unreadable HP cartridges

[Tony Duell]

> 
> > Digital information is stored on the tape using a delta distance
> > encoding scheme.  The "1" distance is approximately 1.75 times
> > longer than the "0" distance.  The magnetic polarity is
> > irrelevant, only the distance is important.
> 
> This is very similar to the Apple II cassette modulation scheme.
> 
> Hmm.. Woz worked for HP at the time...
> 
> If you look at other early cassette tape "standards" they tend
> to be more FSK-like than distance-encoding type.  e.b. Kansas
> City Standard.

The other comman standard at about this time -- used by the HP 9830 on 
cassette tapes, on the Tekky 4051 on QIC cartrisges and by the HP65/67/41 
on magnetic cards was to have 2 tracks. A pulse on one of the tracks is a 
'0' bit, a pulse on the other is a '1' bit. Coincident pulses on both 
tracks might be used for something like a file marker or a byte marker 
(the HP 9830 does the latter). 

-tony