Nerdy Texts of Analog and Embedded Systems Wizardry

No pretty pictures today. The hex for WTPA is up to >10kB, which is quite possibly the most code I’ve ever crammed onto an 8-bit part.

The feature list is full and then some, and the ISRs are beginning to suffer speed wise. When the sampler is cranking out multitimbral stuff at high rates you can hear the audio bog down, which I guess is ideologically in line with the “crusty first” aesthetic, but it bugs the anal programmer in me.

In the waning hours before Bent this means going through all the pristine, general ISRs and doing what I can to save cycles, avoid divides, replace conditional branches with jump tables, etc etc. This usually means the code gets ugly, at least for me.

Tomorrow (this morning, I guess) the proper board Rev comes in — that means I’ve got to make a new WTPA and take pretty pictures, and write the manual. Lots of stuff to do before Saturday…

Oh, and did I mention the limited edition video-synthesis daughter board I’m making for WTPA? Come see it in New York at Secret Project Robot on the 25th.

Oh Snap! There’s the goofy silkscreen I promised last time. But don’t worry, it gets better :-)

So I’ve done my best with the above Still-Life-With-Nerd collage to try and convey, in a meaningful visual way, all the cool stuff that’s happened with WTPA since last week. One really perceptually-difficult-to-convey-yet-great thing is that I’ve put my proper rent-paying clients on hold for the time being to make room for this project full time. It’s scary, but it feels really good! And as rebellion goes I suppose it beats buying a Camaro.

Outside of that, the BOARDS ARE HERE! Sort of.

The bad news is, I found a copper bug (my fault) pretty much right away. With as many hardware revisions as I’ve done this was really embarrassing. It led to a natural dilemma — it was a small bug that could be easily fixed with an exacto knife and some jumper wires, so should I you pass that fix off to you, the kit builder, or should I pony up for an ENTIRE NEW SET of boards so that they’d be perfect?

I debated, then reordered them. This also allowed me to fix a half a dozen little cosmetic and noise-floor / routing things that I wasn’t happy with anyway. I wasn’t nuts about dropping another grand on boards, but I’d be less happy knowing that the final kits were going out half-assed. You can’t pay too much for pride :-)

Silver Lining: This now allows me to sell bare boards, which a ton of people have asked for. Originally I hadn’t planned to do this, but now I can sell the flawed boards (for cheap) with an errata sheet to those among you who are exceptional broke asses and not mess up the math and parts counts for the complete kits. It’s a small consolation, but it’s better than making them drink coasters.
Cosmetically, the boards are flawless! I got a not-quite-perfect board from PCBCart before this (for another job) and let them hear about it, and they bent over backwards to make this one dead-on.

Taiwan Alpha also came through — the pots (all 2000 of them) showed up from China early and perfect.

Then there was Dorkbot which was a blast. Above you can see me in front of a page of C code (looks like the ISR) apparently casting some kind of spell.

People geeked out, I gave away some free PCBs, I laughed, I cried, it was better than Cats.

Finally, social business done, it was back to the Fortress of Solitude with some espresso and a dream. Two great things have come out of this so far:

One:

WTPA now has a banked sample system! This means that instead of holding a loop, WTPA can hold an arbitrary number of loops (theoretically, anyway) and can do anything it is able to do to any or all of those loops independently and all at the same time.
Practically speaking, I’ve pinned that “arbitrary” number at 2 :-). There was a lot that went into this! A huge portion of the code had to be re-written — basically all the audio and memory handling parts. The audio system now grabs data from the ADC and passes it to however many “virtual samplers” for recording, and then sums together whatever output they have before putting it back onto the DAC. Better still, the different banks can use different clock sources! Meaning that you can be triggering events with midi and controlling pitch arbitrarily one one bank, and twiddling knobs and generally being a caveman on the other, with a totally independent samples.
By far the hardest part though was having to wrap my head around implementing a memory manager (ever think you’d have to write malloc? Me neither) which was quite challenging. To be honest, I copped out a little, and it is because of this that WTPA uses only two banks for now.

All this cool functionality also has a totally new (and much more intuitive, I think) menu system. Finally, I’m in the midst of getting rid of “modality” in WTPA so that there’s no such thing as “MIDI mode” and “Manual Mode”. Anything you can do with WTPA you can always just do, without mucking around in a menu.

Two:

I crunched the money numbers! As of today, I know exactly how much all this crap has cost me, and therefore how much these magic beans will cost YOU, dear reader. I’m not spoiling the surprise, but I will say that although I have definitely screwed up some estimates in my time, I was pleasantly surprised with how this one came out. And you should be too.

