chipspeech Diary, Part 2

Last summer I spent a good part of my late night research time on Texas Instrument Voice Synthesis Processor (VSP) LPC10 decoding chips. TI made LOTS of these from the late 70s to the late 80s, both for the OEM market and for its own use, under different names. They usually prefixed them TMC or CD for internal use, and TMS for OEM use, but their naming was not always consistent, which leads to lots of confusion.

The goal for chipspeech is to playback LPC10 streams coming from any Texas Instrument device/software and to do so in a sample accurate manner.

We are very close to that goal, but since I found no less than 8 variants of VSPs - all of which had to be accounted for in my emulation core - I'm convinced in fact, that there may be more.

A variant is a unique combination of the following features:

• Pitch encoding format (5 or 6 bit indexes)
• Energy ROM look-up table
• Pitch ROM look-up table
• K (lattice filter) ROM look-up table
• Voiced chirp ROM look-up table
• Variable frame size (TMS5220 rev A and up)

Those are the main characteristics which can quite significantly alter the tonal quality of the output voices. For instance, if you play a stream made for a variant on another variant you could get anything from a slightly unusual nasal quality playing certain words of phonemes, to a completely alien-like version of it, to typical circuit bending whooshes and 'blurps'.

We could have created even more sub categories to account for other observed behaviors, like for instance what happens when an unvoiced (noise) frame is followed by a silence frame (Energy either fades or not). There are probably even more subtle differences especially considering the influence of the various internal interpolation stages 'shifts' depending on the chip - but nothing that you could easily hear I am willing to bet.

The following is a list of VSPs found in devices that we have acquired and dismantled (some of which are incredibly rare). This list is by NO means complete as we do not have infinite resources... nor storage space :).  And I do plan on revisiting this as new discoveries are made.

TMC0280/CD2801 were found in:

• Language Translator/Language Tutor (same thing)
• Speak & Math
• Speak & Read
• Speak & Spell (UK, that's why the US carts sound funny with it)

TMC0281/TMS5100  were found in:

• Speak & Spell (US - Buttons)
• Dictée Magique (French Speak & Spell)
• Speak & Spell (Japanese - Buttons)
• Century 'Dazzler' CVS arcade PCB.

TMC0281D/TMS5100A  (Energy table changed from the previous) were found in:

• Speak & Spell (US - later membrane)
• Speak & Spell (US - Compact)

TMS5110 (A/C) Found in:

• Stern/Valadron Bagman PCB
• Chrysler Electronic Voice Alert
• Coleco Talking Teacher/K28 Talking Learning Computer (not the Votrax one)

CD2802 Found in:

• Touch & Tell
• Vocaid

CD2501E/TMS5200 Found in:

• TI99/4A Speech Adapter

TMS5220 Found in:

• IBM PCjr Speech Adapter

TMS5220(A/C) Found in:

• Magical Wand Speaking Reader (A rev)

How I categorized the variants:

A) Made a full regression test suite of LPC10 streams, each testing a different edge case. I send the streams to the VSPs, record and compared results.

B) Dumped the internal ROM tables of the chips. When idle, VSPs serially output the current values for Energy, Pitch and K's on its PROMOUT pin.  We can thus read the internal values of the internal ROM tables again by crafting custom LPC10 streams where each and every possible values of Energy, Pitch and K entries are set in succession. But we need a different stream for 5 and for 6bit pitch variants, of course!

Logic analyser dumps of ROMCLOCK,T11 and PROMOUT running these streams were then analysed with custom code, then text editor mangled and compared against each other. (Thanks to Lord Nightmare for the explanation of the PROMOUT logic!)

Each TMS51XX/5bit style chip was placed on a Stern Bagman PCB with a customized EPROM @ 9T, offset 0x1032, BIT6 "channel", replacing the French "Aye Aye Aye", (death sound) with the custom LPC stream. Since there are two form factors for those chips (DIP and SDIP), a ZIF adapter was used when needed:

Each TMS52XX/6bit style chips was placed on a custom made protoboard with custom FTDI programs to send the LPC10 stream using the 'Speak External' command

Portions of this research was contributed to MAME/MESS and hopefully will be used for the introduction of the various Speak & Spell drivers that they are working on.