[sdiy] learning from early drum machines?

[Scott Nordlund]

As others have mentioned, early preset drum machines used a counter with a bank of diodes (later ROM chips) to play the patterns.
 
You could make a mod-16 counter (or mod-32 for more complex patterns) and feed the 4 or 5 counter bits into the lowest address bits of an EPROM.  Connect the other address bits to switches to select the pattern, and put a narrow pulse into the output enable pin so that the proper trigger pulses come out.  The 8 outputs (if the EPROM is 8 bits wide) will trigger your sounds.  You can also make the counter reset at different points, enabling different time signatures (this can be programmed as well).  The downside is that all of the patterns have to be programmed first (on a computer) and burned to the EPROM, and they can't be changed without reprogramming the EPROM.
 
You could replace the EPROM with a bank of switches and diodes, and put logic gates on the outputs to shape the trigger pulses.  This is certainly the easiest way, but it takes a large number of switches (depending on how many steps/sounds you want).  There's also no way to store patterns or rapidly change them.
 
There are only 2 programmable drum machines I know of that don't use CPUs, the Boss DR-55 and the Paia Programmable Drums.  Both of these can store drum patterns, but they seem to be clumsy to program compared to more modern machines (though I haven't used either).  Both of these are obviously very simple, but could be expanded relatively easily to use much larger RAM chips.
 
The DR-55 is programmed by selecting a sound and then entering either a note or a rest for each step.  The note/rest buttons increment the counter by one step while storing a 1 or 0 in an SRAM chip.  The trick here is that the SRAM chip outputs are fed back into the inputs via a large resistor while the write signal is briefly pulsed to enter the new data- the RAM data lines for the other sounds hold the old values (due to high impedance CMOS inputs) during the write cycle so that one sound can be stored while the others are retained.  It's hard to say how well this would work with other RAM chips, but a latch (or something) could be used to get the same result (or you could use an array of 1-bit wide RAM chips).
 
I'm not really familiar with the Paia Programmable Drums thing, but it seems to rely on a similar trick to write one bit at a time, while having a cruder (?) programming interface.
 
Of course, none of these allow the patterns to be arranged into a song, MIDI in/out, etc.  All the really interesting territory opens up only when you start with a microcontroller.


> From: subjectivity at hotmail.com
> To: synth-diy at dropmix.xs4all.nl
> Date: Sun, 11 Jan 2009 01:04:21 +0000
> Subject: [sdiy] learning from early drum machines?
> 
> 
> 
> OK
> 
> i have a goal and it might be crazy.
> to make something that generates a beat (with multiple sounds) a primitive drum machine.
> 
> now my plan was to check out the early (roland, korg,etc) drum machines from the 70's (maybe before they started using CPU's to make the beat) so i could figure out if the pattern making circuitry is even possible without microprocessers or pics.
> 
> i am scared of code so this might be an exercise in stupidity.
> 
> i KNOW i can make the circuitry for the drum SOUNDS.
> i know that they can be controlled by a trig/gate.
> 
> what i dont know how to do is program gates with buttons, that trigger 4 or 5 different sounds at different times (without using 4 or 5 giant 16 step seqs)
> 
> ideally an 808 style seq would be great but i KNOW that is way way too advanced. memory would be nice but i have never used memory before
> 
> anyway..
> 
> any tips on where to find old drum machine schematics?
> or even, which models to look into?
> 
> thanks guys
> 
> 
> right the sky is the limit till i figure out it's too hard) ]]
> 
> 
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc 
> 
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle 
> 
> 
> 
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