Whence 556?

[Tom Gardner]

Bache says it was introduced with the 729 II and that the density was determined by keeping the tape speed ips the same and upping the data rate to 62,500 Hz.  The latter was controversial
Tom

-----Original Message-----
From: Chuck Guzis [mailto:cclist at sydex.com] 
Sent: Saturday, June 02, 2018 11:27 AM
To: CCtalk
Subject: Whence 556?

List:

This is one that's bothered me for most of my adult life.  As you may or may not know, there were three industry standard densities for 7 track 1/2" tape:  200, 556 and finally 800 cpi/bpi.

So 200 and 800 are nice decimal multiples of 10.   But 556 doesn't fit
that pattern--it's not a "nice' number, being the product of 4 and 139 and doesn't correspond to any computer-related characteristics that I know of. It's not metric.  So why 556 and not 400, 512 or 600?

There's got to be a story there, somewhere.   I think the 556 density
came in around the 1401/7090 time with the 729 II.

Anyone know the story?

--Chuck