Nerdy Texts of Analog and Embedded Systems Wizardry

God, it’s been so long I hardly know what to say. But I believe the big banner above says the most important thing :-)

It is with excitement and not a little trepidation that I am announcing WTPA’s booting out of workbench land and into the hard, cold world.
That’s right: The wait is almost over — the real deal kits WILL BE out for the Bent Festival in New York this April! Not my usual, waffly “soon”. But on a real, fixed, finite date!
That’s exactly one year since the proto came out, and would be embarrassingly slow if the latest WTPA wasn’t head and shoulders better than its forefather. But. Lest you think I’ve been idle, know that since my last post I have done several things:

• I have become a business. Not a sissy nonprofit either.
• I have used my extensive pull in the “business” world to do some international commerce, and have a boat full of components on the way from Taiwan.
• I’ve set up (and am continuing to set up) an “E-commerce solution”. Although I like these about as well as I like “blogs” and “hipster runoff” they are all totally unavoidable in 2009.
• I’ve revised the dickens out of the boards.
• And all of this is to sell you kits!

The first run of kits will number 0-199. This figure made the most sense considering all the number crunching I did, at least for now. The retail price will be announced on or around the release date, but for the curious it will be nestled snugly somewhere between Mssrs Grant and Benjamin.
There may, at some point, be another distributor besides Narrat1ve for this kit, but they will not likely be onboard by the release date. The kit will come BYO-enclosure, although I will probably sell some parts on the site to make it easier for those of you who want a little of the guesswork taken out. The website will (has to, basically) get beautified to accomodate info for the people building the kit, and not so much my gibbering. Finally, on (or right around) the release date, you’ll be able to get ahold of the source, schematics, instructional videos, manual, and theory of operation docs all from right here.

The hot mess you see above is the test pressing of the final WTPA boards (it’s a picture of the gerber files, complements of GEDA’s Gerbv, which (sadly) unlike most of GEDA, totally rules). The real thing gets here Monday for requisite twiddling.
This is the third and last (hopefully) major hardware revision WTPA will go through. The second WTPA totally slayed the first in audio quality, latency, functionality and pretty much everything else. This last rev cleans up the rough spots that were still left on the second rev, and adds a sprinkle of small new features.
Mostly, however, it is the physically smallest and most simplified of the WTPAs to come out yet and is geared to make the kit easier to build and cheaper to sell. There is very little fat left on this design in my opinion.

Technically, WTPA has returned to single-MCU design. Shortly after I made the two-MCU rev I learned about a device called an I/O expander, which is cheaper, simpler, and more robust than a goofy UART link with a second MCU. Shortly after beginning to do battle with the I/O expander I did some much needed forehead-smacking and figured out how to rearrange the databus and replace the whole nasty expander / MCU / mess / with another parallel latch. Which of course was the Right Answer. Pie-simple, cheap, effective.
This WTPA also has its two analog subsystems (the input/output amps and the VCO / CV section) isolated with separate ground planes and ground returns to the main regulator. The last WTPA was not noisy, but I expect this one to be even better. The goofy emitter follower in the front end has been removed. This should result in better audio response, more headroom, and fewer parts.
The jitter generator has been moved into the realm of software. This does a lot for us. Most importantly, it makes the jitter work right (and sound cooler). It also introduces less HF noise onto the board, and further reduces the parts count. Finally it allows the jitter to be controlled by MIDI! This is obviously good for playability reasons, but it’s “orthoganally good” as well because the jitter clock was the only effect left exclusively in the analog domain. Other than the gains, everything on WTPA is now controllable from both the knobs and switches AND the MIDI in.
Finally, all this hardware juggling allowed me to free up a few extra pins which I assigned to a spare UART / SPI module. I have no immediate plans to expand WTPA, but if I ever did, this would be a great way to do it (say, to talk to other instruments or to add an SD card interface or the like).

This all means that the hardware is basically done changing. It also means I’m getting down to the wire on software business, too. WTPA will not go gently into the aforementioned good night — expect to see LOTS of updates here for the next month while I pull all-nighters getting this monster ready for her catwalk. There will be some ugly fixes, a huge rush of features, sample banks, scope traces, the whole nine. Exciting stuff!

And hey, thanks so much for sticking around this long. Xoxoxo.

Online in 2k9, my ducks.

