From: Scott McDonnell (NetSamurai_at_comcast.net)
Date: 2005-08-15 04:37:58
----- Original Message -----
From: "Laze Ristoski" <cybernator@mt.net.mk>
To: "Cbm-Hackers" <cbm-hackers@ling.gu.se>
Sent: Sunday, August 14, 2005 7:20 PM
Subject: About tape-loaders and the flag input
> Oh, another thing comes to mind. I tried saving some clicks
> on the tape with a delay of about 3 seconds in-between, so I could
> check how the cassette-read line behaves (using a volt-meter).
> But I could easily recognize occasional noise there. Those
> extra clicks were certainly not generated by my code. The
> tape-drive was far away from any source of interference. So
> how come data can be loaded successfully? Obviously this
> was very low frequency, unlike data which is saved at
> way higher frequency. Does this (the higher frequency) eliminate
> the noise? Btw, that noise was generated during the recording process,
> since it wasn't random during playback.
>
> (that pesky analog technology :P)
>
> --
> Laze
I am not very sure what is going on with the first scenario other than
a guess. But the second one, I should be able to answer. First of all, the
amplitude of the actual data far exceeds the noise. The output of the
tape head is run through a filter which helps reduce out-of range freqs.
Then it is run through an amplifier stage. The amplifier is used to
clip that signal, so it somewhat resembles a square wave at logic levels.
This
reduces distortions and provides a signal that can be used with a comparator
to determine if it actually data instead noise. The other safeguard is the
encoding
sequence which uses different pulse widths for 1's and zero's. Usually a 0
is twice
the frequency of a 1. This is probably why you are perceiving the long/short
pulses.
Since only the transitions matter, frequency is being measured, not pulse
width.
A byte is usually padded with zeros, to get the clock to synchronize. This
is also
why the bits are repeated twice. This also takes care of problems with
varying speeds
in the tape due to many factors. It's not the specific frequency that is
important, but
the fact that one value is twice the frequency of the other. Now that I
think about it,
this probably answers your first question as well, but I am not really sure
what the flag
does in a commie tape interface. I am referring to just about all magnetic
(data) media
in general in the above. They pretty much all work exactly the same, even
your ATM
or credit cards. They usually use a NZRI, FM method. This is because the
mylar may
stretch, rubber belts stretch, motors lose thier effiency, the voltage
driving the motors
could change, etc... Since there are so many variables that could affect
speed, either
temp or permanently, the data integrity is protected (to some degree) using
these methods.
This is a really really general coverage of the method.
Check here for a more detailed discussion of magnetic data storage (meant
for cards,
but as I said, the process is very very similar.)
http://www.phrack.org/phrack/37/P37-06
Hope this helps.
Scott
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