Film-Tech Forum ARCHIVE: How do I get 4 channel non-sync from a CP55?

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Posted by Robert Harrison (Member # 3202) on 06-18-2005, 04:26 PM:

Every so often, we show movies from DVD with a video projector. I have been hooking up the sound to the non-sync on the Dolby CP55. I could never figure out why the signal obviously goes through the Cat. 150 (all 4 LEDs light up) but the center channel does not go through to the output card. When we have a mono movie from DVD (like last December when we couldn't get a 35 print of "The Bells of St. Mary's"), I run the sound into the center channel mag input. But, with stereo tracks, I sure would like to be able to use the 2:4 decoder. Does anyone know how or if this can be accomplished?

Robert

Posted by Louis Bornwasser (Member # 3063) on 06-18-2005, 04:41 PM:

Look at the 242 card (which has a series of diodes installed corresponging to the format buttons) One of these diodes turns off the center channel in non sync.

I will leave it to others to be more specific, but adding or removing a diode turns on or off a particular feature.

If you make an error it is easy to "undo."

Louis

Posted by Ken Jacquart (Member # 369) on 06-22-2005, 04:37 PM:

To enable the center channel of nonsync on a CP55, do the following... On the Cat. No. 243 card, unsolder and lift one end of diode D84. You can't see the "D84" silkscreen as it is covered by the diode itself. I'll try and upload a picture of it's location.

Posted by Robert Throop (Member # 61) on 06-22-2005, 06:21 PM:

Ken,
How would one also enable the subwoofer for non sync on a CP55?
Thanks,
Bob Throop

Posted by William Hooper (Member # 71) on 06-23-2005, 12:35 AM:

Ken, this is excellent infomration.

Do you know of any techniques for suppressing the format change pop on the CP55 ?

Posted by Steve Guttag (Member # 268) on 06-23-2005, 01:08 AM:

Watch out on the Non-sync decode. The Cat 242 has Non-Sync route Left and Right straight through...no decoder. So merely lifting the "Inhibit Center" diode will not perform a true 2:4 decode. Center and Surround will be going through the Cat. 150 but Left and Right will not.

As for the format change "pop"...that is intrinsic to the CD4000 series CMOS chips used on the Cat. 242 (you will hear the pops on both the CP55 and CP65).

There is nothing easy you can do to supress it. Note, on the Cat. 240, where a CD4000 series chip is used for projector changeover, Dolby did an elaborate feedback loop to nix the pop so you would not hear the changeover. This was not the case for the Cat 242. It was always considered insignificant since often one is changing audio in non critical situations where the snap would be tolerated. This theory fell apart though when the DA10 came out and it had to switch between SR and Digital...it is one thing I truely hate about the CP65 (or CP55) with digital...the change from analog to digital has too many tell-tale signs....one is the "SNAP" the other is that the surrounds jump 3dB in optical.

Posted by Ken Jacquart (Member # 369) on 06-23-2005, 06:29 PM:

To answer your subwoofer question... On the CP55, to enable the SW duing a nonsync decode, unsolder and lift one end of diode D83 on the Cat. No. 243 card. It is located immediately to the left of the visible "D73" marking.

Posted by Steve Guttag (Member # 268) on 06-24-2005, 02:25 AM:

Ken, Why must you get this guy's hopes up?

By enabling the Subwoofer (lifting its "inhibit" diode) is only half the solution...you still won't have any subwoofer on non-sync. In the CP-55, the Cat 241 is only looking at the post NR Lt/Rt signals and the external subwoofer signal. The Non-Sync "Lt/Rt" line is coming in after the NR but on the 300mV inputs of the Cat 150 (the Cat 150 has both 500mV and 300mV inputs to deal with the various vintages of buss levels that Dolby used).

The Cat 242's job is to disconnect the signal of the Non-Sync while in film mode and to disable the film sound while in Non-Sync mode (it switches the Cat. 222 to "Dolby Tone" inputs meant for an external Dolby Tone generator). These inputs came into play for theatres when the SRA-5 (or other external SR adapter) were used in combination with a DA10/DA20.

So if you REALLY want the subwoofer(s) to play during Non-Sync, you need to get a summed signal of the Non-Sync Lt/Rt line that only is active when in Non-Sync. Furthermore, one would also need to ensure that the external Bass Extension line is clear of any loading when in Format 04 so the Cat 85 pink noise generator will allow one to properly calibrate the subwoofer level (it uses the external bass extension line).

So your best bet is a summing amp (to get the signals mixed and the levels right at 500mV nominal referenced to a 300mV nominal Non Sync level) and a relay that opens the line when ever one is not in non-Sync.

In short, some additional work is in order.

Now, if you don't have SR (shame on you) or if you don't have External SR...then the "Dolby Tone" inputs are available...I would perform the modifications as detailed in the DA10 or DA20 manual for use with an SRA-5...then send your non-sync to the Dolby-Tone inputs (soldered on the backplane) and come up with your own attenuation (pots)...this will give you a proper 2:4 decode, with subwoofer. Note, when the Non-Sync format is selected, that both the "Dolby Tone" and Non-sync input signals will be fed to the Cat 150 simultaneously so use one or the other or come up with a proper relay system to disable the "Dolby Tone" signal when the Non-Sync format is selected.

Again, in this discussion, "Dolby Tone" refers to an input name, not the actual warble tone.

Posted by Ken Jacquart (Member # 369) on 06-24-2005, 03:22 PM:

Thank goodness you saved me from spelling out all those words! Thanks... Yes, to get a true Pro Logic decode, one would come in through the Tone inputs on the NR card backplane and modify accordingly. I simply hesitate to get into such detail in a public fashion while worried that those who perhaps aren't so qualified (but have an internet connection) may start to hack up their CP and do some damage. [Wink]

Posted by Jason Miller (Member # 2341) on 06-24-2005, 06:13 PM:

Robert,

are you using the Projector 2 inputs for anything?

Posted by Michael Schaffer (Member # 1204) on 06-24-2005, 06:34 PM:

The problem with projector 2 inputs is that it will apply NR to the signal which will make it sound funny. Consumer sources are not SR or A encoded. I think the easiest and safest way for Robert to get adequate stereo is to get a DVD player with analog outputs and hook these up to the mag/external input like he already does for mono.

Posted by Robert Harrison (Member # 3202) on 06-24-2005, 07:22 PM:

Thank you all for your replies. I wouldn't be doing the rewiring work myself. I would get a tech to do anything of that nature. As for the projector 2 inputs, I figured that the noise reduction would be a factor, so I avoided that. Recently, we had a DTS unit hooked up into the mag inputs (mono surround, subwoofer out going directly to the amp), but I suppose I could piggyback onto those as the DTS would not be in use at those times. Dolby made their later CP65 capable of 2:4 decoding of non-sync. We have one of those in another house, but only once have we run video in there. One other item: I saw in an older post that the CP55 had a 4,500 hertz filter on the mag inputs to the surround channel. Does that mean that our current DTS set-up is going through that filter? We should get full response up to 20,000 hertz, shouldn't we? There were instructions in that post on how to bypass that filter. Has anyone else had to do this?

Posted by Gordon McLeod (Member # 33) on 06-24-2005, 07:28 PM:

A few years ago with a Cp55 and a SR adapter wired as per lonnys instructions in the DA20 manual we then routed the video into the NS input of the SR adapter with the correct diodes to turn it on to get it throught the matrix I no longer have the notes on how Lonny said to do it though

Posted by Robert Harrison (Member # 3202) on 06-24-2005, 07:42 PM:

Our SR adapter is wired up the old way.

Posted by Steve Guttag (Member # 268) on 06-24-2005, 09:18 PM:

Ken,

Don't get me wrong...I'm not endorsing the hacking up of a processor...in fact I was trying to point out that it is not a simple task to have a full 2:4 non-sync decode with subwoofer on a CP55...that many modifications are in order and that some external logic is also needed to properly pull it off. Note that I did not supply detailed instructions on this.

With an SRA-5 hooked up the way detailed in the DA10 or DA20 manual, then the Non-Sync inputs on the SRA-5 could be used as a viable 2:4 decode input.

Generally speaking...I go with the "if you can't figure it out, you probably shouldn't be doing it" attitude on these sorts of modifications. Because, if you can't figure it out...you won't be able to get out of trouble if you do something wrong.

Steve