4
1. Presentation
1.1.
What is the EPSi?
The Logidy EPSi Effect Processor (EPSi from now on) is a DSP powered effects
processor packed in a small pedal format. The interface and the inner workings
of the EPSi are not tied to a particular application. Actual parameter names only
show on the 3 character LED display and can be changed. And because the
firmware running on the pedal is easily replaced via an SD Card, the EPSi can be
turned into many different things.
As of now there are two versions of firmware available for the EPSi:
- Version 1.xx: This version is optimized for reverb sounds.
- Version C.xx: This version is optimized for cabinet speaker and room
sounds.
Both versions make use of convolution and the SD Card to load sounds.
1.2.
What is Convolution?
The behavior of a linear time invariant system (like an acoustic hall or a linear
EQ) is fully described by its impulse response. An impulse response can be
obtained by recording the sound coming out of such a system when an impulse –
a kind of short pop – is applied to it. Once this impulse response is acquired, the
behavior of that system can be recreated by convolving that impulse response
with an input signal in a process that is equivalent to filtering. To simulate
acoustic reverberation sounds the time window of that filter is as long as the
decay of the effect. This leads to high computational requirements for long decay
times. In the reverb version of the firmware we are able to reach windows of
about 6 seconds in duration, which is long enough to simulate an acceptable
variety of sounds found in nature or in devices trying to mimic it.
Version C of the firmware is optimized for zero latency convolution and dual
channel processing. This leads to a reduce time window of 1.5 second.
2. Plugging In
If you have used effect pedals in the past, you will find that incorporating the
EPSi works pretty much the same way as with any other pedal. The EPSi
features 4 audio connectors on the back - 2 stereo pairs, one for importing
