Skrasoft Dev Blog

Just a quick post this time… The Pokey module panel graphics are finished and being sent off to get made. Here’s a rendering:

I never really noticed this until I started getting into really low frequencies on the Pokey, but it only actually changes the output pitch on transitions of the current square wave signal. The chip itself can read hundreds of thousands of pitch settings per second, but they are only applied at the next transition of the current square wave. For example, going from a low pitch square wave to a high pitch square wave looks something like this:

The first line is the current square wave output, and what it would do if it ran forever. The second line is an instant pitch change, and the third line is along the lines of what the Pokey actually does. Note that the new pitch data in that example was sent soon before a transition in the low frequency. The worst case is when the new data gets written right *after* a transition, and doesn’t kick in for half a cycle of the low frequency wave:

This implies that the clock dividers driving the logic only update their divide value when they output a pulse. Pokey square waves come from pulsing a T flip-flop, so there are two pulses per output cycle. That means if the current square wave is 1 Hz, the next pitch written won’t happen right away. It can take up to 500 milliseconds to change.

This all came up because of a new feature in the Pokey module design. A Pokey square wave in 16-bit mode with the usual pitch control ranges from about 13.5 Hz to hundreds of kHz. However, the Pokey itself has a bit that can be flipped to drop the range about 4.8 octaves and go down to 0.5 Hz! Initially I planned to have switches on the back of the module for this because I ran out of space on the front. More recently I’ve updated the pitch handling routines to automatically flip that bit as necessary to make the desired pitch. This initially created a slight problem, though, because flipping that bit takes effect almost instantly and drops the current pitch by 4.8 octaves. To go from 14 Hz to 13 Hz, that bit has to get flipped at some point. If the current pitch data is for 14 Hz and the bit gets flipped, the result ends up somewhere inside a 0.5 Hz square. Then, even if the pitch data for 13 Hz is written instantly, it may not take effect until a full second (half of a 0.5 Hz cycle) has passed! In other words, there may be a full second of silence inserted when going from 14 Hz to 13 Hz.

Eventually I came up with a workaround that reliably shortens the silence to about 43 milliseconds when flipping the aforementioned octave-dropping bit. This still creates some audible artifacts when switching between frequencies just above and just below 13.5 Hz, but it lets the pitch CV and knobs control a much greater range without adding any switches.

I made some recording of the worst case switching artifacts. Going from a high note (hundreds of Hz) to a note below 13.5 Hz is not really noticeable. It’s the low frequencies where it becomes clear because of missed/extra pulses when crossing the threshold.

Here’s an example playing two pitches an octave apart that straddle the crossover point:

Click here for audio clip one

This next example is some sweeps that go through the crossover point:

Click here for audio clip two

The more I use the module with this change, the more I like it. It only applies to a channel in 16-bit pitch mode, but there is already a switch on the front panel for that.

There is now one completely-built Pokey module prototype (though without panel artwork). The FPGA code is around 90% done. It still needs some minor usability tweaks and top secret configuration features.

More importantly, I made a few audio clips to show some of the current features.

This first clip plays a major scale on the 6 different wave settings. I’m running a Kenton Pro Solo into the pitch CV input on channel 1. A Doepfer A-141 VCADSR is going into the channel 1 volume CV input. The different types of noise have different amounts of pitchability. The notes that sound like they’re missing in a couple spots are a side-effect of how the Atari chip generates noise. There’s a little more info about why it happens in this post.

Click here for clip one

The second clip runs a Blacet Micro LFO into the CV wave input for channel 1. It’s set to a sawtooth wave to sweep through the different noise types. I’m tweaking two knobs: LFO rate and Atari channel 1 pitch. I also change the pitch resolution of the channel from 8-bit to 16-bit about halfway through, making the trip back down sound less steppy. Each channel has a switch on the front panel to jump between pitch resolutions.

Click here for clip two

The previous two clips have both been one voice by itself. This third clip starts as one voice, but then adds in the second voice slightly de-tuned. I flip the “highpass” switch that gives a nice octave effect, and then over-drive the channels via their volume knobs. The louder one channel is, the more it will modulate the other. The very end is back to one channel.

