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From: Crown (all posts)
Date: 1998-09-15
Subject: Re: FF30, timers, FF11

Hi,

>The output of TED itself only has two states: high and low. In every way
>it is a digital (1 bit) output. Therefore, if we know when it is high and
>when it is low (or to be precise, the ratio of high to low) we can
>predict what the analogue voltage will be at the output after filtering,
>without having to measure it (and incur inaccuracies), and thus emulate
>the "final" analogue voltage based on what we know the digital one is
>doing.
Hmmm, this digital behaviour is true only inside the TED, each tone generator is a simple digital flip-flop. The output of them are added together and then multiplied by the volume setting, and this is what goes out on the SND pin. This is an analog DC sygnal, and definitely has more than 2 voltage levels. After this the sygnal goes through a series of condensators which change the DC sygnal into AC. This also functions as a separator and as a shield, and it has the property,
that with time it converts a non-zero level DC sygnal into zero voltage on its output. For emulation purpose this latter part can be discarded,
as every sound equipment (PC soundcard, amplifier, active-speakers etc),
has this hardware part. This is why you have to measure at a DC point,
also don't forget to set your scope to DC mode, as it has also this.

>I am paying no attention whatsoever to actual analogue voltages, only
>observing how long the signal is high and how long low under different
>settings of $ff11 etc. I should really use a digital storage oscilloscope,
>though :-)
The tone-generators change their inner counter at every 8 cycle, this is the smallest period when they can change their output. The rising and lowering edges takes 32 cycles. You should get the same results.
If you can get hold of a digital scope, there are somethings which would be cool to measure. One thing is the 11010... series of the noise generator, I have an idea how to measure this without scope,
but haven't realised it yet. Don't forget to start with setting then clearing bit 7. Also I suspect that the generators doesn't change their inner counter on the same cycle (like the timers...) It might be also possible that a write to the sound registers doesn't take effect at once, so they might be aligned to the 8 cycle boundary. Sound reload should be definitely aligned, but also possible that $ff11 writes are also aligned.

Tibor Biczo / Crown of GOTU ICQ : 15989510