Inside my DMX

Inside my DMX

There’s still some dust in there that the camera’s flash brings out. The inside of this DMX looks very different than most, because of the lack of solder mask on the board, typical for very early models like mine. The MRAM upgrade is sitting underneath the Z80 (which has also been replaced, apparently as a precautionary measure against potentially unreliable CPU chips installed in these machines). Notice the missing battery and empty RAM chip sockets.

On the left you can see the row of voice cards, which contain all the drum samples in the machine. The DMX has a modular approach to playing back its sampled sounds. The computer simply sends out pulse triggers to these cards, which are self-contained devices capable of playing back 1 to 3 samples stored as 8-bit companded data on one or more EPROM’s, with analog filtering and enveloping in the signal path after the DAC. This is in contrast to something like a LinnDrum, where many of the voices are multiplexed together and can share the same circuitry and filtering. The cards are totally capable of working on their own with any decent trigger signal and power source.

This kind of card-based modular design seems to be inherited from Oberheim’s polyphonic synthesizers like the OB-Xa and OB-X, that featured 4- to 8-voice configurations via individual voice boards. It has its genesis in the company’s SEM-based systems, where the modules that comprised each voice of the synthesizer were complete monosynths, with the capability of working on their own. However, by the time Oberheim released the DX, a scaled-down drum machine featuring six voices instead of eight, this type of design was abandoned in favor of simply swapping out individual EPROM chips, similar to the machines produced by Linn Electronics.

Posted by The Swill Man on 2011-06-01 21:57:47