Nerdy Texts of Analog and Embedded Systems Wizardry

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.

The lone and level sands stretch far away from this nasty Nas, I’m afraid:

Not that you can tell from gazing on this benighted silicon Ozymandias, but no pretty waveforms will happen tonight. It’s not a total loss; there’s some badassery that came through and TON of code, but since I said I’d post today here’s the breakdown:

The body of the code is done for both MCUs, at least at first blush, and the ISRs are now based on the parallel interfaces. The LEDs look crazy hot. Messy little resistors and whatnot are dutifully appearing on the board. The square waves are almost square.

The bad news is that all the old timers who I didn’t pay attention to were right — UART communication on internal RC oscillators is dodgy at best. The real (and unforseen) challenge of the last 16 hours has been massaging the serial link between the two MCUs enough that it works. It limps along now but it sucks. My studied conclusion is that the second MCU really needs its own crystal, and it might as well be at a UART-friendly frequency.
I probably have some laying around somewhere, and will dig them (and exacto blades) out this weekend.

Other fun bugs and riders of the nitpick train include:

The AVR toolchain doesn’t seem to be very excited about the 48p/88p/168p MCUs, and although you can fool it, doing so raises the “bad idea” error flag. Further, the STK500 seems to only want to program the old-ass m168s I dug out to rectify this problem ONE TIME, before getting totally weird. This is a good one: These parts are socketed in the STK, mind, and the fuse bits remain totally unchanged through this whole process. You put code onto the part just fine and boom, it becomes unprogrammable. I did this THREE times. Furthermore, reading the Vtarg on the STK500 shows 6+ Volts, and the device times out when you try to communicate with it. Say what?
I started shopping for a new STK, until further fiddling revealed you can a.) physically pull the RESET pin to ground on the socketed target device using an alligator clip (or stream of foul language) which allows it program just fine OR you can socket and connect to another WORKING part, and move the Vtarg down to 3v, whereupon the old “broken” part works just fine again. Gremlin central.

Writing the code was, thankfully, pretty straightforward and I discovered a kickass feature in avr-libc: ISR_ALIASOF() — this reduced the accumulated ISR code to half the size — thus doubling its beauty.
Finally, a point of consideration: Naming an electonic device on your website “Where’s The Party At” makes for no small amount of schoolgirlish giggling when looking at your keyword referrals in Google Analytics. In keeping, I have decided to name my next piece “Hot Hipster Tang For Altbros”.

OK, OK dusty and neglected friends. For you, today: Twinkles!

What does this mean? Well, several things. The most important of which is that the new version 0.95 PCBs are up and executing code. There are 2 MCUs on the new boards that communicate via a UART. Mostly I did this because I wanted to keep this sampler “kit-friendly” (meaning big DIP parts) and ran out of pins on the main MCU to do all the stuff I wanted to do.

What you’re seeing right now is the smaller helper MCU (here an Atmega168, but it’ll end up a 48p or 88p depending on how much I shave down the code) making the lights go. This is his reason for being, in addition to reading the control pot / CV in, the switches, and generating white noise for the clock jitter generator circuit. More importantly, he’s pretty much done. Besides freeing up the main MCU for more important stuff, this extra MCU allows a lot more foofy lighting effects —

I’ve coded in a bunch of little strobe effects and fades and chases that ought to help keep your sampling experience trippy.

More importantly this update means that I’ve got a bigass hole in my freelance work right now and I’m pretty sure I can use it to nail the code/hardware debugging for WTPA once and (almost) for all. There’ll have to be another PCB rev (I’ve found hardware bugs on this one already) but the code should be pretty much exactly portable to the next one.
A couple days ago I set a deadline of, er, tomorrow to get this revision up, sampling, and tested. The code for the main chip is already well underway, and I think this is realistic. Stay tuned…