[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Votrax chip SC01/SC02
Mark McDougall wrote:
Allen Bong wrote:
In the game "Dung Beetles" by Bob Bishops (???), he was able to
pronounce the phase "We've got you!" without any extra hardware. How
did he do it?
Sample play-back? You can re-create any sound, including speech, using
something as simple as a 1-bit PWM output.
I haven't examined Dung Beetles, but most 1-bit players didn't use PWM
in the sense that term is usually used. They just used the speaker
toggle to recreate the zero crossings of the sampled waveform (usually
sampled by the 1-bit cassette input).
Since this process was still a sampling process, the zero crossing times
were quantized to the sample loop period.
The resulting output was "infinitely clipped" sound--that is, sound
that in which only the zero crossings are preserved, and the waveforms
are all full-scale.
PWM generally refers to a system in which pulses are generated at a
constant rate, and their width is modulated to correspond to (several)
bits of a sampled signal.
Scott Alfter's SoftDAC and my DAC522 both reproduce sampled sound
using PWM. His latest SoftDAC uses pulses at 11KHz with 16 possible
widths, for 4-bit fidelity, while DAC522 uses pulses at 22KHz with
32 possible widths for 5-bit fidelity. Raising the pulse rate (not
the sample rate) to 22KHz has the effect of making the otherwise
very loud pulse train much less audible--unless you have very good
high frequency hearing. ;-) Or if you play it through an amplifier,
it means that just a little high frequency "cut" with the treble
control will almost completely eliminate the pulse sound without
reducing the high frequencies in the sampled sound (up to 4-5kHz--
think AM radio).
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."