Talker.v1.1.demo.soft.mp4
Quoted from General Instrument Application Note 0505D: CTS256A-AL2 Code-to-Speech Chipset
The Code-to-Speech chip set consists of two chips: the SPO256A-AL2, an allophone-based single chip speech synthesizer, and the CTS256A-AL2, an 8-bit microcomputer programmed with a letter-to-sound based algorithm. This chip set translates English characters into LPC synthesized speech sounds.
The SPO256-AL2 is General Instruments's standard allophone chip and is based on the SPO256A speech synthesizer. This synthesizer consists of a 10 or 12 pole second-order cascaded LPC filter, a controller, and a 16-bit ROM in which 59 allophones (speech sounds) and five pauses are stored.
The CRT256A-AL2 is a PIC7041 whose on-board ROM is masked with our code-to-speech algorithm. This algorithm converts English text (in the form of standard ASCII characters) into SPO256A-AL2 compatible allophone addresses, using letter-to-sound rules.
I honestly hadn't expected the level of interest that was shown in this board when I posted it on r/electronics. So I hadn't prepared a repository. This effort is therefore a little rushed, and I beg your forgiveness for any obvious in retrospect errors.
The schematic is based largely on the schematics included in the "Technical Data" sheets that used to be stapled to the back of the Radio Shack blister packs. They were also included in the Archer Semiconductor Reference Guides, specifically the 1988 edition.
Those schematics appear to have been based in turn on those included in the aforementioned GI Application Note.
Since I wasn't expecting anyone else to want to use this, I've customised the schematic to suit myself. There's no parallel port interface, and the serial interface is programmed not by onboard DIP switches, but by the contents of the exception word EPROM.
The active filter was cribbed directly from David C. Alford's article in the December 1985 issue of Micro Cornucopia, "Speech Generation With Radio Shack's SPO256". Although I seemed to have it working well enough when I breadboarded the circuit, the first version on the PCB was pretty horrible. It sounded like it was clipping - a lot. It turned out to be a dumb mistake on my part. Just because the LM324 is spec'd to work on a single supply, the circuit wasn't designed for that. Without a negative supply, I was losing the entire negative half of the signal. How on earth I didn't hear that on the breadboarded version, I have no idea. I toyed with the idea of pulling V- from the MAX232A and regulating it down to -5V; the current requirements of the LM324 were modest enough I could have gotten away with it. But I couldn't quickly find a through hole negative regulator that didn't draw much more current than the LM324 would have. I had a few ICL7660s in my parts box, so I went that route instead.
The board layout was inspired by the CTS256A-AL2 Text-To-Speech Board. It wasn't until I saw that one a couple of years back that I realized that my oh-so-much-larger solderless breadboard version could be tamed into something more reasonably sized.
In the gerbers subdirectory, you'll find all the Gerber files, as well as a zipped compilation suitable for submitting to JLCPCB, which is where I had my boards made.
| Sch/PCB # | Detail |
|---|---|
| U1 | CTS256A-AL2 DIP40 600 mil |
| U2 | 74ALS573 DIP20 300 mil |
| U3 | 74LS138 DIP16 300 mil |
| U4 | SP0256A-AL2 DIP28 600 mil |
| U5 | LM324 DIP14 300 mil |
| U6 | LM386 DIP8 300 mil |
| U7 | MAX232 DIP16 300 mil |
| U8 | 74LS14 DIP14 300 mil |
| U9 | ICL7660 DIP8 300 mil |
| U10 | 2716/32 2K/4Kx8 EPROM DIP24 600 mil |
| U11 | 6116 2Kx8 SRAM DIP24 600 mil |
| Y1 | 10MHz crystal HC49-U |
| Y2 | 3.12MHz crystal HC49-U (3.58MHz color burst crytal will work) |
| R1,R14, R15 |
10K 1/8W |
| R13 | 10 ohms 1/8W |
| R16,17 | 1K 1/8W |
| R18 | 20K 1/8W |
| R2,5,12 | 4.7K 1/8W |
| RN1 | 4.7K bussed resistor array (DigiKey# 4607X-101-472LF-ND) |
| C1,C2,C3, C5,C13, C20-C29 |
100nF ceramic capcitors, 0.1" lead spacing |
| C10,C11,C31 | 10uF electrolytic capacitors, 2mm lead spacing |
| C16 | 22nF ceramic capacitor, 0.1" lead spacing |
| C4,C8 | 220uF electrolytic capacitors, 2.5mm lead spacing |
| C6,C7, C14,C15 |
22pF ceramic capacitors, 2.5mm lead spacing |
| C9,C12, C17,C18, C19 |
47nF ceramic capacitors, 2.5mm lead spacing |
| C31 | 2.2uF tantalum capacitor, 0.1" lead spacing |
| D1 | 3mm LED |
| J1 | DC Barrel 2.1mm Jack |
| J2 | DSUB-9 Female Horizontal (Assmann A-DF 09 A-KG-T2S) |
| J3 | 3.5mm audio jack(CUI SJ1-3544) |
| J4 | 2 pin header, vertical, 0.1" lead spacing |
| J5 | Molex KK-254 AE-6410-02A 2 pin header, 0.1" lead spacing |
| J6 | 3 pin header, vertical, 0.1" lead spacing |
| P1 | 10K audio taper pot (Bourns PTV111-1415A-A103) |
| SW1 | SPST 6mm sq. tactile switch (CUI TS02-66-60-BK-100-LCR-D) |