So sometimes, contrary to common blogline belief there are times when I’m quite busy even when I’m not posting. This New Year / Holiday period was one of them. There are some big advances on the WTPA front:

First, all circuits and modes are totally functional. The resampling problem I was having had two roots. First (and lesser of the two) the input impedance of the AVR’s A/D converter is actually a lot lower than I ever realized. In the datasheet Atmel recommends that you don’t drive the ADC with anything less than a 10k impedance but I actually found that an input resistance as small as 470 Ohms still
produced a noticeably smaller waveform at the ADC input pin than driving the pin directly from the output of an opamp. This was easy (though annoying) to fix — it requires another op-amp section and a redesign of the summing amp into the resampling ADC input. The only unfortunate thing about this is that it will drive the cost of the final device up for you, dear customer, and this kit is already, as they say in France, tres cher.
Still, I made a decision a long time ago that when I design things for me (as opposed to the toy world) I will always err on the side of badassery rather than cost, because I can. And because everybody wants to be a baller, right?

The other (more significant) problem was a Read-Modify-Write issue in the audio ISR. OMGWTFBBQ, was it hard to find! When is a NOP not just a NOP? When it lets you sync an output port you’ve just written to with its input latch. Weird, weird bugs from that one, to be sure. And fixing bugs in my code is, like, totally free!

In other important news I bothered to scope out the speed of the audio IRQs and compare them to the old WTPA. As I had hoped, there was mondo improvement on that front:

• WTPA 0.9, Time in IRQ:
• Recording / Playback: 22-24uS
• WTPA 0.95, Time in IRQ:
• Recording: 5.8uS
• Playback: 7.5uS
• Resampling/Overdubbing: 9.0uS

This means that we could now sample at 44kHz (CD quality — remember CDs?) without breaking a sweat if we wanted to! Of course, I’m not sure how much good that would do us since the ADC on the AVR is already “not-so-accurate” at the paltry 24kHz I’ve got it set to now. But hey, the headroom is there if you all want to get your overclock on.
Practically this means that the processor now actually has some time to get some work done other than audio, which is good.

So the final hardware related issue in this animal is the jitter generator. The way it was originally designed the jitter generator used a PRBS white noise source to mess up the clock signal, and it worked! Sort of.
It became apparent that when the “jitter” control was at maximum, the knee frequency of the filter on the white noise became the dominant interrupt frequency. In English: The fastest noise waveform actually determined the clock frequency in a totally deterministic sounding way.
Since this frequency was different than the clock frequency, you would hear a “jump” in sample rates when adjusting the jitter. This sucked. Right now I’m trying to get the jitter to mess up clocks of all frequencies equally well without inserting a dominant frequency of its own. Meaning a jittered-up 600Hz clock will still have a frequency of 600Hz, it will simply have a totally random duty cycle. (You know, like peak-to-peak displacement, like Wikipedia says under “Jitter”).
After fiddling around with another analog solution (pictured above) and then with some circuit sketches using AND gates and JK flipflops I realized that the best way to do this (everybody all together now) was IN SOFTWARE. I can’t decide if it’s sad or not that it always seems to be that way. Anyhoo, once I’m done with that it’s time for new boards.
Oh, and my birthday is coming up in a little less than a week. Send me naked pictures, or LOLCATs or something. Xoxoxox.

As a matter of fact that IS an Akai MPK49 inquiring where the party is.

In brief: all the MIDI business is working great, although I’ve run into some annoying problems resampling (what guitar players might call overdubbing) which are both hard- and firmware related.
I’m late to haul my freight back to the mid-midwest for the holidays as of now, but you can see the hardware fix I did in the form of that gnarly looking perfboard. That part at least works great. Maybe I’ll nail the firmware issues over some wassail, Kwanzaa candles, or something. Happy Holidays!

WooHoo!

Today is momentous. It samples; from the shoulders of giants it does. I even made a snarled up little video of the first experiment in the act. Scandinavians vs. USA in the bass-less wastelands of Camcorder Flats. Or perhaps holding hands: Glasnost or whatever the equivalent is with stank cheese, progressive design, and socialized medicine.

