Subj : Play audio from a serial port
To   : AKAcastor
From : Bob Worm
Date : Thu May 09 2024 07:45:15

  Re: Play audio from a serial port
  By: AKAcastor to Bob Worm on Wed May 08 2024 15:42:24

Hi, AKAcastor.

 > The low pass filter would probably be filtering out the start bit if it is
 > the only 0 bit?  A single bit at 115.2 kbps would be filtered out at 57.6
 > kHz (it's a pulse lasting 8.68 microseconds, that would be half the cycle),
 > I think - though I really don't have much knowledge of analog electronics.

That's a good question. I suppose the instantaneous pulse would be 57.6 kHz but an alternative way to look at it would be as a 11.5 kHz signal with a really wonky 9:1 duty cycle... I further suppose since it's a "square" wave the instantaneous frequency could be considered to be infinity (obviously not really).

I will ask my good friend Martin - he loves this kind of thing.

 > I'm actually surprised to have not seen this kind of trick in use before.
 > Seems like it could have fit in during the 80s when any (cheap) audio more
 > than a beep was highly desirable.

There were a good number of consumer and homebrew projects for doing this with the parallel port. In theory a nice clean 8 bit output, I made one with my school friend back in the day and it sounded *awful*. I suspect our quality control was not good enough as other people reported good results.

Oh for the days when you could just walk into Maplin and buy qty. 1 components... In fact we just turned up with a list of "0.1uF electrolytic" etc and handed it to the poor guy behind the counter to rummage through the catalogue and find the 20 or so part codes we needed for us and then fetch them from the back!

BobW
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