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SP-12®: Technical Description
Architecture
Two years after the original Drumulator E-mu Systems released the SP-12. This new drum machine was based very closely on the Drumulator but added 12-bit samples, user sampling and sound storage to diskette. The technical architecture is very similar, being based on a custom design micro-controller, Z80 processor and multiplexed audio.
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 1.2mS 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 eight voice channels are internally mixed into one, along with the click channel. A TL084 is used.
Sample Memory
The SP-12 has a combination of EPROM and RAM memory for storing samples. The standard samples are stored in the 24kbytes of EPROM's and any user sampled sounds reside in the 48kbytes of RAM (SP-12) or 192kbytes of RAM (SP-12 TURBO). The sample RAM is powered by a Lithium battery when the power is off, so you don't lose your samples. The sample memory is actually 16-bits wide, and therefore has some de-muxing to fit the various 12-bit samples across all the chips.
Sequence Memory The SP-12 has RAM memory for storing sequences. The base model has 8kbytes, whilst the TURBO has 32kbytes. The sequence RAM is powered by the same Lithium battery that is used for sample and program memory.
Program Memory The SP-12 has an Operating System of just 32kbytes held in ROM, with an additional small amount of RAM.
Digital Sampling A single 12-bit linear Digital Analog Converter (DAC) chip replays
the samples (an AD7541), 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.
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.
OS Versions
The SP-12 has 24kbytes of EPROM based OS, which was upgraded a few times during its product life. You can determine the OS version by watching the LCD display at power on. Version 2.1 was current in 1985 - the last version was 2.6. It can also be found by looking at the two OS EPROM's.
Eight Voices There are a total of eight voice channels, through which the 24 drum samples are played.
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Analog Filters 
The SP-12 uses DS/DD soft sectored 5.25" diskettes, which are becoming hard to locate. High quality diskettes are recommended (Imation and Maxell). Do not try and use HD diskettes, or
DD diskettes that are hard sectored or IBM formatted. They will not work. We recommend Imation DS/DD.
* Debug EPROM