Everyone plays guitar (Part 2)

It has been over a year, and what a year it has been. Fortunately I don’t have to go into any of that here.

The TV project has all but died. I still have the spare PCB kicking about, but I’ve not got the motivation to fix my BusPirate probes and try to add USB to my TV. The Rasperry Pi2 is out now, and I don’t think my TV could supply enough current regardless. So one rainy day I might look into this again, but for now – everyone plays guitar, right?

As this post is going to deal with the guitar pedal project I think we should go over some history.

The MP-01 design was released 6th July 2013 (wow, that was a long time ago). It used a single PIC32 with external SPI DAC and ADC. Due to quite a lack of foresight, the rotary encoders were not on change notification pins, and any rail noise was put straight onto the guitar signal. That was a major issue that resigned this version to the junk pile, and the less said about this the better.

MP-02 was to be the savour. The design for that was released almost 1 year ago to the day (tomorrow in fact). I learnt my lessons from the first design. The op-amp stage was re-architected to utilise the op-amps supply noise rejection capabilities. I’d been hit by scope creep quite severely by now. I had moved to a 2-PIC solution; a PIC24 to handle the non audio functionality (which went from just LEDs to include an external port for an expression pedal and USB to allow direct loading of patches from a PC), and a dsPIC33 to handle the real-time audio activities (using it’s in-built ADC and DAC along with the rotary encoders, footpedal switch and EEPROM). After building the pedal I had real difficulties getting the thing to work as I expected it to. For some reason I just couldn’t sample quickly enough and there was still far too much noise. I finally fixed the sampling issue by changing how I was clocking everything, and I was having a pedal that was actually passing the guitar signal through, albeit with a lot of white noise on top. This noise was causing me some heartache as I just couldn’t pin down what was causing it; poor layout, inherent ADC and DAC noise issues, op-amp selection, dodgy power supply. I was fighting against so much that I decided that enough was enough. Plus I blew up the dsPIC twice as apparently the metal enclosure is made of metal.

MP-02 v1.1 was to be the savour! I refused to play any more games. The dsPIC range do have inbuilt ADCs and DACs but they just can’t compete against a proper chip. So I enlisted the help of something made for audio; the Wolfson WM8731. This blows the 10 and 12 bit capabilities of the dsPIC out of the water with a whopping 24bits at 192kHz. It supports pass through of the audio without requiring it to even enter the dsPIC. We have volume control, anti-thump mute, and filters coming out of our ears. This is definitely going to go some way to helping the sound. The LMV321 op-amps are out. I looked over the specs and the limited slew rate is fighting against us. I am looking at a couple of different op-amps to use instead, and I am favouring the LMV793. A slew rate of 24V/us (instead of 1V/us), and a noise density of 6.2nV/rt(Hz) (instead of 46 nV/rt(Hz)) means that this pedal will be singing. I’ve added some external SPI SRAM into the mix too. At 512Kb we have enough storage for over 5 seconds of audio (16bit mono 48kHz). This doesn’t sound a lot but it is more than enough for a decent echo. Plus unlike EEPROM, SRAM can be written to again and again and again… I’ve still got the EEPROM though, boosted to 512Kb too. This allows me to bootload both the PIC24 and dsPI33 from SPI or UART without having to support the entire USB stack in the bootloader. Scope creep got me again, and now we support USB OTG. This means that whilst you can plug the pedal into a PC to download new patches, you can download from your phone instead or use that as an expression pedal, or simply plug in a USB memory stick to load patches directly. This just opens up the possibilities. Finally I’ve added filtering and protection. I probably could’ve added extra but it will hopefully that will be enough.

These changes complicated the design no end, but I still managed to get everything on two layers (this may be the problem, we shall have to see), within my 50x100mm outline, and mostly single sided population. I want this pedal to be silent, as well as whisper, sing, and scream. I’ve done all I can to help with this task from additional components to filter the noise to segregation to help keep any noise where it can do no harm. I’ve even gone to a dedicated audio chip. This marks the beginning of the end of my attempt to design a guitar pedal. All will be revealed in a week or two when the new design gets posted through the door.

I have been fairly lax in updating this website, but hopefully the next post will bring good news.

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