In late '97, I finally completed my home-built copy of the sound generating circuits from Rolands TR808 and TR909 drum machines.
They are now built into the unit pictured above.
The front panel of it's 4 unit high, 19" case houses all the controls of the 909 sounds along the top, the controls for the 808 sounds below, and the individual outputs from all the sounds at the bottom, plus a power switch and master volume for the main outs. The rear panel has a mains power input, midi/parallel data input, main audio outputs and a multi-way cv/gate output. Internally, as well as the 808 and 909 sound circuits, there is a 6502 based midi to cv convertor. This provides 24 accent and trigger lines for the drums (with one acc/trig spare), and an additional 16 cv and 8 gate outputs to drive my analogue synths via an external breakout box. It took a big part of my spare time over 2 years to build, but remains the centre of my music making setup - so it was well worthwhile in the end.
I use a Roland TR-626 to sequence the 808/909 sounds. There's some extra info on this here.
Crash, Ride and Hi-Hats
One of the biggest problems I had was in getting hold of the original sample data for the 909's cymbals and hi-hat. To cut a long story short - these sounds are generated by a counter that outputs addresses to ROM chips which store each sample. The data from the ROMs are fed to a simple digital to analogue convertor (DAC), then thru a voltage controlled amplifier (VCA) to shape the envelope of the sound, then a low pass filter to remove the sample frequency.
After a long wait, I finally got hold of a real 909 and permission to open it up and solder wires onto the PCB to extract the sample data.
Each of the three ROMs are 32k in length. There is one each for the crash and ride, and one shared by the open and closed hi-hats, which has a bit of extra logic to select the address range to be played back (the hi-hat can't be open and closed at the same time).
The samples have 6 bit resolution, but this isn't so bad, as they are heavily compressed to make maximum use of the bits, and the original envelope is then restored by the vcas.
The envelopes for the hats are generated by simple decay only circuits. The cymbal envelopes are generated by extra DACs on the address lines to the ROMs,which are converted to an exponential decay with the same length as the sample regardless of tune setting.
I have created three wav files for the hi-hats, crash and ride using the actual data from the roms. Each sample is shifted by two bits to occupy eight bits(unsigned), so the lowest 2 bits are not used. I chose an arbitrary sample rate of 32kHz as this is pretty close to the rate used for the hats, and the cymbals are tunable over a wide range.
More technical details on using this data to build your own 909 cymbals and hats are here.
The schematics for the 909 are still available from Roland.
You'll also find them online here.
This site does not quite include all the schematics: Hi Mid and Lo Toms Common Noise and Tom Noise Generator
Note that the interconnections between the diodes in the Tom Noise generator, and the tom circuits themselves are indicated by triangles, squares, and outlined squares on the schematics.
I have made some modifications to the original 909 circuitry - details are here.
A 2.2M zip file of samples of my drum module is here.
Last update: 14th June 2002
