Re: Mark space ratio in 1531 data signal

Den Mon, 9 Dec 2019 14:06:18 +0000 skrev smf <smf_at_null.net>:
> On 08/12/2019 23:00, Nejat Dilek wrote:
> > My question is simple : what is the significance of a full period
> > pulse signal's mark space ratio being 1:1 when actually zero
> > crossing of the signal matters?

At the point where you reach the maximum possible frequency, you can
kind of think of the recorded signal as sets of sinus half waves, sort
of. The limit on what can be stored on tape is how short each half of
the waveform.

So not having 1:1 means that you can't use as high frequency as you
could with 1:1, and thus you can't store as much data.

Or in other words, with a given frequency you have less margins if you
don't have a 1:1 ratio.
 
> I've read a couple of conflicting explanations of how the +/- analogue
> waveform gets converted into a digital signal that the c64 or c16 can
> read, but I'm not sure that is particularly important to the answer.

The schematics has all the answers (for us who can read schematics):

http://www.devili.iki.fi/Computers/Commodore/C2N/Service_Manual/page_17.html

What it does is that it first amplifies the analogue signal to a large
enough level (IC1-2, IC1-1, IC1-4), and then it runs it through a
schmidt trigger circuit (IC1-3) that outputs a digital signal and also
requires a certain minimum signal level to react. The result of that
circuit is that the noise of an empty tape won't give random garbage
output but the output signal will instead stay in whichever logic state
it was last time it did see any reasonable signal. After IC1-3 there
are two 74xx inverters which makes sure that the signal conforms to TTL
logic levels.

Btw I'd say that it seems like bad practice to not have a capacitor in
series with the signal going to the rec/play head when writing to
tape. Leaving the datasette in it's rec position might eventually
magnetize the head(!). A perfect save routine should really output a
signal with a 1:1 pulse quote after it'r ready and has stopped the
tape, and wait until the user presses stop on the datasette until
exiting the save routine :O Maybe this doesn't matter too much though.

> It's probably that if you keep the ratio the same then you only need
> to write one timing routine when saving.
> 
> The analogue side might also get upset if you have extreme ratios, as
> there is a low pass filter in there.

There are actually no purpose built low pass filters. However both the
components (the quad OP amp) and the combination of tape and tape head
have their bandwith limits.

There are however high pass filters in the playback amp, and there is
also a lower limit on what frequencies the tape heads can handle. (You
could in theory add a hall effect sensor and get away with detecting a
DC signal on a tape, but that's totally uninteresting as the high pass
filters in the playback amp together with the schmidt trigger will make
sure that a super low frequency will be read back correctly as long as
you at least have read one full waveform cycle (to make sure the
schmidt trigger is in the correct state).

Btw notice that the component list doesn't match the schematics. The
component list lists two dual OP amp ICs while the schematics has
one quad OP amp. Doesn't matter when reading the schematics but would
matter if you need to trace the signals and take measurements on an
actual PCB.


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