I’ll keep you all posted on the nerdy revelations better for this last week. Don’t expect total coherence or an Infinite Jest command of grammar, but I got you. Oh, and I think I’m going to LA to talk about WTPA at the end of the month, so if you’re on the West Coast come say hi! More details on that to come.

Xoxoxo,

TMB

These last days of March have been decidedly un-lamb-like. Last week I got in the test boards for the final WTPA and yesterday night I assembled them. They look clean. Nice and small. I did some preliminary testing, but decided to played hookey tonight and bend up this Magick Tuxedo out of ABS. She looks good in black, right?

WTPA makes her first pre-release public debut at Dorkbot Chicago, the first DB meeting in fact since Deadtech closed its doors. I’m honored and excited and also nervous.
The version seen above is a barebones copy of the final version I ordered so I could keep coding with the latest hardware while I was waiting for the real-deal-Hollyfields to come from China, but those crafty fabricators at PCBcart are fast! They’ve already shipped! Expect this cursed soldermask-drought to end and pictures of something with a goofy silkscreen soon.
Unfortunately (???) March has seen an upswing in this whole “Business” thing which I feel obligated to pursue in order to “keep organic peanut butter on the table”, and I’m not as far along on the new audio functions as I’d like. But I’m obligated to have at least some cool new effect (and not just smartly registered MIDI jacks) to show off at the dorkbot meeting, so I’m thinking I’ll work out this granular synthesis thing I sketched out a couple weeks ago. Stay tuned for clips….

xoxoxo, TMB

Free Technical Advice: When the Fluke suggests you change its battery, change its battery.

Better: I ordered the REAL BOARDS from China today! I’ll spare you some more pretty gerber files, even though the latest ones are by far the most lovely. Summarily filed under the “breaking some eggs” part of the process. That having been said, my people at PCBCart have so far surprised and pleased me with being fast and friendly. I am now Significantly Monetarily Committed to Not Screwing This Up. I also ordered another set of bare test boards from Advanced Circuits to continue to mess with while I wait for the Mona Lisas to show up.
In other news, my tube amp is on the fritz again. Why I thought tube gear was romantic and cool at some point in my life is beyond me. I swear I’m this close to replacing the chassis bolts with thumbscrews, or better yet replacing the 6L6s with MOSFETs.

Finally, for those of you who haven’t noticed already, there’s a little link above which gives the details about WTPA in a nutshell as the uninitiated had a really hard time figuring out what all this gibberish was about.

So I got the test PCBs Monday, bare as the day they were born. The last couple days I’ve been going through all the circuits and checking the hardware for problems, bugs, things to optimize, and stuff that would otherwise be Bad To Deal With in a run of 200 units.
The good news: The third rev of boards does more or less all the stuff it is supposed to, and has most of the performance improvements I’d expected. However, nothing is perfect.

First, let me give you young nerds a piece of advice. Become rich. Then use your dough to buy prototype boards that have a soldermask. Good lord. You will save yourself hours in debugging stupid soldering problems, and gray hair in debugging other problems which you may at first erroneously attribute to solder bridges.
Bare PCBs do offer an advantage in that they’re easy to probe wherever you need to, and all that exposed tin makes it easy to solder fixes in, but in general they will make you crazy at least once with some weird connection that you didn’t mean to make. That was the first round of problems I had to solve.

With those out of the way came the first real, serious, problem: The VCO. The VCO got incorporated in the last board rev and I dutifully wanked on it a few times and then set it aside to deal with other problems. This in retrospect was quite stupid, because if I had paid more attention I could’ve saved myself a lot of time and a little false advertising.
It turns out that the VCO design I used (ripped wholesale, with a few changes, from Horowitz and Hill p. 240, who I’m pretty sure in turn ripped it wholesale-with-changes from the LM324 datasheet) is good in many regards. It is single supply and simple. It requires only opamps and a fairly generic transistor or FET. The problem came in trying to tune the monster over a wide range. Though the TLV2474 opamps I’m using are pretty good at RRIO, they aren’t perfect.
All the drama occurred when the CV was some small number of millivolts from the bottom rail. The ‘2474s don’t really want to drive that low, so I made the little Merce Cunningham looking mess of dancing resistors (pictured above) to help pull them there. Simple enough, but a little ugly.
Approaching REALLY closer to the rail (say within 10mV or less) the VCO stops responding in anything approaching a linear fashion and gets jittery and tends to drop out. Normally I wouldn’t care and would just bias the CVs above 10mV, but that ends up being a significant amount of tuning in the low-sample rate range. Which is silly. _NO_ op amp is that good, really, and no good design should try and expect one to be assuming it isn’t colliding particles or something.
With a marathon all-nighter / all-dayer the best performance I could get from the VCO was a not-so-great 2kHz to 16kHz, a tuning range of 8-to-1. This sucks. In retrospect the humble relaxation oscillator of the first WTPA did something like 60-to-1.

