General Instrument SP0256-AL2

GI-SP0256 refers to a family of closely related NMOS LSI chips manufactured by General Instrument in the early 1980s, able to model the human vocal tract by a software programmable digital filter, creating a digital output converted into an analog signal through an external low pass filter. The SP0256 includes 2 KB of mask ROM. The various versions of SP0256 differ primarily in the voice data programmed into their mask ROMs.

The SP0256 (and its predecessor, the SP0250) implement a 12-pole, Linear Predictive Coding (aka LPC-12) all-pole Vocal Tract Model (VTM). The SP0256 generates speech with a 10 kHz sample rate. The SP0256 realizes its 12-pole filter with a series of cascaded 2-pole IIR filter sections. This is in contrast to its contemporaries, such as Texas Instruments LPC Speech Chips, which used an 8 kHz sample rate with a 10-pole model, and realized their VTM with a lattice filter.

The SP0256 combines the VTM with a simple controller that loads compressed coefficient data into the VTM, either from the on-chip ROM, external speech ROMs such as the SPR-128, or in the case of the Intellivoice, an SPB640 speech data FIFO.

General Instrument made several variants of the SP0256. These variants differed primarily in their mask ROM content. Listed below are some known variants. Others likely exist that aren't listed on this page.

The SP0256-AL2 is perhaps the most commonly encountered variant. It contains 59 allophones primarily intended for use with English language phrases and five pauses in its internal 16 KB ROM. The SP0256-AL2 needs the control of an external microprocessor to concatenate allophones into words.

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