SPEECH CHIP COLLECTION

SP0256-AL2

SP0256 speech processor with the factory-programmed AL2 internal ROM: 59 allophones and five pauses.

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General Instrument SP0256-AL2 speech synthesizer chip

Overview

The SP0256-AL2 is an SP0256 with the AL2 speech program manufactured into its internal 16-kilobit ROM—2,048 bytes, not 16 kilobytes. AL2 supplies 59 allophones and five pauses, selected by 64 codes. The suffix identifies those built-in contents; there is no separate AL2 ROM chip to plug in. It is also important not to confuse it with the CTS256-AL2, a separate text-to-speech companion. The internal ROM is mask-programmed during manufacture, so an ordinary programmer cannot erase it or replace its vocabulary after purchase.

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Why allophones made AL2 useful

An allophone is a particular realization of a speech sound. By choosing these building blocks in sequence, a computer can attempt new words without a stored recording of every word. This is the basis of AL2’s “unlimited vocabulary” description: it means words can be assembled, not that the chip understands written English. Software—or the CTS256-AL2—must convert spelling into sound codes. The stock allophones have programmed pitch and timing rather than a set of normal host commands for arbitrary intonation. Sound selection and pauses help phrasing, and changing the clock shifts both pitch and speed. Concatenation does not provide all the transitions of fluent speech, which helps explain AL2’s recognizable robotic accent.

How a tiny chip reconstructs a voice

Rather than store a continuous recording as thousands of waveform samples, the SP0256 stores instructions and compact descriptions of how a voice changes over time. Linear predictive coding (LPC) models speech as an excitation signal shaped by the resonances of the vocal tract. For voiced sounds, the excitation is a repeating pulse train whose spacing determines pitch; for unvoiced sounds, such as hiss-like consonants, it is noise. A 12-pole digital filter shapes that source into speech. Changing its coefficients changes the simulated resonances; amplitude, pitch, and timing parameters control other features of the sound.

Why provide an external ROM interface?

Two kilobytes is a small budget for complete phrases. An appliance might need only a few announcements, but a talking game, clock, or computer peripheral could need much more. External speech ROMs supplied additional encoded speech programs without redesigning the synthesis engine. They could hold complete words and phrases, specially prepared voices, melodies, or effects. A custom phrase program can preserve acoustic changes within a whole utterance that simple concatenation of isolated allophones may miss. Extra memory therefore expands the available speech content; it does not turn the SP0256 into a new kind of synthesizer.