Embedded integration in valve compressor

Habibi · Jul 21, 2015

Hello all,

I am not sure whether this should be in this thread of the Truth Table section but will post here, please move it if you need to.

I wanted to share something I am working on at the moment. I have built a single channel vari-mu compressor which will be largely controlled by the embedded device BeagbleBone Black, I am also using the mBed for some further app control developments.

The compressor has motorised pots which can be read by analogue input pins, and controlled via the digital output pins via a Half H driver.

I have written a biquad filter class that is being used for the sidechain EQ. I am currently working on taking the sidechain signal a little further to manage the attack and release duties but I have not got there yet - this is mainly due to me needing to build a negative voltage follower to drive the input grids.. The analogue inputs on the BeagleBone Audio cape are being used, the cape also has some speaker output terminals which I will be playing around with to drive the side chain.

Attached is a pretty rough schematic of the project so far. Some values are not accurate and I have left a few smaller details out until I have a finalised design, but hopefully this gives a clear idea of the goals so far. I can upload the code as well if anyone is interested.

I am interested if anyone has done a similar project and can offer any further suggestions for where this might end up.

There are a few really interesting concepts possible with this, it will be great to get some collective ideas to push it further!

All the best

Habibi

Andy Peters · Jul 21, 2015

Is the point of this project to enable some kind of DAW-based recall?

Habibi · Jul 21, 2015

Sure, I don't see why it needs to stop there though. It will allow physical and digital variable control over networks and serial comms.

There will also be a number of self testing capabilities that can be in the software. The power supply for this is a shunt regulator that can be adjusted digitally, that also brings up some interesting distortion features if you were so inclined.

Cheers

JohnRoberts · Jul 21, 2015

Over the decades I have given a lot of thought to a soft dynamics engine. Not soft as in not loud, but soft as in extremely flexible. I came up with tens of parameters to adjust. Over the decades I have evolved from digital controlled analog (I was going to save presets to the leader of the multi-track using FSK) , to digital controlled digital, but have yet to acquire the skills to write a cost effective interface. Too many knobs and switches for a hardware front panel so needs to controlled by a PDA or smart phone app.

Of course after the leap to digital controlled digital, this is starting to look and feell like a plug-in and I don't like competing with software products that can be so easily copied.

Good luck... My suspicion is that most customers will be confused by too many adjustments and just want the next sound that is in fashion, until replaced by next +1.

JR

ruffrecords · Jul 22, 2015

Way back in 1973, my dissertation fro my Electronics degree was entitled 'An improved Compressor'. It used those new fangled 741 op amps. There were several techniques used to improve performance. First was the use of precision rectifiers in the side chain. Second was the use of piece wise linear approximations to create variable compression slopes and thresholds. The most .novel' one was to rectify both the input and the output and feed the input through the piece wise linear circuit whose outut was therefore the required output level. This was compared with the actual rectified output and used to drive the gain reduction element. The beauty of this technique was that the compressor characteristics were independent of the characteristics of the gain reduction element. It took a lot of tweaking but it did eventually work.

The reason I mention it is that the system is sensitive to differences between the two precision rectifiers and many times over the years I had thought about using micro-controller to replace the side chains. It seems to me that what you are proposing lends itself doing this.

Cheers

Ian

Habibi · Jul 22, 2015

Sounds very interesting, Ian. Do you still have your dissertation? I would love to read it.

Something you could even do with a set up like this is turn the vari-mu into a feedforward compressor, by taking the input signal and processing through the sidechain software. Lots of interesting possibilities I imagine...