On Tue, 20 Feb 2018 10:28:10 +0000, Dave Plowman (News) wrote:

In article ,
Johnny B Good wrote:
I'm not quite sure of the actual sequence. With twin capstan drive,
the speed the pinch rollers came in might be a problem to match. But
the motors were sync'd so I'd guess the solenoids came in first then
the motors ran.

I'd assumed contra rotating capstans for a reversomatic bi-directional
system rather than trying to maintain tape tension between two capstans
across the heads.

No. The idea was to get the very best tape contact to the heads. And
miniumum weave and flutter etc. A very clever design.

Both capstans were direct drive. And I assume clever electronics to
maintain the correct tape tension.

It was a pro 1/4".

In which case, the capstan motors weren't being synchronised in the
conventional sense. The electronics must have been used to set up the
tiny (and appropriate) speed differential required to generate the
required tension in the section of tape passing the heads. Clever
electronics indeed.

I suppose such clever (or 'smart') electronics could easily include
accurately timed solenoid control of the pinch wheels to allow for the
effect of 'run up time' (no matter how brief) to eliminate the start and
stop pitch slide effect.

Whilst extremely low inertia motors can be made by making the rotor
'ironless' using a rotor made entirely from just the copper wire formed
into a moving coil shape[1] by the use of an epoxy glue, this doesn't
seem to be a good idea when you want to eliminate flutter from even the
most residual of commutation/cogging effects on the motor's angular speed.

Since you need to use pinch wheels for reasons other than to achieve
'instant' stop/start anyway (to prevent deformation of the pinch wheel
rubber tyre and to allow the tape to be threaded), I don't see tape deck
manufacturer choosing an 'inertialess' motor design for this application.

The "Engaging a pinch wheel onto or off a constantly spinning capstan"
technique still offers a tried and tested method to achieve the near
instantaneous stop/start characteristic of 'pause/unpause' we've all
become accustomed to using on even the most humblest of tape transport
mechanisms.

Cassette decks rely entirely on its very slow transport speed and the
tape's elasticity to absorb the brief start up acceleration forces
required to bring the supply spool up to speed but open reel tape decks
designed for much higher transport speeds typically[2] get round this
issue by including additional damped tensioning arms in the supply reel
to capstan path to both absorb the higher forces involved and to
effectively isolate the section of tape passing the record/replay heads
from the high G force involved in using a pinch wheel to 'instantly'
accelerate that critical section of tape up to speed.

I'm not saying such instant start, direct drive, capstan motors are
impossible, just that they seem an improbable solution in this case
(BICBW).

[1] More likely in this case, a disk shape to form the rotor of a
'pancake' motor.

[2] I use the qualifier, "typically" because very highly specified
capstan driven[3] tape transport mechanisms use a variety of active servo
control methods to buffer the critical tape path section from the
mechanical loading effects of both the supply and the takeup spools
(reels).

[3] I make this distinction because there are some very high speed
magnetic tape data recorders where the tape speed is too fast to make
capstan drive a viable option. In this case, tape spool drive is used as
a controlled version of the fast forward and reverse modes used by
conventional tape drives where the data flows are buffered to allow short
term adjustments of the supply and takeup spool rotational speeds to the
correct average linear tape speed via servo control in a similar fashion
to the control of the spin speed of an audio CD player disk.

--
Johnny B Good