Mono to Stereo: Digital Implementation

This is a quick follow-up to the construction of the mono to stereo converter. It seemed like a good idea to have the same filter available as a plugin for a DAW, in this case Ardour. It turns out that it isn’t too hard to create a LADSPA plugin and after a bit of head-scratching we worked out the configuration of an infinite impulse response filter that copied the response of the physical filter. Here’s the signal flow diagram, arrived at using the approximate derivative technique:

mono2stereo

Here the factors a and b are given by the formulae:

mono2stereo-formulae

where F is the sampling frequency and R and C are the values of the resistor and capacitor in the phase-shifting network (in our case having a time constant of 0.159ms). In the physical version there are two cascaded phase-shifters and equally the above digital filter has to be applied twice to one channel (the other channel is left unmodified). Because it is implemented as a simple IIR filter the digital version is extremely efficient and works very well – listen out for both the analogue and the digital versions on the album! You can find the source code here.

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