ac-digital-vc

What is still different

Against real units

The recordings of Virus Cs that exist are phrases on factory programs: Access's own 2002 web demos and videos. E73 PowerStrnJ's chords match within 3.5–5.3 dB per third octave with their notes guessed from the spectrum, and an A/B by ear against them passed. No recording has isolated notes on known settings.

After the DACs

The op-amp stage between the DACs and the jacks isn't modelled, nor the inputs' converter: the photos of the board don't show their parts.

The DACs' mute

After 0.17 s of digital silence the CS4390s mute, and the output is exact zeros. Whether the board enables that isn't known.

The sample rate

46,875 Hz, 12 MHz / 256: Gearmulator's figure and what the DACs' clocking gives, not measured on a unit.

A new instance

Its RAM banks A and B are copies of flash banks C and D, as a unit fresh from the factory might hold; what Access put there isn't known.

STORE TO FLASH

It writes the flash, kept with the session. The DSP program that holds the bank meanwhile is stood in for, from what the firmware asks of it, and erasing is instant where the chip takes about a second.

LEDs

12 of 69 are placed by their wiring order alone: LFO 2's and LFO 3's destinations, and BPM.

CPU

The DSP's code is compiled the first time it runs. Code no earlier program used costs up to 8 ms in one audio block: 3 blocks over 2.67 ms across all the factory programs, none the second time.

How it was made

1. 01

   The firmware

   OS 6.6 runs from reset: the 80C515's program on an emulated 80C515 (MAME's MCS-51 core with the C515's own peripherals from Siemens' manual), which loads the DSP's program over the host port into a DSP56362 on the dsp56300 recompiler, Gearmulator's DSP engine. The board around them was traced in the firmware: the key matrix and LED latches, the battery RAM and flash banks, the port pin that selects the host port and the one that lets the firmware write its own flash. Every key and menu page, and the LEDs, were then checked against the owner's manual.

2. 02

   Against Osirus

   Gearmulator's Osirus runs the same DSP engine but replaces the 80C515 with C++, so it checks the control side. The same MIDI goes into both, and what each sends the DSP is compared word by word: the DSP's program, the upload of every factory program, every kind of channel message, sysex parameter changes, single dumps and multi mode. In multi mode 15 of 16 parts match; on the 16th, Osirus loads the wrong program. The comparison also found a bug here, now fixed: part 1's edit buffer read back through the host port.

3. 03

   The DACs

   Photos of a Virus C's board show three Cirrus Logic CS4390s, one per stereo output. Their response is modelled from the datasheet: the filter fitted to its plots within 0.16 dB, flat to 21.3 kHz with 25 samples of delay, the noise floor 103 dB under full scale, and the mute.