Film-Tech Forum ARCHIVE: Dolby remote fader cable length limit?

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Posted by Scott Norwood (Member # 30) on 09-08-2012, 07:52 AM:

Subject says it. How long can the cable run be to the Dolby remote fader with a CP650? Not a cat. 779 or 771, but a regular 100k pot. Is a 50' or 100' cable run likely to cause issues? Does the cable type matter?

Posted by Richard Fowler (Member # 893) on 09-08-2012, 08:07 AM:

We have done several @ 100 foot average using shielded multiconductor cable. I have a rental box with a pot with 75 foot cable for producers to set level for screenings.

Posted by Steve Guttag (Member # 268) on 09-08-2012, 10:35 AM:

The Dolby fader works by putting a resistance between the source and return and thus setting a current via a known reference.

Since Dolby uses 100K as their zero reference...the resistance of a typical cable at 100' (remember you have to multiply the cable's resistance times two to allow for the return journey) will likely be insignificant to the overall characteristic. The larger the gauge wire, the less of an affect. Also note, it will likely only play into the result as one approaches "10" and progressively have no effect as one approaches "0"

So a 20 or 22 awg cable will likely go well over 100 feet without any real noticeable difference. In fact, you will likely find that the particular Pot. to have a greater variance in the big scheme of things. Having worked with a great many CAT 204s with CP200s, it is nearly impossible to get all of the "7s" to agree unless you twist the POT some.

Posted by Louis Bornwasser (Member # 3063) on 09-08-2012, 03:17 PM:

I am assuming this is the same configuration as on earlier Dolby processors. In the 70s and 80s when I did a lot of world premieres, I had a regular xl socket on my dolby box (100k pot). This allowed me to use the existing mike snake installed in the house. Several of these had lengths approaching 500 ft with no problem.

Like Steve said, the reference might drift a little, but usually the Director or Mixer could care less; he was remixing on the spot. Louis

Posted by Scott Norwood (Member # 30) on 10-28-2012, 06:45 PM:

Bumping my own thread here:

I have concluded that I actually need a cat. 771 for this application. I have two cat. 779s in the projection booth, but want to add a fader at the FOH position, while allowing adjustments to be made at both locations at any time, without having to toggle between the local fader and the remote fader (this is a performing-arts house that occasionally screens films).

Can I just steal a balanced audio cable from the booth to the FOH location? Is there any reason why a 100 ft. cable run would be an issue? (The Dolby manual says that the total cable length needs to be under 300 ft, which it will be.) Is there any issue with mixing cat. 779s and a cat. 771 in one system, as long as the IDs are unique to each device?

Posted by Louis Bornwasser (Member # 3063) on 10-31-2012, 07:51 PM:

See my post above.