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Author Topic: What is a 'tach pulse' and how is it created? Douglas Carmichael Film Handler Posts: 18 From: Naperville, IL, USA Registered: Oct 2003 posted 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  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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)  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 01-09-2004 10:33 PM                  I agree Gordon. Don't ya just luv sky-scam and the "spice" system... >>> Phil  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 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.  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 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  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 01-10-2004 09:31 AM                     ...or wait for the movie! 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.  |  IP: Logged Matthew Bailey Master Film Handler Posts: 461 From: Port Arthur,TX Registered: Sep 2000 posted 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.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 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  |  IP: Logged Bill Langfield Master Film Handler Posts: 280 From: Prospect, NSW, Australia Registered: Apr 2001 posted 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  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 01-11-2004 02:24 PM                     Bill it is classified information on a need to know basis we could tell you but....... 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  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-11-2004 03:25 PM                     Gord, I think that he thought we were trying to set up a Dodge Hemi for a tachometer . Mark  |  IP: Logged Robbie Hidalgo Film Handler Posts: 47 From: Los Angeles, CA USA Registered: Dec 2003 posted 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  |  IP: Logged

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