I tell you, I am a sentimental mess like Lawrence Sterne never made. Honestly, when the first phrase of music kicked out of this thing I almost peed. And then I had a beer or two, which explains the tone here. However, I will stand by this: The New Version Sounds Great Or At Least Slays The Shit Out Of The Old One. I think this mostly has to do with the noise floor not gumming up the quiet parts and _maybe_ some improved amp design.
It’s not perfect by any stretch: I still haven’t hammered out anything in the way of new features, and it isn’t benchmarked yet (I’m very curious to see how much time it spends in the audio ISR) but it IS bumpin, and this test has been great fun. The jitter generator works and sounds weird in an exciting way BUT is tweaky and needs to be tuned in both the analog and firmware domains. I think it’s a keeper, though, just from the initial listening.
As I had hoped the noise floor is pretty good! It’s not annoying, or really even noticeable! I can still see some gnarls on the scope, though, and will hunt them to their holes and Make Reckoning. Like “Reckoning” was “Clean” or “All”.
AND there’s more than enough sample time to hang yourself with the new RAM. Booyah.
Next: MIDI, and ferreting out any remaining hardware bugs and revving the board to the FINAL VERSION!

Yesterday was a good day for sampler technology!

Three really important things got done; two are improvements on the old sampler:

First: the serial link between the MCUs is now totally robust and fast. As I suspected the real problem with the serial before was the internal RC oscillator on the helper MCU. For now it’s been replaced with a 7.3728MHz crystal, but Digikey just rolled up with some 18.432MHz crystals this morning which I’ll be using instead, I think (these particular and seemingly-weird frequencies are exact integer multiples of common UART frequencies, and allow 0 baud rate error).

There were a couple other little forehead-smacking moments where I got bit by casting errors and other general programming bugs but they weren’t really bad once the UART worked. Like for instance — if you declare a variable “theByte” as an unsigned char, set it equal to ~’p’, and then later check for equality with ~’p’, it comes back false. Know why? Know how to fix it? I do now. :-)

Second: It makes audio! Since I already put a picture of a noble sawtooth wave up a million years ago and since the old sampler definitely did this just fine, this is less exciting. BUT it does mean a couple things. The new DAC works, and the analog sections (mostly) work and all the volume pots are the right way around. The VCO works, and the jitter generator — Oh boy does it work! All these need tweaking but they’re all rocking solid.

Third: I spent a long long time fiddling with the PCB and tracking down noise demons. This, I am happy to say, was a big success and a _huge_ improvement on the old sampler!

Pictured above are some example waveforms. On the left is a picture of the output of the old sampler with the preamp gain, through level, DAC level, and master volume all the way up. You probably can’t read the V/div knob on the scope, but this waveform is unmistakably clock noise which has capacitively coupled into the audio path and has been amplified such that it is ~220mV at the output. Yikes! This is really bad. Those of you who’ve heard the original WTPA know that this whine is one of its not-so-good-akshully characteristics.

The photo on the right is a noise picture of the new sampler after a little screwing around by me. Under full-gain, worst-case noise conditions, the new sampler has approximately 4mV of clock feedthrough. This is 34.8dB of improvement!

There are a couple noise sources in the WTPA family. One source of LF noise is the LEDs pulling current spikes from the supply when they turn on. This is at its worst when running from a not-so-low impedance supply (like a 9v battery or worse an STK500 through the ISP header) and did manage to find its way into the audio but it wasn’t terrible. The best fix for it was to improve filtering at the op-amp reference terminals (the VDD/2 level at the non-inverting terminal of a summing amp, say) since the noise that got there got amplified by the circuit’s gain. This helped for sure.

By far the worst noise source is clock bleedthrough, although the new design also shows bleedthrough from the white-noise generator. Improving this was a little trickier. A couple things made a big difference. Varying the gain on the preamp as opposed to keeping the gain fixed (and high) and attenuating its input level was one. Re-doing the design with better ground and signal routing mattered A LOT, although I still had to rip up a trace with an exacto — one poorly-thought out connection added nearly 100mV to the output noise here. Filtering the analog references better helped deal with this noise a little, too.

Finally, I think a ground plane (or two) are in order for the final hardware revision. I also think associating the VCO and Jitter Generator with the digital supply lines (or at least isolating them from the quiet amplifier analog supply) would be a really good idea. I might even use a choke input to the quiet analog supply. We’ll see. Either way, this statistic is already A LOT better than it was, and I’m excited.

Next: Sampling / RAM / throughput rate test, and more fun analog tweaking.