13 Dec
2014
13 Dec
'14
10:57 a.m.
On 12/13/2014 12:38 AM, Chuck Guzis wrote:
On 12/12/2014 09:04 PM, Jon Elson wrote:
Yes, quite correct. Turning on or off the erase head
would
leave a "noise burst" that
could be picked up by the read amps. So, in NRZI mode, you
had to ignore them.
Good drives had something like a current ramp-up control to
eliminate these while
writing.
In PE mode, there would be a preamble and postamble of 40
transitions each,
so you have no way of mistaking a little noise for the
beginning of a valid block.
Jon
OK, that narrows things down QUITE a bit. I
worked with
some Exabyte
drives and others that had a lot of constraints on what
you could do,
and you could do some interesting things. I think you
could write
zero-length blocks, and they would show up just like that
when
read. Clearly on 9-track, there SHOULD BE no such thing
as a
zero-length block, but I suppose some drives could
actually write
such a thing at 1600. A preamble, CRC, LRCC and
postamble, with
no data. But, that OUGHT to be an illegal command.
I seem to remember on certain tapes, that very short
records are discarded or reading; something like a
"noise" threshold of 7 bytes. But that could be for
7-track and 800 9-track. I don't recall--but regardless,
mistaking a 0-length record, should one occur, for a
filemark is not catastrophic.