Film-Tech Forum ARCHIVE: Why does the CP650 warble at all?

This is topic Why does the CP650 warble at all? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Mark J. Marshall (Member # 1409) on 12-17-2005, 04:54 PM:

This seems to be a real design flaw in my opinion, but I wanted to see what others thought. I really can't think of any application where allowing the CP650 to vary the PITCH based on how fast the data is coming in would make sense, but maybe I'm missing something. This should at least be something that could be turned OFF in the software (maybe it is, but I don't see it).

In this thread, I'm not really interested in how to mechanically fix the warbling, I'm looking for people's opinions on why the software was designed that way to begin with, and if you think it's a good idea or not.

Thoughts?

Posted by Carl Martin (Member # 1146) on 12-18-2005, 12:38 AM:

never worked with a cp650, but i'll speculate.

could it be so that it will match the pitch of the analog track in case of a dropout?

Posted by Gordon McLeod (Member # 33) on 12-18-2005, 09:11 AM:

You may actually have the unit toggling at a high rate back and forth between analogue and digital that did happen with the odd DA20

Posted by Louis Bornwasser (Member # 3063) on 12-18-2005, 09:47 AM:

I concur with Gordon. To test this theory, either turn off the analog light source or temporarily block the analog LED.

Reversions to analog will then show up as missing audio.

You will then know if a well qualified Dolby tech should be called to repair the Digital. Louis

Posted by Demetris Thoupis (Member # 833) on 12-18-2005, 10:16 AM:

This happens on the DTS too. I remember when we installed some units we checked the film path while the film was running and we were "forcing" the film to go faster than it should. It was speeding up the pitch on the DTS discs. But a properly alinged soundhead and a proper projector should not do such warbling. Maybe you should check your film path if you have a problem like this.
D

Posted by Brad Miller (Member # 2) on 12-18-2005, 10:18 AM:

I've seen projectors not running exactly at 24FPS exhibit the complaint he has. I've also seen this from platters with a bouncy payout, particularly with dts.

Posted by Mark Gulbrandsen (Member # 72) on 12-18-2005, 08:17 PM:

quote: Brad Miller
I've seen projectors not running exactly at 24FPS exhibit the complaint he has.

I've seen this happen with Sony's but not with Dolbys. Dolby has a pretty wide tolerance for speed as does DTS.... something like +/- 5% or thereabouts. The Sony 2000's seem a but more touchy as regards speed accuracy.

Mark

Posted by Ken Jacquart (Member # 369) on 12-18-2005, 08:40 PM:

The CP650 (and DA20 and CP500D) has a Dolby Digital decoder speed tolerance of +7% to -11%. This is a wide latitude. This allows for variances in AC voltage, projector / motor gearing, and it's simply good practice. If you are actually hearing warbling on a regular basis, then there is probably something wrong... check to see if you hear it in analog playback (Dolby SR... Format 05). If you do, then something is wrong with your projector drive train. If it's okay in SR, then you may have a problem with your Cat. No. 773 (Dolby Digital film decoder) card.

-ken

Posted by Mark Gulbrandsen (Member # 72) on 12-18-2005, 09:51 PM:

quote:
+7% to -11%

Ken,
I knew it had wide speed lattitude but thats practically good enough to be able to hand crank in an emergency and still maintain SRD playback!

Mark

Posted by John Hawkinson (Member # 1135) on 12-18-2005, 10:10 PM:

I am confused by the focus of recent responses. Why does the digital decoder's speed tolerance have anything to do with warble? Of course digital needs to track speed changes in the film, otherwise the sound would be out of sync.

And if the speed of the film rapidly changes, producing a warble in analog, you'd see the same warble in digital. But it wouldn't be digital's fault.

If you exceeded the decoder's speed tolerance, then presumably you'd have dropouts, which are completly unrelated.

--jhawk

Posted by Ken Jacquart (Member # 369) on 12-21-2005, 04:41 PM:

John,

There were some "early" Cat. No. 773 cards (Dolby Digital film decoder) in CP650s that exhibited undue pitch changes because of some capacitor problems. If someone were to experience a described "warble", the first thing I ask them to do is switch to SR and see if it's still there. If it is not, then yes, it could be "digital's" fault (at least in this case).

Posted by Robb Johnston (Member # 239) on 12-21-2005, 11:29 PM:

are the tension arms bouncing? if so, you might find that the airpot is busted in the reader

Perhps the bounce is enough of a spee dup/slow down over and over again to cause it to cut in and out.

If that is the case call Dave H. for replacement