The Magic of Digital Effects Pedals

Those of us interested in guitar effects pedals are spoiled for choice in the current age. There are hundreds, possibly even thousands of small and large effects pedal makers producing all manner of different effects with a wide range of features and price points. However, one thing is notable by its absence, or rarity anyway. How many boutique digital effects pedal makers can you name?

In this blog, we’ll look at some history and reasons why this is still a specialist business, cover some basic building blocks, and provide some pointers as to how you might go about getting started with your own digital effects.

Firstly, to answer my own question, I came up with five manufacturers I consider known for their digital effects: Line 6, Eventide, Strymon, Boss, and Digitech.

Line 6. Parent company Yamaha Corporation. Annual revenue over $4 Billion.
Digitech. Parent company, Harman International Industries. Annual revenue over $7 Billion. Boss. Parent company, Roland Corporation. Management buyout in 2015 and no longer publishes revenue figures, but in their last year as a public company in 2104 posted sales of over $850 Million. Seeing a pattern here?

Eventide is a private Corporation and there are no public financials, but they have a history of innovative digital products in audio, broadcasting, and aviation going back to the early 1970’s. That leaves Strymon as the only relatively recent arrival from my list. Certainly there are others; for example, I saw Robert Keeley has some very interesting looking new digital delay and reverb units, but with new boutique builders appearing on the scene almost every week, new digital effect pedal manufacturers are rare.

The obvious explanation is that digital products are just much harder to design and build, and that’s true, but why? And is there still a path for the start-up builder to create their own digital effects pedals?

It’s In the Software

To go to the trouble of flipping an analog signal like an electric guitar pickup into the digital domain and then back again to analog to be heard requires a good reason. The reason is that sometimes, doing what we want to do with the signal in analog is just going to result in something too large, expensive, noisy, and frequently all three. Analog components are not perfect; they have tolerances and add noise. They are quite large and get costly. As you add more and more of them, these factors soon get out of hand. By comparison, I can pick up digital devices with millions of transistors smaller than a fingernail for reasonable cost, and every one of them will do the exact same calculations every time. There are trade-offs. First, is that to use digital devices, I need software.

The Sauce is in the Source

Many people come to working with music electronics from the role of tinkerer. Maybe they swapped out some op-amps for lower noise variants in a boost, or heard that changing some resistor values would increase the gain of a fuzz. Many aficionados love looking at ‘gut shots’, and pointing out the orange drop caps or NOS transistors and how those might impact the tone. It’s often a great way to get started.

Looking at a photo of a microchip is not going to help you much. The magic is in the code. This is what makes the pedal a chorus or a delay, a reverb or an amp. Often the hardware is similar or even identical between models of digital pedal, it’s just the software that’s different. In most cases, the code is hidden behind the digital equivalent of goop. Most manufacturers are not going to share the software ‘source code’. This is their secret sauce into which they put much of their investment, and what makes it their sound: It’s their intellectual property.

Even if they did share it, in most cases you’d have no clue what you were looking at, or what to do with it unless you already had expertise in that area. We’ll talk about DSP here in a minute, but DSP code is usually highly specialized and requires a fair bit of education and experience. Not to mention you’ll need the tools to program and debug it. Again we’ll cover this in more detail later, but just for some click bait; a copy of Visual DSP++ and an in circuit emulator for the Analog devices Sharc DSP costs about seven thousand dollars. Add some development boards and other tools and we are already in for around 10K, and we haven’t even started. That’s assuming we already have tools like high bandwidth scopes, signal generators etc available. If not, then you can at least double that. We are beginning to see why digital effects are not for the faint of heart, nor small of wallet.

So remember that this is the first key difference between digital and analog effects; most of the unique audio function of a digital device comes from the software, the hardware is mainly just there to support it.

TLA’s

Since we are basically dealing with computers here, we need some computer terminology. Computer folk love Three Letter Acronyms. Here we go.

ADC – Analog to Digital Converter
Converts the analog signal from the input to digital so it can be processed by the DSP.

DAC – Digital to Analog Converter
Converts the processed digital signal back to analog for sending to the output.

