XRJDM@SCFVM.BITNET (Joe McMahon) (02/22/90)
A post from EMUSIC-D... Gan Seum-Lim inquired recently: >Yesterday a friend showed me a computer detective game "Mean Street", it is >run on a IBM PC compatible and it is able to drive the build in speaker >to produce high quality music (infact, any sound) just like the Macintosh. >Does anyone know how this is done ? I am very curious about this ! The question essentially is how to get reasonable sounds out of a 1-bit D/A converter. There are several names for how this is done, but they all essentially boil down to pulse-width modulation. Given a continuous, complex waveform, if you use it to pulse-width modulate a high-frequency (beyond 20khz) squarewave oscillator, the output would have narrow up-going spikes where the input wave was low in amplitude, and narrow down-going spikes where the input was high in amplitude. If you then lowpass filtered the output, it would closely resemble the input. This is what's happening on the IBM PC programs that make music with the builtin speaker. Except the lowpass filtering part is done by your ear. If they don't make the basic squarewave frequency high enough, you'll hear a lot of high-frequency "hash" along with the music, but it will still be okay for game use. It's funny you should ask this question because it turns out that this may be the A/D conversion technique we'll all be using in the future. There are companies that are now building 1-bit A/D converters that work this way. They sample audio at a very high frequency (a few hundred KHz) and get 1-bit per sample, then digitally filter the samples into 50Khz 16-bit samples. Cost of the hardware is supposedly cheaper, and the S/N ratio is much better than standard 16-bit A/D's. I think Crystal Semiconductor is one of the companies doing this. They call it "Delta- Sigma" modulation.