Re: 3480/3490 Cart BPI

R.S. said:
It rather cannot be 38000.

If looking for an exact number, I think this is correct. But not for the reason
described.

According to the ECMA standard, "due to the ECC bytes" (etc.) "the maximum
density of data bytes is ...1,491 bytes per mm"
= 37,871.4 BPI
If the maximum is 37,871, then it cannot be 38,000, or 38K
(for K >= 1,000)

However, if you want an easy-to-remember number that is a common
denominator for the 3480/3490/3490E and compatibles family, then 38000 or
38K suffice.

Note that BPI is a linear density; 3490 has higher capacity than 3480 due to
arial density - number of tracks recorded across the tape width.

For interchange purposes, this density is not very relevant,
because it's a given for 3480/3490/3490E.
(whereas in 3420, it varied).
Some important factors are tape length, number of tracks, compression,
and then operational and application requirements.

Shmuel Metz said:
you're not taking gaps and checking data into account.

If you want capacity then you need
to take, e.g., checking, framing, gaps, into account.

Also correct. Rather than deriving density from capacity, you could start with
the maximum density to derive some theoretical capacities.

According to the ECMA standard,
"The length of the tape shall not be less than 165 m." (3480)

1,491 data bytes per mm (maximum, above)

= 246,015,000 bytes per tape

So a minimum length 3480 cartridge holds at most 246 MB
(assuming 1 pass of 18 tracks, uncompressed).
In reality, tape lengths are longer than 165 m.
However, there will also be gaps and tape marks that reduce
the practical capacity. Also, perhaps the data does not contain all 1's,
thereby falling short of maximum theoretical density. <g>

In summary, tape cartridges are not recorded at 6250 BPI.

Regards,
Mike Baldwin
Cartagena Software Ltd.
www.cartagena.com

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