SP1200^® : Techncial Overview

Architecture The SP1200 is based very closely on the SP-12 and Drumulator, and the technical architecture is very similar, being based on a custom designed micro-controller, a Z80 processor and multiplexed audio. It was also designed to be fully compatible with the SP-12 samples, which can be read in via the cassette interface.

The 8-bit Z80A processor runs at 3.3MHz, it reads the controls, writes to the display, and communicates to the outside world via an Serial I/O chip. It also runs the sequencing software. A CTC (counter timer circuit) is used for generating timing interrupts for scanning the controls every 1.2 ms and keeping the sequencer accurate and sync'd to MIDI or SMPTE.

The micro-controller is the E-mu magic which enables the sample memory to be read for each drum sample. The hardware design was all done by Dave Rossum, and this was the last solo project by Dave.

Sample Memory The SP1200 uses dynamic RAM memory for storing samples. The sample memory consists of twelve 32 Kbytes memory chips (total memory size of 384 Kbytes). The sample memory is actually 16-bits wide, and therefore has some de-muxing to fit 12-bit samples across all the chips. With a sample rate of 26,040Hz the SP1200 has 10.07 seconds of sample time, split over four separate sample banks. 

Digital Sampling A 12-bit linear Digital Analog Converter (DAC) chip replays the samples, and is also used for sampling sounds via successive approximation. This saves the cost of an ADC chip. The sample input goes via an anti-aliasing filter to remove unwanted frequencies that are above half the sample frequency, the cutoff is brick walled at 42dB.

Operating System The SP1200 has diskette based OS, which is stored on all sample diskettes when they are formatted. The on board Boot EPROM instructs the micro-processor to load the diskette based operating systm into program memory, which is a mere 64 Kbytes in size and is used as the sequencer memory as well. Sample and sequencer memory are volatile and must be backed up to diskette before you power down the SP1200.

The latest versions are:

       OS diskette 1.2
       Boot ROM Version B

Micro controller This is made from discrete logic, although in the future this circuitry was encapsulated into one custom chip by E-mu Systems for the samplers made after the mid-1980's. The micro controller gets the sample out of memory, to the DAC and to the subsequent sample and hold circuits which are needed as the output channel is multiplexed.

Eight Voices There are a total of eight voice channels, through which the drum samples are played.

Analog Filters Yes, the SP1200 has analog filters ! Channels 0 and 1 each use a dynamic VCF. They use the SSM2044 with a simple AR envelope generated by the Z80. They create a 5ms sloping attack, followed by a decay. The frequencies can be internally adjusted.

Channel 6 and 7 are unfiltered. The other four channels are each filtered with a static five pole 1dB Chebyshev filter, set at frequencies to optimise the sounds.The filters can be heard on the MIX output all the time, and on the individual outputs if the jack is half inserted.

Internal Mixer The eight voice channels are internally mixed into one, along with the click channel. A TL084 op amp is used.

Diskette Drive The SP1200 has an integral 3.5" DS/DD floppy drive. It enables sampled sounds and sequences to be stored to diskette.
 

Yes, the SP1200 has analog filters !

Two of the channels the ride/crash and hit hats are unfiltered.

Four others (snare/rim, bass, claps. cowbell) are each filtered with a static five pole 1dB Chebyshev filter, set at frequencies to optimise the sounds.

The remaining two channels (the toms) each use a dynamic VCF. They use the SSM2044 with a simple AR enevelope generated by the Z80. They create a 5ms sloping attack, followed by a decay. The frequencies can be internally adjusted.
 

The SP1200 uses DS/HD 3.5" diskettes.

Unavailable from us
* OS 1.2 Diskette

Available from us
* Boot ROM B
* Slider knobs
* Old style Fuseholders

* Schematics
* Service Manual
* Documentation CD
 
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The debug program is now part of the operating system rather than being a separate EPROM, as on the SP-12.

To activate the diagnostic software, power the instrument up while holding down the SETUP button, or press SETUP before a disk is loaded.

The SP1200 will cycle through a total of 12 tests, you can turn the SP1200 off at any point.

* CPU RAM test
* ROM test
* Sample RAM test
* Buttons/footswitch
* LCD
* LED's
* Sliders
* Sample Trim
* MIDI
* SMPTE
* Dynamic buttons
* Floppy Disk

There are 7 trimmers on the main circuit board. Four are for the DAC's, one is for the sample offset, and the last two trim are dynamic filters.