Film-Tech Forum ARCHIVE: CP55 popping during format changes

This is topic CP55 popping during format changes in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Josh Jones (Member # 447) on 11-16-2003, 01:51 PM:

This phenomena occurs during switching all formats, the optical changes are worse than the rest, but still noticable. Any ideas?

Josh

Posted by Paul G. Thompson (Member # 655) on 11-16-2003, 01:59 PM:

Seems as I had that problem once in Southsound. It has been so long ago I can't remember for sure exactly what it was. However, the format card (Cat 243) comes to mind.

[ 11-16-2003, 08:23 PM: Message edited by: Paul G. Thompson ]

Posted by Josh Jones (Member # 447) on 11-16-2003, 09:29 PM:

any other ideas?

JJ

Posted by Jack Ondracek (Member # 1466) on 11-16-2003, 09:47 PM:

Josh... (unless someone chimes in here...) there's probably not much you'll be able to accomplish in there... unless you can get schematics of the cards... [Eek!]

If the 55 switches between source opamps with unbalanced offsets, you can get that... dried-up caps... switching between outputs with mismatched impedances all can do this... not to mention the possibility the switching pulses might be inducing themselves into a gain stage somewhere in there (maybe bad caps here, too). Probably nothing you couldn't trace out... if you could get extenders & schemats...

On the other hand [evil]

that call to Dolby did the trick for me...

Posted by Josh Jones (Member # 447) on 11-17-2003, 07:15 PM:

Ok, so now what. This problem becomes really annoying when SRD is reverting to SR and back again. any other ideas?

Josh

Posted by Paul G. Thompson (Member # 655) on 11-17-2003, 09:35 PM:

Josh, I spoke to a tech today about your problem. He said it is normal for some older units to do that. We do have some good options to get it taken care of. [Smile]

Posted by Brad Miller (Member # 2) on 11-17-2003, 10:29 PM:

Can you describe the "pop" in more detail? (I've never had a CP55 produce any noise when switching formats.)

Posted by Paul G. Thompson (Member # 655) on 11-17-2003, 10:59 PM:

My experience with popping noises mentioned is similar to relay contact closure noise that is emitted into the audio. That's what I experienced in Port Orchard. It was low-key, but it is still there.

And Brad, I agree with you. It should not be there, period! Maybe Josh can more accurately describe what it sounds like. More than likely, I'll call Dolby Labs tomorrow.

Another description is of an electro-mechanical automation unit cycling if the wiring ground interface is not that great. As you sit in an auditorium, every relay closure can be heard in the sound system.

One question I forgot to ask....

Is it in all channels, or just some? That might shed some more light on the problem.

Seve Guttag, where are you?? [Smile]

Posted by Jack Ondracek (Member # 1466) on 11-17-2003, 11:17 PM:

Josh/Paul...
If the box is prone to this (although maybe raising input levels might help?)... does it have the same mute function that the 65 has?... and could that be used between source switching?

Posted by Paul G. Thompson (Member # 655) on 11-17-2003, 11:20 PM:

Jack, that is a very valid thought. Somehow, that slipped past my feeble mind.

I wonder if it could be a gain structure problem. [Smile]

Posted by Josh Jones (Member # 447) on 11-18-2003, 01:59 AM:

The sound is more of a snapping noise, not so much of a popping.

JJ

Posted by Daryl C. W. O'Shea (Member # 1303) on 11-18-2003, 05:38 AM:

quote:
Maybe Josh can more accurately describe what it sounds like. More than likely, I'll call Dolby Labs tomorrow.

Wouldn't it make more sense for Josh to call Dolby himself, if in fact he wants Dolby's help. As you said, Josh can more accurately describe what it sounds like. I'm sure he's perfectly capable of doing it. [Smile]

Posted by Steve Guttag (Member # 268) on 11-18-2003, 06:28 AM:

Every....and I mean EVERY Cat. 242 produces a snapping noise as it switches between formats. It is generated in the actual analog muxes...the CD4051 (I can't remember the exact number...there are three chips in the series...one of them is an 8x1 mux and that is what is used on the Cat. 242).