Click here for clip three

This last clip is more experimental. It combines the Aux input with low frequency noise. The signal path is a Blacet Micro LFO triangle wave into the Pokey synth AUX input. I’m also running one of the Atari channels set to noise at a sub-audio frequency. The low frequency noise creates rhythmic patterns that vary with pitch and modulate the volume of the AUX input. When the rhythm is changing it’s because I’m tweaking the sub-audio frequency and new patterns form automatically. The changes in pitch and timbre are because I’m also tweaking the Sample Rate knob for the aux input.

Click here for clip four

Keep in mind that even though the channels modulate each other to varying amounts, there are three independent channels on this thing mixed to a single output.
Channel 1: Atari oscillator #1
Channel 2: Atari oscillator #2
Channel 3: Aux input

The two oscillator channels are identical in terms of features and controls. The Aux input has a gain/attenuation control and controllable sample rate.

Still chugging along on the Pokey synth module FPGA code. Last night I finished the first draft of the A/D sequencer. The result: knobs work! Based on the current scheme, the sample rates should be:

Pitch inputs: ~40kHz each
Volume inputs: ~40 kHz each
Wave inputs: > 40kHz each
Aux input: ~40 kHz
Sample rate input: ~20 kHz

The pots are sampled in the single-digit kHz range.

Here are a couple quick audio demos:

Audio 1

Audio 2

I made these to check how the channels modulate each other at different settings. I’m just turning knobs on the front panel; there isn’t anything connected to the inputs yet.

Layout finished for the Pokey module main board prototype a month or so ago, and this past Monday two bare PCBs came in. My weeknights have been spent soldering parts and testing power supplies. There are five power rails for running the digital and analog stuff combined. When I was looking at Altera FPGAs, it looked like the design would need seven, but Xilinx parts are more forgiving. I wanted to use a Xilinx Spartan 6, but those are only available in BGA format (ie, impossible to solder by hand) until August. I settled on a Spartan 3A part.

The Soldering Zone

Anyway, this is just a rambling status update. The power supplies are doing what they should and nothing is getting hot. The first audio spewed forth last night, and here’s a clip of what that sounded like:

Click here for a .wav

It’s the same test code I used in this post, just migrated to a Xilinx FPGA and self-contained. It’s good to see that it’s working on a standalone module now instead of just on a dev board. Progress!

Knobs 'n' Lights 'n' Stuff

The three LEDs show “pitch clipping” and whether or not the aux input is available. The pitch inputs take -5 to +5, giving a 10 octave CV span. Since the Atari chip is somewhat limited in its frequency response, there are LEDs to show when the requested pitch is either too high or too low for the chip to handle. There’s a switch on the PCB that lets you control if a clipping channel should either mute or just play its most extreme frequency. Muting is useful for keeping chords in tune.

My next step is to get the A/D converters talking. I’m shooting for at least a 30 kHz sample rate on each of the Pitch and Volume CV inputs. That means pitch input 1 will be updated at least 30k times a second, pitch input 2 will be updated at least 30k times a second, and so on. The finalized sample rates, along with other tech specs like input impedance, will be in the manual.

In terms of overall time frame, I ordered 1000 potentiometers in early May. They take 12-14 weeks to get, putting actual release of the Pokey module in the range of August/September. The FPGA code is currently about 25% complete. Bifurcating technobabble flux capacitor.

Prototype PCBs came in this week for the Pokey synth module front panel. Everything matches the drill test panel nicely. The next step is to get the main board prototype finished, ordered, and built.

Bare Board

Placed Parts

Mechanically Maniacal

I’ve been making hardware changes to the Pokey module so that it can actually be manufactured. Yesterday evening I took a break to write some more Verilog FPGA code for the Pokey controller. The previous test was to write to the Pokey as often as possible, which came out to 1.79 million times per second. This test added a finished start-up routine and tweaks frequency/volume registers to check for glitches.

