Let's Design and Build a (mostly) Digital Theremin!

Posted: 2/8/2017 7:28:51 PM

From: Northern NJ, USA

Joined: 2/17/2012

All Theremins Need Mains Hum Filtering In The Signal Path

While farting around integrating the truncation filter into the CIC hum filter this morning (I don't think anything else will need it as a subroutine, and one can compute the truncation bit width using the CIC depth parameter) I wondered if I'd broken anything, so as a check I removed the hum filter from the signal path (I should note for completeness that removal of the CIC introduces 1024 / 800 = 1.28 or ~2dB of gain below, which is negligible):

Completely unacceptable spectra.  And you should see the waveform view: +/-15k peak and the hum is loud as hell in my headphones.  Numbers coming from the UART show at least a 4 bit degradation in the pitch operating point.  With the CIC hum filter in the signal path the waveform peak is +/-500, and there is no hum whatsoever in my headphones even at full volume (with a Presonus HP4 driving them that's a lot of gain at full volume!).

I'm not sure I would be inspired enough keep going much further in this project if it weren't for the squeaky clean data and relatively spur-free noise floor the CIC hum filter provides.  It's a real game changer.

Posted: 2/8/2017 10:13:09 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Dew your TW anniversary will be here soon, "Come Back Over to the Light"

Then people will come to New Jersey for reasons they can't even fathom. They'll turn up your driveway, not knowing for sure why they're doing it. They'll arrive at your door innocent as children, longing for the past when electronics were warm and fuzzy. Shortwave listening will be all the rage once again.

dew said: "All Theremins Need Mains Hum Filtering In The Signal Path"

I think you mean All Digital theremins, when properly done in Analog it is not an issue. Here is my random sound sample using the noisiest switching wal-wart for evaluation. Also notice I do not have a thin digital theremin whistle sound. A fuller sound is more natural with analog.

Someday if I find a local musician I may pursue things.

I would post a picture but TW has evolved into grumpy old men... me too!


Edit: Yes it can happen if the antenna impedance is too high, that is why we all have our favorite oscillator.

Posted: 2/8/2017 10:21:03 PM

From: Northern NJ, USA

Joined: 2/17/2012

"I think you mean All Digital theremins, when properly done in Analog it is not an issue." - Christopher

I'll stand by my statement, having experienced my EWS growling like an unmuffled Harley-Davidson due to mains hum AM/FM intermodulation.  One could conceivably lower mains hum interference quite a bit in an analog Theremin by lowering the impedance at the antenna (I've seen this on the bench, the intrinsic C and lower R form a high pass filter) but that generally hurts Q and voltage swing.  It's best, I think, to let it in the door and then immediately bang it on the head with a CIC cast iron frying pan.

[EDIT] Here is one spectral grab of the sound sample you just posted:

Note the 60Hz hum about 30 dB below the signal.  Granted, this is the most egregious point I could find in the sample, and it's not the end of the world by any means, but who wants that mixed into (or worse, intermodulated into) their musical instrument output if it can be eliminated?

Posted: 2/20/2017 6:46:08 PM

From: Buenos Aires, Argentina

Joined: 9/14/2008

Cheers again dewster.

I came back from my holidays, with a DS1054Z in the cabin bag :)

So what I need now is the homework you promised :)

Posted: 2/20/2017 11:04:18 PM

From: Northern NJ, USA

Joined: 2/17/2012

Progress Report

Been plugging away.  I removed the operating point low pass SW filter, and redesigned the CIC hum and high pass SW filters.  Also revisited the LC DPLL SV code and simplified the constructs and  parameters somewhat.  Lots of experimentation, trying to understand the basis for the things I'm seeing via SPDIF.  Complex systems are complex, and there's much to come up to speed on, discover the nuances of, and become accustomed to.

Above is the pitch operating point through just a CIC hum filter, and a first order high pass filter to remove DC.  This is me moving my hand from far away, to maybe 1" away from the antenna plate, and back.  There is an attenuation of 4 bits, 1/16, or -24dB, so that the 32 bit values applied to the 16 bit SPDIF interface are contained sufficiently to not roll over and cause false peaks.  The view above is vertically zoomed to +/-8000 or about 1/4 full scale.

The larger noise levels with my hand very near the plate are due to insufficient HP filtering (for this test) - so any small jitter on my part causes a signal; environmental noise coupling through the increased hand to plate capacitance; and perhaps a lowering of the Q as my hand parasitically draws energy off the tank, significantly lowering oscillation voltage swing.  I don't believe it will be an issue because the numbers are changing hugely when my hand is near the antenna, so even if the noise is larger, the signal is larger as well (i.e. the SNR is probably fine).

I'm getting a better feel for SW filtering; when I'm working with more than 32 bits and need two 32 bit regs to hold them, I'm now aligning the input/output with the most significant 32 bit register rather the bottom of the least significant register.  This makes the shifting manipulations clearer, and the signed ALU operations work as they were designed to.

Above is full gain, with me fluttering my hand about 0.75m away from the antenna.  There's a ~15Hz component in the noise floor that's rather clear here if you squint, not sure what's causing that but I think it's environmental because it comes and goes.  The main takeaway is the ~4000 count change with only a far field hand gesture, on top of a ~600p-p noise floor (and some of the "signal" - my hand flutter - is likely being removed by the high pass filter).

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