From: Scott McDonnell (NetSamurai_at_comcast.net)
Date: 2005-08-15 04:37:58
----- Original Message ----- From: "Laze Ristoski" <email@example.com> To: "Cbm-Hackers" <firstname.lastname@example.org> 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 Message was sent through the cbm-hackers mailing list
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