time in (dB/s) is written to the DSP according to this formula: 1 - RMS/(10*Fs)
Hold (ms)PeakRMSCombo1holdThe RMS detector hold time in (ms) is written to the DSP according to this formula: Fs *Hold_Time/1000
Decay (dB/s)PeakRMSCombo1decayThe RMS detector decay time in (dB/s) is written to the DSP according to this formula: Decay / ( 96 *Fs)
Soft KneePeakRMSCombo1


Clicking the Soft Knee button on the compressor control, writes the 33 gain table point parameters to the DSP. This allows a soft or smooth transition at any threshold point defined in the compressor curve.
Show GraphPeakRMSCombo1


Clicking on the “Show Graph” button, opens the compressor curve editor. In this window, 33 points can be dragged or edited to a precise compression curve. Each time a point is dragged or edited, the 33 points are re-written to the DSP.

Algorithm Description

The PeakRMS Combo algorithm offers a dual-detection path. The RMS and Peak envelope of the detector input signal are calculated. Depending on the time constants used for each detection (Peak and RMS) and ADI proprietary code, the algorithm switches between using RMS and Peak detection in order to apply the compressor gain adjustment. This results in getting the “best of both worlds” with both Peak and RMS compression. Typically, there are complaints that RMS is too slow to react to fast spikes in signal input. Peak detectors can handle these fast spikes, but then do not have a natural sound response to the increase in signal level. The generated images below show the difference in output signal for RMS, Peak, and Combo compressor.

The clipped brown signal is an input sine tone at a low level, that immediately increases in level. All three compressors have a limiting compressor curve set at -20dB. The Red curve is the RMS output, showing the first initial peaks still getting through to the output because the of the slower attack time constant. The purple curve is the Peak output, showing the quick attenuation of the peak, but unnatural ripple introduced by the quick attack. The light-blue output is the Combo compressor. Note that the first peak is attenuated quickly (not as severe as the Peak) but much more than the RMS. However, there still is a natural decrease in signal level from the initial attack, which subjectively sounds better than the harsh peak detection.

Playing with both the RMS and Peak detection time constants on this control can yield different results, but the default settings on the control are recommended as a starting point. The graph below was generated with the default time constant settings.


Example

The following schematic image shows the PeakRMS Combo algorithm being used on a stereo signal. The external detector path is fed with the larger signal between L and R. This ensures that any rise in signal from either channel will ensure that the output level is reduced. Other applications may be more lenient and can use the average signal between L and R to drive the detector path. The schematic below uses the Input, AB in/out Condition, and Output cells.



Algorithm Details

Toolbox PathDynamics Processors - Combo: RMS + Peak - Stereo - PeakRMS Combo (no gain)
Cores SupportedAD1940
ADAU170x
ADAU144x
ADAU176x
ADAU178x
“Grow Algorithm” Supportedno
“Add Algorithm” Supportedno
Subroutine/Loop Basedno
Program RAM70
Data RAM24
Parameter RAM39
resources/tools-software/sigmastudio/toolbox/dynamicsprocessors/peakrmscombonogain.1340638028.txt.gz · Last modified: 25 Jun 2012 17:27 by William Jahn