Motorola DSP56004 (1992)
A 24-bit audio DSP that traded the family's serial port for a dedicated audio interface. It ended up inside everything from home-theater receivers to high-end CD transports.
Overview
Motorola introduced the Symphony DSP56004 in October 1992. It was the newest member of the 56000 family, a line built around 24-bit internal processing. The 56004 was optimized for audio and positioned to provide low-cost DSP for both professional and consumer products. It stayed fully code-compatible with the original 56001, so code written for the earlier chip could move to the new one.
Motorola called the architecture Dual Harvard. Program RAM and data areas are separate, and data memory is further split into X RAM and Y RAM. Filter algorithms that mix variable data with fixed coefficients keep the two in separate RAM areas, which cuts processing time. The 56004 also carries two additional data-memory areas, X ROM and Y ROM, that store sine wave and logarithm tables on-chip. Internal clock speed runs at 40 MHz and above, which works out to a processing rate of 20 million instructions per second. A phase-locked loop converts virtually any external clock frequency up to 40 MHz. That reduces clock noise and lets the whole system run from a single clock source.
The interface is where the 56004 departs from its siblings. The 56001 and 56002 have a general-purpose synchronous serial interface. The 56004 has a serial audio interface (SAI) instead. It connects directly to as many as four mono or two stereo ADCs, and six mono or three stereo DACs, with no external glue logic. Multi-channel audio applications benefit most. An external memory interface addresses and refreshes up to 4 MB of dynamic RAM directly, again without glue logic. That suits digital audio files and the long delay lines behind cathedral and stadium simulations. Four general-purpose I/O lines handle LEDs, front-panel buttons, and the like.
The trade-off is MIDI. Unlike the 56001 and 56002, the 56004 has no asynchronous serial interface, so implementing MIDI is less straightforward. It can be done through one stereo I/O channel of the SAI (two audio channels combined on one pin) and/or the GPIO lines, but it takes additional software. A designer coming from the 56001 will notice this first.
Collectors chasing the chip today will mostly find it soldered inside finished gear rather than on its own; it's a component to be identified on a board, not a box to put on a shelf.
eBay Listings
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.