DoubleTalk PC — standalone emulator

A software emulator for the RC Systems DoubleTalk PC, an early-1990s 8-bit ISA text-to-speech card. It runs the card's original 80C188EB firmware on a vendored CPU core, so the firmware itself does all the speech synthesis — you feed it text bytes and it produces the same PCM audio a 1993 screen reader would have heard, no ISA slot or physical card required.

The card's firmware ROM is proprietary and is not included — you supply your own (see The firmware ROM).

How it works

The DoubleTalk PC is essentially a tiny embedded computer: an Intel 80C188EB CPU with 512 KB of firmware that turns ASCII text into speech and streams it out an 8-bit DAC. This project emulates that CPU and its on-board peripherals (timers, interrupt controller, the host mailbox) accurately enough that the unmodified firmware boots and synthesizes speech. Nothing here reimplements the speech algorithms — they live in the ROM and are executed as-is.

The CPU core is vendored verbatim from MAME and compiled against a small compatibility shim so it builds and runs outside MAME. See notes/ for the full reverse-engineering write-up.

Repository layout

Path	What
doubletalk/mame/	Vendored MAME 8086/80186 + 80C188EB CPU core (BSD-3-Clause)
doubletalk/shim/	Minimal MAME-compatibility shim so the core builds standalone
doubletalk/doubletalk_board.{cpp,h}	Board wrapper: CPU + I/O ports + DAC + host protocol
doubletalk/dtalk.{cpp,h}	C API over the board
doubletalk/dtalk_cli.cpp	Command-line harness (boot trace, synthesize to WAV)
doubletalk/nvda/	NVDA screen-reader add-on (README)
notes/	Reverse-engineering documentation
LICENSE, LICENSING.md	License text and per-component licensing/attribution map

Building

Requires a C++20 compiler (g++) and make.

cd doubletalk
make                 # builds build/dtalk_cli and build/libdtalk.a

Windows DLLs for the NVDA add-on (via mingw-w64 cross-compilers):

make windows         # needs g++-mingw-w64-i686 and g++-mingw-w64-x86-64

Command-line usage

dtalk_cli takes the path to your ROM dump as its first argument:

# Synthesize text to an 8-bit WAV
./build/dtalk_cli doubletalkpc.bin say "Hello, world" hello.wav

# 16-bit WAV through the modeled DAC output stage (optional low-pass corner in Hz)
./build/dtalk_cli doubletalkpc.bin say16 "Hello, world" hello.wav 3000

# Boot the firmware and report state / instruction trace (diagnostics)
./build/dtalk_cli doubletalkpc.bin boot
./build/dtalk_cli doubletalkpc.bin trace 100000

Run dtalk_cli with no arguments to see the full command list.

NVDA add-on

An NVDA speech-synthesizer driver lives in doubletalk/nvda/. It maps NVDA's rate/pitch/volume/voice controls onto the card's own commands and plays the firmware's 10.5 kHz PCM.

Status: working on NVDA (2023.1+, Windows).

See the add-on README for build and install steps.

The firmware ROM

doubletalkpc.bin (512 KB) is proprietary RC Systems firmware. It carries no redistribution grant and is not included here or in any release. Following standard MAME practice, this project identifies the ROM by hash but gives no sourcing guidance — supply your own dump (e.g. read it from a DoubleTalk PC card you own) and verify it:

• CRC32: 66685631
• SHA1: bf7e78d6381c76d291ee069971873347a314ffff

The repository .gitignore blocks *.bin / rom/ so the ROM can't be committed by accident.

Licensing

The code we wrote is BSD-3-Clause — free to use, modify, and redistribute (including commercially); just keep the copyright notice and don't use the authors' names to endorse derived products. It does not require you to open-source the rest of your project.

The vendored MAME CPU core is also BSD-3-Clause, so the entire repository is BSD-3-Clause. (The card's protocol was reverse-engineered against the Linux kernel's DoubleTalk drivers, which are GPL-2.0 and live upstream — the notes link to them, but they are not included here.) See LICENSING.md for a complete, per-file breakdown of licenses and copyright holders.

Credits

• David Sexton — standalone port, shim, board wrapper, C API/CLI, NVDA add-on.
• Christopher Toth — the 80C188EB CPU support in the MAME core and the firmware-to-audio reverse engineering (see notes/investigation-audio-path.md).
• MAME and its contributors — the 8086/80186 CPU core this builds on.