Some are worse than others but they all do it whether you have noticed it or not. It is a "Feature" of the card. In fact, "Dolby Snaps" are kinda of an inside joke for those in the know...

For the Cat 240 and 240A though they used the same series of chip (CD4053) for projector changover...to kill the "snap" they did a form of feedback kill it off...thereby admitting that they knew of the problem.

For the format card, the Cat. 242 it was not considered an issue since it would normally be switching between two sound sources and once the feature had started would remain where it was. The flaw in the plan came when the CP-65 came out and you have a DA10/20 connected and the snaps would "announce" a digital change.

Dolby may have run out of real estate to put in the feedback circuits for the format card since they used all "through-hole" technology...with SMT of today...it could be more effectively addressed...not to mention there are better MUXes out there. Note on the Cat. 441 that the DG series of analog muxes were being used. If the Cat. 242 were being designed today as an analog card...probably a MUX like the DG400 series would have been given the nod...it doesn't snap as bad in its natural state and could also have its snaps severely reduced.

Steve

Posted by Josh Jones (Member # 447) on 11-18-2003, 09:02 AM:

instead of using muxes, why didnt the build discrete VCA's to control the audio path? That would have been quiet and reliable.

It sounds like a nickname, it sounds like a snack, Dolby Snaps!! [Big Grin]

Josh

Posted by Steve Guttag (Member # 268) on 11-18-2003, 11:49 AM:

Previously, Dolby did use descrete FETs to do analog muxing. You don't need a true VCA persey...just bias the Gate of the FET and as you switch change the gate voltage using a simple RC circuit to control the time-constant for a cross fade. For a study guide...refer to Dolby's Cat. 108 and Cat. 201.

As to the descrete thing...for a cinema application...you would consume way too much real estate for what you are doing. Remember, in a CP-55 and earlier processor, you PC Card was limited to a very definate dimension to work with. You can't just add space without adding a card (like the Cat 150D and earlier)...then you bring with you all the challenges of moving the signals around between the two...for the Cat. 150 it wasn't too bad since only one channel was on the second card.

Steve

Posted by Paul G. Thompson (Member # 655) on 11-18-2003, 12:12 PM:

Thanks for the information, Steve. I can use that in the assistance of others who have the CP-55 should the situation arise. Your information is what I was looking for. [Smile]

Posted by Josh Jones (Member # 447) on 11-18-2003, 01:55 PM:

As for the discrete thought, I thought that discrete VCA's consisting of two transistors/fets and a couple of resistors could serve the purpose just fine. such a circuit consumes little space and power.

Another thought, are the muxes doing hard switching, or are they indeed crossfading. If they were not, couldn't you lengthen the time constant by adding capacitance to the control line , forcing it to crossfade?

Josh

Posted by Phil Hill (Member # 371) on 11-18-2003, 03:01 PM:

Josh, Josh, Josh my very good friend, why put up with all that digital crap and the headaches? Just run optical and sell both the CP-55 and the DA-20 and buy something you enjoy.... Ummmm, how 'bout another motorcycle? [beer]

>>> Phil

Posted by Josh Jones (Member # 447) on 11-18-2003, 03:28 PM:

Hmmm, you may have a point now phil, after all, I'm sure that damn card would make a lovely kickstand plate [evil]

Josh

Posted by Phil Hill (Member # 371) on 11-18-2003, 03:42 PM:

Yes Josh, it would be "lovely".... [Razz]

>>> Phil

Posted by Steve Guttag (Member # 268) on 11-18-2003, 05:27 PM:

Josh,

For each input into the Mux, you would need a FET. You would also need the logic circuit to control the gate voltages and some additional capacitors and resistors (the time constant for raising/lowering the gate voltage will not necessarily be the same)...you are now up to some 8 FETs plus the resistors and the capacitors...compare that to the footprint of the CD4051...a 16-pin IC with an inhibit line. You need just a 3-bit binary control circuit that can be TTL level switching with the audio signals having a swing of 15 volts.

