Motorola DSP56002
The 40 MHz square wave at the heart of Lexicon's PCM 80. This DSP never got its own front panel.
Overview
The DSP56002 is a chip you mostly learn about by opening something else. It shows up in the service documentation for the Lexicon PCM 80, where it sits on the DSP board next to two of Lexicon's own ASICs - the Lexichip-2 and the Tacochip - and a Zilog Z-80 doing housekeeping for the Lexichip. In the audio world it's a 24-bit fixed-point digital signal processor that manufacturers dropped into gear that needed serious number-crunching. Most of the people who heard it never knew it was there.
The PCM 80 documentation makes the division of labor clear. The host side runs a 10 MHz NEC uPD70208 - the V40 - which handles the front panel, MIDI, the footswitch and footpedal jacks, and the digital I/O. The host loads the DSP56002's program code and data, then controls its reset, interrupt, and DMA functions. The DSP56002 itself is the tightly-coupled engine that does the actual effects processing. It's clocked by a 40 MHz square wave that the service notes say should read clean at R32 on the DSP board.
The architecture around it is unusual. The PCM 80 doesn't hand the Motorola chip the whole job. A Tacochip swaps memory banks between the Lexichip and the DSP56002 buffers at the beginning of every wordclock period, driven by a TACOSWAP signal at half the selected wordclock frequency. A Lexichip-2 and the DSP56002 both receive the frame-start signal - a high-going pulse once every 32 64FS bit clocks - and both get the falling edge of WC/ to mark the start of a sample period. So this is a multi-processor pipeline, with the Motorola part as one piece of a tightly synchronized set. The DSP board is its own island, connected to the host board through a 72-pin connector, with the DSP56002 reset line (56KRST/) pulled low on power-up like everything else in the chain.
The PCM 80 selects 44.1 kHz, 48 kHz, or a PLL frequency, and the clock tree fans out from a 256FS master - 11.2896 MHz at 44.1 kHz, 12.2880 MHz at 48 kHz. The DSP56002 takes its bit clock through that chain. It also feeds the digital side: a Crystal Semiconductor CS8402 transmitter accepts 24-bit serial audio data from the DSP56002 and encodes it into a biphase-mark stream for the S/PDIF output, with a 75-ohm output impedance and provision for an optional 1:1 output transformer. So the Motorola chip makes the reverb tails and also supplies the data that leaves the box digitally.
The supplied documentation doesn't date the DSP56002 as a standalone product. What's documented is its use inside the PCM 80, and the manual's signal-tracing notes show how it was expected to behave in the field: a 40 MHz clock at FCLK, a reset line that should sit high and drop low only on power-up, an interrupt mask bit (FINTMSK/) in the host's Control Register 1 that gates DSP56002 interrupts, and a host-to-DSP control bus (HST56KCB) carrying commands back and forth. It's a chip meant to be commanded, not operated directly.
Period coverage of the DSP56002 as a component is thin. The trade press that mentioned it did so in passing, as part of a booth or a product line rather than a review. Studio Sound (October 1992, p.44) noted Motorola SPS showing active audio demonstrations at Stand 1039, with the DSP56401 AES-EBU transceiver among the parts on display. Motorola's DSP family was being pitched to the pro-audio market as building blocks, not finished goods.
What that means for the collector is a strange kind of value. Nobody hunts a DSP56002 the way they hunt a PCM 80, but the PCM 80 is the reason the chip is interesting, and the chip is the reason the PCM 80 does what it does. Pull a DSP board and you're looking at a snapshot of early-nineties pro-audio engineering: a host CPU, a slave Z-80, two Lexicon ASICs, and one Motorola DSP holding up its corner. The part number on the chip is the least glamorous thing on the board, and it's doing some of the most important work.
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