I fiddled around trying to get another VCO design breadboarded for a little while, but all the good ones I know use an OTA. I’ve seen some designs with analog switches and of course plenty with specialized VCO ICs, but I had three op amp sections and a dream. The final decision was to drop the VCO, return to the traditional relaxation oscillator, and reduce the opamp from a quad to a dual. Even Saving You Money(tm) in the process.

For those of you crying “Noooo! How will I make it talk to my Modular?” first let me say — samplers usually don’t talk to modular synthesizers, you’re spoilt. Second, let me say, because I love you, and because that’s a weird and cool idea, I’ve worked out a way to make the sampler’s oscillator clock sync up to audio inputs — so you’re losing a VCO, but you’re gaining a sync! This means that now you can synchronize your samples to your vocal line, synth line, guitar, or any signal in the audio frequency range.
This should work pretty reliably — I’ll know soon. I can no longer say “The only sampler with a VCO” but I think the new effect will probably be cooler anyway.

Everything else: I found a hardware bug in the in-circuit serial programming section related to changing around the data bus. I fixed with some pullup resistors. After that I made sure the switches and LEDs worked with the new bus structure. It’s technically still possible to PWM the LEDs and maybe I’ll do that sometime before release, but it’s low on the list. Next I quantified the bus-turnaround time related to the bus routing changes, and they’re almost invisible. It took some NOPs in the ISR, but I bet I can optimize those out later. I tested the MIDI output — so far WTPA has only received midi messages. But today it took charge, such as it can.
Finally, I took a look at the audio path. This was important and enlightening. Using a function generator and a fancypants benchtop meter set to take dB readings, I figured out the f3dB points for all the different audio paths through the system. They were pretty good — the first reading showed the audio range of the sampler’s pass-through and DAC paths to be about 14Hz-356kHz (pictured above). Not bad!
Still, as a rule of thumb (also ganked from H&H) I try and position f3dB points 10 times greater or less than the frequency of interest to avoid phase shifts in the audio range. You know what they say about phase shifts, right? “They make for weak ass west coast bass, not that Miami shit.” That’s what I heard them say.
Anyway, I got the range for those paths to be more like 2.8Hz to 370kHz with a few changes. In the overdub path I managed to actually DC couple some of the connections which means that there should be no loss in that path relative to the rest of the sampler.
Giving it the listen test, it sounds Really Good. I’m happy with the audio quality. It’s nice and quiet where it should be, with good frequency response, leaving it free to Expose Da Crust in all the important parts.

Finally, there was one more valid thing I found. In an effort to keep impedances (and costs) low I settled on two values of potentiometer for the final thing — a 500k audio taper for the input and output amp gain controls and 1k linear pots for everything else. In all the audio controls on the board, the 1k pots have a resistor from the wiper to ground to approximate a logarithmic curve (and make your life silky smooth). This means that the total resistance in some places in circuit ends up being pretty darn low. Although this does keep noise pickup low and the top f3dB point high, it also ends up working out the driving opamps something fierce. My hot rail-to-rail output doesn’t swing nearly as rail-to-rail as it used to. This of course means less usable bits in our sample. I can fix this a little by chaning the tapering resistors on the pots, but the right thing to do would be to bump the pots up to 10k and scale their related circuits. Easy enough, if I didn’t have a boatload of pots on the way from China already. The ENOB isn’t terrible, but it could be a little better. Hey, what’s a design if there isn’t room for revision?

Lastly, before I leave the soapbox, I’d like to take this opportunity to show this off as yet another example of an old TMB truism: Analog Be Harder Than Digital. Not better, but harder, and perhaps by association more noble and romantic.
While I was frantically breadboarding crappy VCOs, I came across this app note in which that continuous font of inspiration, Jim Williams, designs like 10 analog circuits which are complicated enough that they make my head spin even after reading his explanations, and he figures them out for shits in his garage or something, while on vacation. There’s a badass VCO in there which tests my understanding, and whose makeup is also (or consequently?) not quite right for WTPA.

Oh, and it also made me always remember to refer to VCOs as “Voltage to Frequency Converters” again. Saying VCO makes you sound like Jean-Michel Jarre; saying VFC makes you sound like a curmudgeon with a white beard and a bunch of Teflon caps and bandgaps. Who would you rather be?

Hello, big bag of Chinese boards? Perhaps you would like to come over?

Xoxoxo, TMB