Real tape drive densities

[Jon Elson]

On 02/14/2016 05:36 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.
>
Well, that could be a malfunction.  Some tape 
drive-formatter sets cannot do this without hardware 
problems.  But, some had software-controlled density, and so 
the software could override the detected density when the 
tape was mounted.  800 NRZI had nothing recorded over the 
BOT marker, 1600 and 6250 had distinct patterns written over 
the BOT marker to ID the density.  Once the density is 
identified, it SHOULDN'T allow you to change it in the 
middle of the tape.

Jon