DSP – Digital Signal Processor
A specialized microprocessor optimized for signal processing. Under software control, this manipulates the digitized signal from the ADC to create the effect.

PCM – Pulse Code Modulation
The most common digital encoding method for audio.

CP – Control Processor
A general purpose processor used for control functions such as handling the buttons, knobs, and display.

SoC – System on a Chip
A specialized integrated circuit that includes multiple components such as an ADC, DAC, CP and DSP on one physical device.

I2C – Inter Integrated Circuit
A standard mechanism for chips to communicate with each other

SPI – Serial Peripheral Interface
Another standard mechanism for chips to communicate with each other

Although it’s possible to use a general purpose processor to do audio processing, this job is normally performed by a DSP which is a specialty type of processor designed for real time processing of signals. DSP’s are generally more complicated to program, but are much more cost and power efficient for this specific task. Sure, I can do audio processing on the Intel Core i7 CPU on my desktop. This is what Windows and Mac based systems such as Protools and Logic do after all, but it’s not really practical to run a huge power supply, large heatsinks, fans and a $300 processor in an effects pedal.

So to build our digital pedal we are going to need an ADC to convert the analog to digital, DSP(s) to do the processing, a DAC to change the signal back to analog. We’ll also likely need some sort of control processor for booting the system and handling things such as control knob inputs, digital display and LED’s, and maybe software updates and interfaces such as MIDI and USB. We’ll also need some flash memory to store the code and save our presets.

Think of components of this type as scientific geniuses, or world champion athletes. They are high functioning, but require a strong support group to remind them to eat, and to stop them from driving their cars into a swimming pool. Without the right structure around them, they’ll be waking up in a Vegas Hotel with a bizarre tattoo and all their credit cards missing. They need high speed clocks and often advanced PCB routing techniques, such as impedance matched traces and differential pairs, otherwise they just won’t run. Then there are various DC to DC converters as different parts run at different voltages, and voltage regulators as they are very picky about stability. Getting all these parts into a tiny effects pedal form factor probably means you are going to end up with a multi-layer board, and, of course, many of these parts are small pin pitch surface mount designed to be machine assembled, so the boards have to be sent out to a contract manufacturer for assembly.

The bar is set pretty high for digital pedals, and producing something like a Strymon Timeline or Eventide H9 requires a tremendous amount of work in many different disciplines. It takes a team to design, build and support something competitive for the commercial market. Add to this the fact that because of the high speed clocks, the device may need regulatory approval, it’s becoming clear why we don’t see many one-man band digital effects pedals.

So does this mean there’s no chance for small scale digital effects? Not necessarily. Recall our acronym SoC? The system on a chip puts many of the components needed for a specific application on a single chip. The choice is limited, but a lot of the work is already done. The Spin semiconductor FV-1 is an example of this. It’s getting a little old now, but check out the block diagram and you can see that most of the key parts we talked about are there. And because they are altogether on the same chip, we don’t have to worry about different voltages and chip to chip interconnects. You can order a ready to go development board that you can start testing with right away, and they provide a bunch of free algorithms to get going with. You could conceivably build a commercial product from this.

Building Your Own

If you are more interested in experimenting at home, there are a few projects on the web for the Arduino. Arduino is an open source prototyping platform based around the Atmel AVR (and some other) microcontrollers. There’s no real DSP on the basic Arduino, but as we mentioned before, there is no reason you cannot use other systems for audio processing. The fact that it’s not optimized for price performance on signal processing doesn’t really matter to us for a home project. Many Arduino base boards can be expanded with plugin daughter boards known as ‘shields’. There are a few projects on the web that provide documentation and kits for Arduino based guitar effects. The Electrosmash pedalShield look especially fun, and their circuit analysis of classic effects pedals are very well done.

For a while Line 6 had the ToneCore product that provided a pedal chassis into which to plug different modules. A development module running on the Freescale Symphony DSP was available on which you could develop your own DSP code. The concept was that you could purchase the pedal bases and modules from Line 6, load your effects code on the modules, and then resell them. I don’t think you can buy these any more but, it sure was a fun idea.

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