[vcf-midatlantic] Zilog System 8000

Dave McGuire mcguire at neurotica.com
Tue Nov 17 14:18:56 EST 2015


On 11/17/2015 12:38 PM, Oliver Lehmann via vcf-midatlantic wrote:
>> Keep in mind: it's the metal drive roller which determines tape 
>> speed, not the roller.
> 
> This is exactly why I know as well. It is just there to get the "speed
> on the tape" but it is not mission critical to get the dimensions 100%
> correct.
> Remember - the capstan dimensions change over time anyway because the
> material will be constantly removed during operating the tape. This is
> a normal process. IF the capstan dimensions would be vital for the
> correct function of the tape drive, it would have been unable to read
> the times not far after it was shipped - probably.

  Actual tape speed over the head is actually not that critical for many
types of tape drives.  Most modulation/encoding schemes are
self-clocking, either through separate clock tracks (DEC TU56) or
embedded clocks that are recovered via a phase-locked loop.  Getting
close will do it.

  Speed variations, on the other hand, can be bad if a PLL is used for
clock recovery.  If the period of the speed variation falls outside of
the clock recovery PLL's loop bandwidth, the loop could lose lock,
resulting in the loss of the clock.  So, keep those replacement rollers
as round as possible, but don't worry so much about the exact diameter,
unless you know for a fact that there is an absolute tape speed
requirement, which is rare.

                -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA



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