Listen Here

It is just a simple test. The volume is modulated by a digital triangle wave, and the pitch is modulated by a slower digital triangle wave. Everything works just as expected. There is a lot more stuff I need to port from the microprocessor into Verilog, but it’s going faster than I originally expected. I probably won’t be using dsPIC chips for audio projects anymore after finding out how much more can be done with an FPGA.

In other news, my company is now official. The new name has a better ring to it than “skrasoft,” but I’ll save it for when this module goes on sale. I’m working as fast as I can to start manufacturing without sacrificing any of the features. There are several as-of-yet-unmentioned new features in the FPGA version of the module.

For those who missed it, I posted a test video of the old microcontroller-based module on Vimeo here:

http://www.vimeo.com/9258219

Tonight I solved a hardware problem with the Pokey module that had been a mystery for quite some time. In celebration, it only makes sense to post about an unrelated change from last weekend.

When the Pokey adjusts pitch on a noise waveform, it can often change timbre completely or even go silent. With analog tuning control, an incoming voltage can end up teetering on the brink of stability between two different sounds. For very slow pitch changes, it can end up sounding like a bad connection.

Click here to hear some “before” pitch changes

A common solution would be to add hysteresis. That is, keep a record of *how* the signal is changing. If it has been increasing and keeps increasing, let it pass as-is. If it has been increasing and tries to decrease, ignore the change unless it is more than some set amount.

That sure is a lot of logic to add, though. It requires looking at the last direction moved, the current direction, and how far it is trying to move. Instead, I went with a simpler approach. I’ll spare the math, but the resulting process looks like the following graph. Red is the noisy input signal and blue is the smoothed output.

Red represents the eternal struggle between order and chaos... or something

It looks just like it’s been low pass filtered, but the overall frequency response has actually changed very little.

Click here for some “after” pitch changes

Listen to how sudden the transitions are now. There is no more loitering between timbres.

I’m currently doing an updated circuit board layout with all the minor changes I made to the prototype. Hopefully in a matter of weeks I will have the first few completed modules to use for demos and beta testing.

Things have grown a little convoluted because of all the projects I’ve mentioned up until now. I’d like to narrow it down to just a few until I can make enough money from selling finished devices to fund further designs. Prototyping is not cheap, so I need to be more realistic.

Pokey synth(s)

The Pokey module for use in Frac and Euro modular synthesizers will be the first thing I release. Right now I have the first prototype up and running, but there are still some circuit changes to make. The good news is that these are all minor. The overall design is working how I envisioned it, including the speed-sensitive tuning controls.

Along with that, I am checking out local manufacturing houses for stuffing the circuit boards. I am trying to have this done locally. There is also the metal work, which got a little more complicated with the addition of the Euro form factor.

I’ve mentioned a Pokey MIDI synthesizer a few times. That, however, is being put on hold. It still needs a considerable amount of DSP written, and I still do not have MIDI working 100%.  It works about 99.99% of data transfers, but there is still an occasional glitch every thousand commands or so that needs to be fixed.

2A03 synth(s)

The 2A03 MIDI synth is on hold until I get a lot more time or some outside help. The 2A03 module (also in Euro and Frac) is the second module I plan to release. On the upside, it should take about 1/4th the development time of the Pokey module. On the downside, it’s been more than a year and a half since the Pokey design was started.

ODB

This is a module I’m making as a break from soundchip modules. Without giving too much away, it is based around concepts of high-fidelity 1-bit audio.

Since every post needs a picture, here’s one from the digital sketchbook:

Still needs a lot of work

Pokey Shot One

Panel Shot Two

Pictures on this blog will be somewhat blurry until the sun comes back or I buy some decent apartment lighting.

This panel actually came in weeks ago, but I was without anything to test it. It mates well with the front panel control PCB, so that was a sigh of relief. The laser etching is not very eye-catching, though. When light shines directly on it (first picture) it shows up well, but otherwise it comes through as more of a gray color. The contrast ratio on black is not there. Looking at my modular synth off center or with bad lighting, my white-on-black screen printed modules are much easier to see/read.

There comes a time in every person’s life where he/she realizes the short comings of laser etching and must shed their naivete. It is time to move into the messy and tumultuous world of screen printing.