The Logidy EPSi is a pedal that was introduced around 2014, and I consider it a groundbreaking pedal because it is the first affordable pedal that can do real-time convolution to process Impulse Responses (IRs). Typical units that can achieve such processing are found in racks or as plugins in your DAW. This allows standalone processing. Now, what does this all mean?
IRs are sort of like a fingerprint of a linear audio signal chain. As an example, let’s take a guitar cabinet. You apply an impulse signal (a short pulse) to a guitar speaker cabinet and capture the sound (Impulse Response). In mathematical terms, this impulse response convolved with an input signal (say your guitar signal) will then produce a signal that is exactly what would be produced if your guitar signal were sent through the guitar speaker/cab. Most guitar cabs only need about 20-30ms to capture the essence of their sound signature. IRs longer than this capture early reflections and sound characteristics of the environment. You can use this technique to also capture reverb signatures of spaces.
Using this technique, you can then produce IRs of various guitar cabinets and speakers, as well as the sonic characteristics of the audio chain used to capture the sound (i.e microphones, mic-pres, etc.). It can also be used to capture sonic signatures of sound spaces (like concert halls, etc.). Many people have already produced IRs, and are available free or commercially.
Convolution involves a lot of DSP processing, and was usually reserved for powerful desktop computers or dedicated rack units. But DSP processing has become more powerful and cheaper, and hence is now available to the masses via the Logidy EPSi at a price of $200 USD.
In general, IRs are WAVE files. These can be loaded into the EPSi via an SD card. Though the interface is primitive, it’s pretty logical how to name the files so that you can access them on the pedal. You can also use stereo WAVE files so that the left/right IRs can be applied to the stereo input.
There are actually two different firmwares that can be used with the pedal. And they reside on the SD card, so using different SD cards on power up can change the function of the pedal completely. Here’s how it is described by Logidy:
Version 1.00 is optimized for long time windows turning the EPSi into a high quality reverb pedal. The SD Card lists 114 sounds from acoustic spaces to spring and plate reverbs to various special FX sounds.
Version C.00 is optimized for low latency dry path convolution which allows the processing of Speaker Cabinet + Room type impulse responses. Together with an onboard guitar amp style EQ, the EPSi alone can simulate stunning clean channel sounds from a mic’ed guitar amp.
Now — why would you want to use this pedal? It’s obvious from the FW that for 1.00, the pedal can be used as a high quality reverb pedal, using IRs included on the SD card. Or others which can be found free on the Internet, simulating a wide variety of sonic spaces.
For Version C.00 — it can be used any time you need an accurate simulation of a guitar speaker and cabinet. You can take a signal chain that consists of distortion pedals or amp simulations and use that as the input to the EPSi to get a great guitar sound as if it is going through a guitar speaker cabinet. This signal can be recorded direct or amplified using a PA FRFR (full range full frequency) speaker system. No microphones or guitar amps needed.
Many amp simulator pedals have reasonable amp models but their speaker modelling is poor. The cheaper units don’t have the DSP power to use convolution and load IRs, hence their sound is not as good. And — it cannot be overstated — the speaker/cab simulation is a very important aspect to the sound of a guitar amp. Enter the EPSi. You can take older amp sim units and turn off their speaker cab sim — or use guitar overdrive/distortion pedals — and use the EPSi to do the guitar speaker sim. And take the sound from mediocre to spectacular. I’ve used this technique on a variety of guitar pedals or a Zoom G3 into the EPSi, then direct into some FRFR speakers, and the results are impressive.
The EPSi also has EQ controls on it as well, enabling more sound sculpting post IR processing.
Hats off to Logidy for making convolution affordable and available to the masses.
Here is a demo of using the EPSi for reverb:
Demo for guitar speaker/cab simulation. I like the use of the Sweet Baby OD from Joyo as the distortion generator — one of the sweetest sounding chimey/grind pedals available at an amazing price (about $40 USD).
Another video demo of the sim vs the real thing, simulating spring reverb. It’s almost impossible to tell the real thing from the EPSi: