Film-Tech Forum ARCHIVE: What is a 'tach pulse' and how is it created?

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Posted by Douglas Carmichael (Member # 1930) on 01-09-2004, 09:36 PM:

What is a 'tach pulse' and how is it created from the projector?

(I've seen devices like the Sky-Skan GARLIC card that can accept a tach pulse from the projector and output SMPTE timecode.)

Also, I haven't seen Adams-Smith synchronizers anywhere on the web. Are they still in business?

--Douglas

Posted by Gordon McLeod (Member # 33) on 01-09-2004, 09:46 PM:

I would recomend you first understand the basics of biphase transport control before you start delving into any timecode generation issues
The tach pulse is a set of pulses in quadrature that give position and direction of rotation from a positioning sensing device (typically but not always an optical tachometer with quad outputs)

Posted by Phil Hill (Member # 371) on 01-09-2004, 10:33 PM:

I agree Gordon.

Don't ya just luv sky-scam and the "spice" system... [puke]

>>> Phil

Posted by Steve Kraus (Member # 476) on 01-09-2004, 10:47 PM:

Now for an answer you can understand.

The pulses come from a shaft encoder which is a small device which resembles a motor which is attached to a projector and driven by a timing belt. If you're talking about a conventional projector they can be attached whereever it's convenient. On Century or older Simplex making use of the shutter shaft emerging from the front of the projector (1X speed) is handy; on X-L / 5-Star setups it's common to attach to the back of the soundhead and put the driving pulley on an extension of the soundhead sprocket shaft (1/4 X). The timing pulleys need to have whatever ratio is necessary to drive the encoder so it puts out however many pulses per frame needed to drive the dubber, digital dubber, timecode generator etc.

Now, about quadrature. The typical encoder puts out pulses (often they are TTL level) with a 50% duty cycle...on off on off...so many pulses per turn...50...100...whatever. But there is also a 2nd signal wire and pulses are sent out on that one, too but the are 90° out of phase with respect to the first wire. What this means is that the transition from high to low on one signal is exactly halfway between transitions on the other wire. (By comparison 0° out of phase would have both wires doing the same thing while 180° out of phase would have both wires transitioning at the same instant but oppositely.)

So you have two wires carrying pulse streams out of phase as described. The purpose of this is to convey direction. Which wire is leading and which is lagging tells the driven equipment which way to go. Which is why sometimes when hooking up a temporary encoder installation in whatever way is physically convenient it can happen that the encoder is being driven the wrong way but interchanging the two signal wires corrects the issue if the driven equipment doesn't have a switch for this purpose.

Posted by Phil Hill (Member # 371) on 01-09-2004, 11:12 PM:

Well, there is some good info there as well as some mis-information.

I STILL recommend you "read the book"...

>>> Phil

Posted by Manny Knowles (Member # 1171) on 01-10-2004, 09:31 AM:

...or wait for the movie! [Wink]

Curious about how the question arose in the first place.

Coincidentally, I was "this close" to getting one of these theatres hooked up to sync with a mag dubber. I was looking forward to it.

Posted by Matthew Bailey (Member # 601) on 01-10-2004, 07:37 PM:

Anytach pulse is generated by whatever device used,but the only
three classifications I can only think of basically is an optical or mag transducer-P.C.,coil or mag head-or a switch,sometimes a magnetic switch-alarms for example.

Posted by Mark Gulbrandsen (Member # 72) on 01-10-2004, 09:16 PM:

Encoders are usually done by reading photographicaly made divisions on a glass disk. In a Quadrature type encoder two photodetector pairs look through this spinning glass disk to generate the pulses. Here is a good link to the US Digital site with some good basic information on the operation of Encoders.
http://www.usdigital.com/articles/universal-encoder-technology.shtml

Mark @ CLACO

Posted by Gordon McLeod (Member # 33) on 01-11-2004, 12:57 AM:

This is a link to the manufacturer of one of the more popular encoders
http://www.danahercontrols.biz/dynapar.htm

Posted by Bill Langfield (Member # 818) on 01-11-2004, 09:24 AM:

Perhaps it's just me(proberly), but...

What on Earth are you guys talking about?

You've got me totally lost, clueless even.

Bill

Posted by Gordon McLeod (Member # 33) on 01-11-2004, 02:24 PM:

Bill it is classified information on a need to know basis we could tell you but....... [Big Grin]

This is how a dubber or twin machines are interlocked for sync playback of 3d for excample or a seperate mag film on a dubber

Posted by Mark Gulbrandsen (Member # 72) on 01-11-2004, 03:25 PM:

Gord,
I think that he thought we were trying to set up a Dodge Hemi for a tachometer [Eek!]

.
Mark

Posted by Robbie Hidalgo (Member # 2090) on 01-11-2004, 04:23 PM:

Hey Doug...

FYI, the assets of the synchronizer manufacturer Adams-Smith were bought by its German distributor Peter Struven Audio Vertrieb the day before the 1998 AES Amsterdam Convention opened.

Since then, Adams-Smith seems to have just "faded away" in the market. Shame. I've still got a Zeta 3 that locks like glue.

You can also take a look at Lynx for synchronizers if you're in the market for one. They've been the studio benchmark for years, and IMHO, significantly preferrable to an Adams-Smith if you've got the budget.

-- Robbie