PeakRMS combo compressor suitable for low end (<40Hz)
Hi,
I have been using the PeakRMS combo compressor in ADAU1701 with mostly great succes in many fullrange speaker projects over the years
I use it primarily to limit too high levels below 120Hz - all the way down to 30Hz.
Though I can sometimes notice that the compressor still passes the very low end content (or if it is distortion ??)!
My questions are:
1. Is this specific block suitable for such low frequencies - down to 30 Hz?
2. Could it be that I have to high input levels to the compressor? The "graph" in the GUI shows that it is limited to +6dB input level! What happens if the input sometimes signal is above +6dB?
Thanks and best regards
Markus
Audio Signal Processors
software
sigmaStudio 4.7
SigmaDSP Audio Processors
adau1701
Hello SpeakerDude,
You bring up some goo topics and questions.
1) Yes, frequency does not exactly have a direct correlation to the block itself. It is the time constants settings that are important. I will attach a document that I wrote a couple of years ago about the subject. It is kind of a long document and you have to read it carefully to follow the logic. But basically I can sum it up. The time constants have to be at least twice the period of the signal you are running through it. Three times is the best but at low frequencies twice seemed to work just as good. This is because you do not want the gain to change during the period of the actual sine wave. You want it to change over several periods of the wave. If it changes during the wave it changes the shape of the wave and that is the definition of distortion.
2) This is a danger with the compressors that are not full range. If you know the input signal level will never to above 0dBFS for a 24 bit signal then they are fine. But, if you are adding gain or EQ somewhere in the DSP signal flow then you might have a signal that is higher than the +6dB in the table. What happens is that it will just subtract 6dB and continue going louder!! If I were here when the programmers wrote this code I would have suggested they give it an infinite ratio and do not let it get louder. So the compressors that are not full range WILL be able to output signals louder than what they are set for. If you have 10dB of gain reduction at the top of the graph, then it will reduce the level by 10dB for the rest of the time the level is louder than +6dB coming in. The +6dB is the top point in the graph.
I setup a project that can show this and test it if you like to experiment with it.
Dave T
PDFADAU1701 Compressor Range Test.dspproj
Hi Dave,
Excellent document! - very intersting reading!
Regarding the risk for hitting the +6dB max input lkevel I will try to do the following:
1. reduce the gain before the compressor by -10dB
2. lower the compression graph by -10dB
3. and finally add a +10dB gain after the compressor
That should help keeping the input signal below +6dB and at the same time the system gain will be intact!
If -10dB gain reduction is not enough I will try -20dB!
Thanks and best regards
Markus
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