BarrVerb - a DPF Reverb plugin

BarrVerb is a reverb plugin based on the legendary Alesis MIDIVerb from the mid-1980s. It implements an emulation of Keith Barr's custom DSP, originally implemented in LS-family logic ICs. It could also run the MIDIFex DSP code, if a suitable ROM image could be found and "deinterleaved" to suit BarrVerb's decoder.

I have to acknowledge the work of /u/thement_ on Reddit, who nerd-sniped me into doing this in the first place, Eric Brombaugh on the synth-diy mailing list who reverse-engineered the MIDIVerb, and Paul Schreiber who made a a couple of videos of Eric's reverse-engineering of it from which I was able to get the circuit diagram and a bit of explanation of the opcodes.

Some liberties were taken with the arithmetic in the unit in regards to how it handles twos'-complement arithmetic. Eric Brombaugh explained that it appears to have an off-by-one error that gets corrected over two instructions but to simplify the process loop I just use normal arithmetic. This can cause an error of up to +/- 2 DAC values in the output, which in practice is audibly indistinguishable from doing it "100% accurately". On test it can be measured by compiling the plugin with "accurate" and with "simple" maths, subtracting the output of one from the other, and boosting the gain by around 50dB or so - you're never hearing that difference in practice.

A brief technical guide

The Alesis MIDIVerb has a simple DSP made out of discrete logic which can carry out four operations. Unlike a general-purpose DSP it can only multiply by 0.5, which is implemented by shifting one input to the adder right one place and feeding the leftmost bit to both bit 15 and bit 14 to extend the sign. Bit 15 is also fed to the carry input of the adder chain, for the twos'-complement add.

The "program counter" is an 8-bit counter clocked at 6MHz, which feeds the lower 8 bits of the DSP EPROM address. This is latched into the DSP low byte first. Each instruction consists of a two-bit opcode and 14-bit offset, which is added to the memory pointer on each step to address 16-bit words within the 16kB DSP RAM. The instruction steps are latched in stages so that for a given counter position, the offset comes from the instruction before, and the opcode comes from the opcode before that, presumably to allow time for the latches to settle. In this implementation, the ROM has been pre-"prefetched" so that the value in rom.h at word 0 of any program actually contains the opcode from word 126 and the offset from word 127 and so on, which simplifies the DSP loop.

There are no branches possible so unused code must be filled with a "dummy" write to RAM. Three of the addresses are "magic" - at step 0 the ADC is loaded into RAM at the address in the pointer register, at step 96 (60 hex) the right channel output DAC is loaded with the "adder input" bus, and at step 112 (70 hex) the left channel output DAC is loaded. For each of these "magic" addresses the ALU works as normal but the accumulator register is not loaded with the output of the adder. In general the DSP code used seems to run an instruction to load the adder input bus with the contents of RAM, pointing to a "temporary" address where the effect outputs are stored.

Known limitations

This is the first thing I've written from scratch using DPF, and as such may not actually be very good.

This plugin will work at any sample rate but will only produce approximately correct results at 48kHz because it does not downsample and upsample very well. The input filter does not have the right response and there is no reconstruction filter on the output, which you are unlikely to notice in use.

The DSP engine runs at 24kHz (or really, half the sample rate) rather than the correct 6MHz/256 = 23.4375kHz, which you are unlikely to notice in use.

Building BarrVerb

1. clone the repository

2. git submodule update --init to pull in Distrho Plugin Framework

3. make

You should now have a ./bin/ directory with BarrVerb as a standalone Jack client, BarrVerb.lv2 as an LV2 plugin, BarrVerb.vst3 as a VST3 plugin, and BarrVerb-vst.so as a VST2 plugin. These have been tested on Linux using Carla 2.4.2, but very little else. Further testing and patches would be welcome.

This software is provided under the ISC licence as documented in the file LICENCE which is fairly permissive. The file rom.h contains a permuted version of the MIDIVerb ROM which has already been shared and distributed widely, but must be considered to be copyrighted by the late Keith Barr.