Well I’ve been keeping this under wraps but it appears that the time has come to let the cat out of the bag.  I have been working on a Propeller based guitar effects pedal called “the Howler” for a few months now.  The hardware is up and running, the core OS is done, the chassis design is complete, and I’m working on getting the first couple chassis samples made.

The design has the following features:

-          Propeller core

-          1MByte SRAM standard (room for 1.5MByte on the PCB)

-          44kHz 24-bit sampling (2 input channels, 2 output channels)

-          4 control knobs

-          16x2 LCD screen

-          2 stomp switches

-          2 stomp indicator LEDs

-          1 1/4 inch audio input

-          1 1/4 inch audio output

-          1 multipurpose 1/4 inch audio jack (can be used as either a second output (for stereo effects) or as an additional input (for modulation effects etc.))

-          NTSC Video out

-          Micro USB

-          9V DC Power in

-          Headphone out (mini phono)

-          RJ45 expansion port (for potentially adding things like external foot pedals)

-          Aluminum chassis

The pedal is designed for production, so I’ll be making it available as a completed unit.  All the source will of course be available so that people can create their own effects and hack away to their heart’s content.  I’m also developing a windows application which lets you do a lot of high level control (effect configuration etc) from the PC side.

I have a demo song but I sampled it at a very high bitrate (since it was an audio quality demo) so its too large to post here (6MB).  I’ll have to see if I can resample to a lower bitrate and post it.

Post Edited (epmoyer) : 11/22/2007 5:13:28 AM GMT

Wow, can't wait for the Demosong!
Give us some Rock 'n' Roll!

Happy Thanksgiving!  Hope you all had a good one!

mpark said...
Well, since Mark seems not to mind Eric's total threadjacking I have to ask Eric his impressions of the prop as a signal processor.
You didn't feel too hamstrung by the lack of hardware multiplies?

Post Edited (epmoyer) : 11/23/2007 5:23:24 AM GMT

OK, here’s the first demo recording I did.  The file’s only 80kbps, but that's the best I could do given the file size limitations on this forum.  The background guitar uses the Chorus effect.  The three main parts of the song have a second guitar with the tremolo, followed by the delay, followed by the distortion.  All tracks were recorded using the same guitar with the exactly the same knob settings so that the operation of the effects was not masked by other changes (other than the distortion, for which I cranked up my guitar’s built in compressor so that I would get more sustain).  I would normally have used some reverb but I haven’t written one yet and I didn’t want to add any effects that weren’t coming from the pedal.

Distortion normally uses some filtering to affect which frequency ranges get distorted.  This one doesn’t yet, so it’s a little rough.  I also didn't have these effects written to use the knobs for parameter alteration at the time, so I had to dial in the delay time by iteratively changing my constants and recompiling;  It’s not quite matched to the song tempo, but pretty close.  Chorus usually uses some sample interpolation to avoid the noise of sliding the delay line tap point back and forth at discrete sample steps.  The current version doesn't, but the noise is not very noticeable in this recording.  If I play harder it becomes more evident.

Thank you everyone for your compliments and encouragement!  I'm very excited to see this thing taking shape; and I've tried to make it as versatile a platform as possible.   I must admit that I was a bit surprised myself to see how good the sound quality was;  I had anticipated having to fight some digital noise issues but it ended up being very clean.   I went from a 2 layer design to a 4 layer design near the end of routing because I figured I'd be able to keep the noise under control much better with dedicated power and ground planes.  Looks like it payed off in the end.

The SRAM is accessed in parallel, 8 bits at a time.  There is a PLD on the board which manges the SRAM, LCD, and some other misc glue functions.  You load the SRAM address into the PLD, and then it will autoincrement the address as you read/write bytes.  For sequential bursts you pay the address setup hit once and then just pull or push data a byte at a time.

I have a dedicated device on the board to do a/d and d/a.  The device serializes the sample data, so I use the prop counters to gerate the serial data clocks and to retrieve/export the relevant sample data.  I figured that with a dedicated device I would both get cleaner audio and keep more cogs free for effects processing.

A tuner is in the cards.  Pitch detection will be done in software.

Post Edited (epmoyer) : 1/14/2008 8:54:04 AM GMT