RMS index selectable compressor

Processors & DSP JohnHammer Apr 17, 2026 00:00 3 replies Original on forum ↗

Thread Summary

The user inquired about the functionality of the RMS index selectable compressor with external gain, specifically how to deactivate the external detect input and the range and persistence of the index input. The support engineer provided a project example using the ADAU1787, demonstrating how to control the compressor with a DC cell and a lookup table, and explained the setup for different compression curves. The curves are configured to activate at 0dB, -10dB, -20dB, and -30dB, with the meter reading -3dB for a 0dB signal.
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JohnHammer Question

Apr 17, 2026 00:00

Hi all

I think the RMS index selectable compressor with external gain could be very usefull for my project. I need to select 1 of up to 6 different compression curves. Same timing for all.

I am not sure exactly how it works:
Ext. dectect input L and R - what is this for. How to deactivate? DC input = 0?
Index Input - from 0 to x ? And when it's set to x and I adjust the curve - is that then saved in the project?

Thank a lot.

John

software

ADAU1787

4.7

processors and dsp

Processors and Microcontrollers

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DaveThib Reply

Apr 17, 2026 00:00

Hello John,

You were little vague as to what you are looking for but I will start you out with what I think is most of what you are looking for.

Here is a project with an index controlled RMS compressor.

I am showing two ways to control it. One directly with a DC cell and another with a DC cell set to a real number using a lookup table.

If you use an AUX ADC then you would have to multiply it by the number of sections-1.

Also, if you use an external control I suggest you use the table that has limit protections.

In this project I set the first section to send out unity gain roughly when a 0dB signal is sent to it.

Note, I have an oscillator set to 0dB

Then the next section I set it for -10dB

Then the next one to -20dB and the last to -30dB.

So you can easily see in the meters which section is active. Also, remember that the meter will read -3dB for a 0dB signal so -10dB will read -13dB etc.

The curves are setup like this one example:

Here is the project.

ADAU1787 Index controlled RMS Compressor.dspproj

Dave T