Real tape drive densities

Jay Jaeger cube1 at
Sun Feb 14 20:13:14 CST 2016

On 2/14/2016 5:36 PM, Chuck Guzis wrote:
> On 02/14/2016 02:08 PM, Jay Jaeger wrote:
>> I have never heard of 200bpi or 556bpi for anything other than 7
>> track. I cannot imagine why anyone would ever produce such a thing.
>> The only density I have ever heard of as being available on both 7
>> track and 9 track is 800BPI NRZI, from any manufacturer, and I have
>> seen quite a lot of them over the years.  Nor have I ever seen a 9
>> track tape whose label on the exterior claimed it had been written at
>> 200 BPI or 556 BPI.
> That would agree with my own experience as well.  800 NRZI and 1600 PE;
> 6250 GCR.    There may have been non-computer (e.g. data logging) drives
> in 9-track with lower densities, but I've not run into any.
> Some drives apparently *can* mix densities.  I came across an AT&T
> distro tape recently for SVR4 that started with 6250 GCR and then after
> the first tapemark, switched to 1600 PE for the remainder of the tape. I
> managed to read it in two passes--the first is the drive set to 6250 at
> loadpoint, then with the drive set to 1600 at loadpoint.   All data was
> complete, so it's a puzzlement.
> --Chuck

Yes, some drives can - ones that are not "smart" with their own
microprocessor.  My HP 9 Track drive is, unfortunately, "smart", and
looks for recording bursts at the beginning of the tape and sticks
doggedly to that.

Maybe some yahoo decided to write over the first file of that SVR4 tape
at higher density so as not to clobber the files after it.  Smart, so as
not to clobber the other stuff, but crazy.  Or it may have been just a
case of writing that first file, with no double-EOF EOT on it.

Or maybe someone wrote the first file and then went "OH SH*T that was a
DISTRO TAPE wasn't it..."  ;)

Anyway, I doubt AT&T would have written the tape that way intentionally.

Curious: What processor was that distro for?  Maybe donate the files to


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