Re: A question about the PSG & samples

[Ricardo Bittencourt Vidigal Leitao]

On Mon, 22 Jun 1998, Alex Wulms wrote:

> Are there any more undocumented features in the PSG, the MSX1 VDP and any 
> other chips used in the MSX which all programmers and other MSX fans should 
> be aware of?
>

        Sean Young's page (www.msxnet.org) has a description of a
undocumented MSX1 VDP feature, when you set the address for reading. You
should read that.

        But one thing I ever wanted to do and never had the chance is to
transform your MSX1 into an AM radio.

        The idea is simple: remember that fourth PCM method, when you set
the PSG to a very low frequency and send bytes faster than that? Now think
of it backwards, setting the PSG in a very high frequency and sending the
data slowly.

        Let's take some values: if you write 0001 to the first pair of PSG
registers, the oscillator will generate a square wave of ~ 110 kHz. And
now you modulate the output at audio values, like 8 kHz or 11 kHz. Notice
the word here: "modulate". What you're doing is an AM modulation of the
initial square wave.

        Since the carrier is a square wave, your spectrum will have
signals at 110 kHz, 330 kHz, 550 kHz, and if the signal is strong enough,
this 550 kHz signal can be detected by a common AM-DSB radio (at least in
theory, I never tried it). 

        If anyone wants to try it, don't forget to wire the antenna
directly to the PSG pins. The MSX has an amplifier with a low-pass filter
that cut signals higher than audio.

------------------------------------------------------------
Ricardo Bittencourt           http://www.lsi.usp.br/~ricardo
[EMAIL PROTECTED]    "Save the trees: eat more woodpeckers"

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