Re: Roll velocity - a new theory (?)
By Zoltán Jánosy
>
> Date: Wed, 14 Jun 1995 07:17:33 -0400
> From: KBELLISON@aol.com
> To: automatic-music@foxtail.com
>
> Subject: Sock-drawer Vaults and Roll Velocities ...
>
> ******************************************************
> On Bill Jellen's comments about roll velocity:
> ******************************************************
>
> > "since the increasing mass (of paper on the take-up spool)
> > slows down the low torque pneumatic winding motor."
>
> A mass in motion tends to stay in motion, so it would take very little energy
> to increase a velocity from 70 to 85 tempo over a 3 minute time span - I'd
> think it to be a no brainer even for a low torque pneumatic motor. Anyway,
> the *total* mass of the system that the motor sees (feed spool + in-between
> stuff + take-up spool) never changes ... as one spool is relieved, the other
> is taken up proportionally, and the sum mass effect would be zero. The motor
> does not see any load-change due to mass per se.
This argument sounds reasonable. However, it *is* so that the roll velocity doesn't increase to the theoretical value (at least on some systems). So maybe some other explanation would be more appropriate. How about the following one (I haven't thought about it seriously, it's just an idea):
Let's suppose the force needed to pull the roll is constant (it is caused mainly by the friction on the tracker bar and it is more or less the same all the time - not counting the variation caused by the opening and closing of the holes). However, as the roll diameter increases, the you need greater torque to balance the same force since the arm of the force gets longer. Thus, if your motor cannot supply the needed torque, you cannot pull the roll with the same constant force - and the roll velocity will decrease. If the motor is designed to balance this effect, then the roll velocity will stay constant.
It can happen, that originally this was not intentional. But if it was, then this raises a new problem with restorating old players. If a new motor is built into the instrument, then these assumptions may not be valid anymore. And then the tempo will change during the playback (and there is an even worse effect: on systems with integrating dynamics controllers - such as the Welte-Mignon - the dynamics will be damaged as well).
One argument supporting the design for constant roll velocity is that encoding the dynamics is much easier in this case.
One idea I had (it was discussed in our AES paper) is to analyze the dynamics curves drawn on some rolls at the factory. These would show with great precision the change rate of the roll velocity. Of course this would give information only about the player, not about the recording (that we are more interested in) :-(.
I invite you again to do your own measurements on your players. Of course the information we get would be valid only for the current (restorated) players, but if we have lots of data, we might be able to draw some conclusion.
Zoli |
(Message sent Thu 15 Jun 1995, 14:12:51 GMT, from time zone GMT+0200.) |
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