As to the switching on the CD4051...it is pretty much instant (I think the switching time is in the nanoseconds).

As to slowing down the logic...that won't help you...you would only be slowing down the reaction time of when you select until when it switches...once the prescribed level is reached...bang, it switches.

Steve

Posted by Josh Jones (Member # 447) on 11-18-2003, 06:34 PM:

I seem to recall that the dual projector expander, cant remember who makes it, used a pair of muxes to crossfade the audio from one machine to another. How was this accomplished?

Posted by Steve Guttag (Member # 268) on 11-18-2003, 10:29 PM:

Using feedback circuits and/or controlling the charge on the in/out of the Mux you can effectively silence them. It just requires more circuitry and board real estate.

Steve

Posted by Phil Hill (Member # 371) on 11-28-2003, 08:11 PM:

So Josh, WTF is the progress/final disposition?

I have 758 customers that want to know the answer... Shall I just recommend they get a CP-200? How about a CP-65? Or a CP-500? Perhaps a 650? Or just junk it and get a bike? [beer]

Or suggest that they just go to (hell) and get SDDS? [thumbsdown]

Hope to see you again at xmas....

>>> Phil

Posted by Gordon McLeod (Member # 33) on 11-28-2003, 10:38 PM:

Simple solution buy a StarScope [Smile]

Posted by Phil Hill (Member # 371) on 12-01-2003, 06:40 PM:

YooHOO! JOSH! WAKE UP! See above! Inquiring minds want to know all!

Why are you ignoring us? And don't give us some lame excuse like "I've been busy", or "I'm bored", or "I'm only ignoring you", or "It's soooo cold here I froze my balls off", or...whatever! (as Adam would say)

>>> Phil

Posted by Phil Hill (Member # 371) on 12-03-2003, 08:31 PM:

Ummmm...JOSH!!!!!

SEE ABOVE and ABOVE!

>>> Phil

Posted by Adam Martin (Member # 641) on 12-03-2003, 09:29 PM:

Josh can't come to the phone right now, his tongue is frozen to the sign post out front.

Posted by Phil Hill (Member # 371) on 12-03-2003, 10:13 PM:

Adam, you mean the one that sez "Place Tongue Here"?

>>> Phil

Posted by Josh Jones (Member # 447) on 12-04-2003, 09:23 AM:

Frankly, I vote for a CP 65, I've got a starscope. They suck

Josh

Posted by Steve Guttag (Member # 268) on 12-07-2003, 09:48 AM:

The CP-65 uses the same Cat. 242 (only some revisions later) as the CP-55...You'll have the "snaps" on it as well.

The CP-50 was silent..the CP-200 can be silent. Some muxing was done with descrete stuff (Cat. 108 and Cat 201)...some was done with TDA series chips (Cat 141). Most were pretty silent but I have had a few that popped.

The CP-500 "thumps" as it switches formats and has an annoying habbit of reselecting the current format when you come out of the menus making it hiccup.

The CP-650 seems a bit better all around in the noiseless switching department...especially with signal present....gotta love that D I G I T A L technology!

Panastereo seems pretty good too though if you pay attention without signals present...you can hear it too...I've yet to pick it off with anything playing.

Steve

Posted by Jean-Michel Grin (Member # 1360) on 12-08-2003, 07:44 AM:

Tell to the barman at hte candy bar to no "popping" pop-corn during the show [Big Grin]

Just kiddin' guy !

Posted by Frank Angel (Member # 248) on 12-08-2003, 01:42 PM:

What Steve said. We have a CP50 = not a sound, and a CP55 = popitty pop pop pop.