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| From: Richard Atkinson
Date: 1998-09-10
Subject: Re: FF30, timers, FF11
|
To answer the $FF11 question once and for all, I looked at the output of the SND pin on TED with an oscilloscope. It is digital, using PWM (pulse width modulation) and for each value of $FF11 (assuming bit 7 set) there is a ratio which corresponds to the analogue voltage at the video connector, after the filtering has been applied. When one voice is enabled, the period of each cycle is twice as long (exact value in clock cycles to follow) as when both voices are on. The width of the pulse is determined by the volume setting, and is an integer number of clock cycles. (14 or 17MHz clock)
I will need to check my results, as it's rather hard viewing it on my crappy old oscilloscope, but two things I did notice were: the width is not linear with the volume setting (the step between 0 and 1 is less than the others), and because of this, volume 1 with both voices is not the same as volume 2 with just one.
LUCA: thanks for the 1551 upload info; I like this drive! And I may have a possible source of another one :-)
Richard.
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