Tag Archives: guitar pedal

Everyone plays guitar, but more people watch TV

Guitar pedals are harder than they look – at least this one has been. I’ve learnt some valuable lessons that I’ve never had to worry too much about before. The most valuable lesson is that noise is not fun. Living in the digital domain is brilliant because is it far less affected by noise. Analogue is the complete opposite.

If you remember from last time, I had to bias my guitar input at Vcc/2, so that I could feed it into my first op-amp. This meant that any noise present on the voltage rail was put straight on my input, and amplified – not a good start. Thankfully, all is not lost as I just need to convert the input stage to an inverting op-amp with the reference set on the non-inverting input. It should just be a case of moving a few components around as I has enough forsite to scatter passives around the op-amp.

In the meantime I want to talk about my TV. It is a low-end 32inch LCD Hitachi with IO that fits the price tag – no USB, only 2 HDMI, Component, Composite, and VGA. It’s pretty basic, but it fulfils MOST requirements of it, and was a bargain when we bought it. However, over the last two years my needs have changed, but I cant justify buying a new TV. I’m sure modern TV’s are extremely complicated, but like all good hackers/engineers, I needed to know just how complicated. More than a few years ago I would have just taken the back off and gone oscilloscope crazy, but I’ve learnt my lesson by poking around inside said oscilloscope so instead started with the service manual.

In fact I started by searching for spares for my TV (Hitachi 32LD30UA). That gave me a part number of my main control board – the Vestel 17MB35. I can only assume it is a popular control board, purely based on the amount of information I can find about it. Amazingly I found a service manual on the ArgosSpares website. I don’t know why I say that’s amazing, as Argos is where I bought the TV, but I didn’t expect to find a service manual there. Anyway, hunting through the manual I found out that my control board should have footprints for 4 (not just 2) HDMI connectors, 2 USB connectors, and even a “Ye Olde” DSub9 connector. Thinks were looking up, as I needed a USB port just to power my Raspberry Pi. I have been tempted to open the back of the TV and piggy-back off of a voltage regular, but not all I need to do it fit a USB socket instead – a nice and clean modification.

At this point, I still haven’t actually opened up the TV, but I have ordered and received a spare control board. I figured when I eventually open the TV I can copy the contents of my TV’s EEPROM into this board, and see what happens when I switch them over.

My aim is to have the Raspberry Pi powered from a USB port on my TV, have a third HDMI channel, and enable HDMI-CEC. I am pretty confident in the USB power, but who knows about the rest – wish me luck.

Everyone plays guitar…

It has dawned on me that whilst I don’t pay a lot a month for hosting, I do actually pay something and so not keeping this blog up-to-date could be construed as a waste of money.

With that in mind I have an update – hooray.

I built my first guitar more than 10 years ago, whilst still at secondary school. I had only been playing for a few years, and I wanted something that would stand out both visually and aurally.

I started with a strat body, and a random neck I bought off eBay. I cut the scratch plate out of some clear acrylic, and mounted combination of a P-90, humbucker and single coil pickups. These were all wired together through a vast array of switches to enable me to switch any combination of pickups in both parallel or series, and either in or out of phase. Looking back it was a hideous rats nest of wiring, but that was nothing compared to the paint job.

I sat in the garage with my sister, who was 5 at the time, and we painted it with all the paints I could find. It was gloss, matte, and metallic, with a crackled finish in areas.

Since then I’ve bought 2 more guitars, and modified the wiring each time, but now I am more selective over the components. I try to select parts that match the current hardware, or add functionality without adding to the part count. All of this is leading to the real point of this post. I am designing a guitar pedal.

I have been meaning to design a pedal for a number of years, and I had no real reason to not do it. Every time I would sit down and work out what it needed, but I would always be put off by one aspect or another. There are hundreds of questions that stop me from progressing. What do I want the pedal to do? How do I control the modification to the sound? Is a 12bit ADC and DAC going to be enough? Do I have an analogue front end to mix the effect in, or deal with it in digital? Do I want a “true bypass” pedal, or is all the signal going into the processor at all times? Do I stick with what I know and use Microchip’s PIC32, go for a multi-core Parallax Propeller, or try my hand at one of Freescale’s DSP chips? Instead of letting the questions buzz around, I took the plunge and started the design of my pedal.

The signal is first passed through a capacitor, effectively removing the DC component. The signal is the biased at Vcc/2, and pass through an op-amp – TI’s LMV321. This will apply some gain to the signal, before feeding into the ADC – Microchip’s MCP3202; a 12bit ADC. It’s not the best ADC on the market, but it’s pretty cheap and will do the job for now. The digital data is then read in by Microchip’s PIC32MX764F128H. With a core frequency of 80MHz, this should be more than enough to perform some basic effects. Following manipulation, the data is then sent to the DAC – Microchip’s MCP4822; a 12bit DAC. Again, it’s not the best DAC around, but I am not aiming for that yet. Finally, I used a unity gain buffer to match any impedances, remove any DC component, and allow the next device in the chain to handle the signal. The parts selected were spares left over from previous projects. The only new purchases were the 6.35mm jacks and metal 1590A enclosure. It should be obvious that this is not going to be the best pedal in the world. I’m sure that any analogue aficionado will berate me for my choice of op-amp, and any audiophile will say that the minimum number of bits to consider would be 24. But they would be missing the point.

This pedal is my start line. It will allow me to see the weak points in the design. I should have mapped the control rotary encoders to an “Interrupt-On-Change” pin as the are a bit slow to respond, or sometimes appear to run in reverse. I should have use a codec IC with build-in 24bit ADC/DAC. I should have used analogue switches to bypass the circuit when disabled. By this time next week, I will have some answers.

After antagonising over this pedal for years, I have finally started. I spent around 2 days drawing a schematic and laying out a PCB. There’s some code still to be done, but a lot of it was written in the 2 weeks waiting for PCB’s to arrive. I’ve got PCBs from iTeadStudio for $27.59 (under £20), and all my components for around £25. For under £50, I have designed a programmable multi-fx guitar pedal. And I have no doubt that I’ll be doing the same next week.