Film-Tech Forum ARCHIVE: Modifying a CP55 for Drive in use

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Posted by Jon Bartow (Member # 241) on 05-08-2003, 09:38 AM:

OK here is what is required, and what I have to work with

· Drive-in with two screens running reel-to-reel on both screens
· Currently mono sound on both fields for both FM and the pole speakers
· Owners want Stereo for the FM while keeping the mono for the speakers
· They also require non-sync and a microphone for announcements
· Existing sound system includes Altec 1598B’s for field speakers, Kelmar AS-8800 for the preamp and exciter supply, and a no-name stereo FM transmitter (up until last summer a complete Century JRA system was in use with AM transmitters using the field speaker wiring as the AM antenna)
· Owner has two CP55’s and two stand-alone Kelmar exciter supplies for use in this project

Here’s the problem: We need a straight L/R unmatrixed signal (Post NR) for the FM transmitter AND a summed mono signal for the field speakers at the same time as well as being able to switch to N/S (format 60) or a mic (using format 22 mag inputs with a mic preamp)

Here’s what I have come up with:

Use format 03 (Dolby Stereo Optical with no surround) and add a diode to the 243 card in the F1 line for “Stereo Decode Inhibit”. This will create a mono signal on the Center channel, but leave the L and R connected to the cat 150 matrixed L/R outputs not connected to the center mono channel like format 01). Then on the backplane cut the trace from the cat 150 pins 4 and 6 (L/R matrixed out) and add wires from Cat 150 pin 10 to cat 242 pin 27 and cat 150 pin 7 to cat 242 pin 26 (thereby feeding Lt/Rt into the 242 card instead of L/R, giving Lt/Mono/Rt on the L/C/R main outputs, and still giving me NR and EQ on all three channels)
For the mic I will use format 22 mag inputs and just connect the mic preamp output to the L/C/R mag inputs. Thereby giving me an announcement mic across all three channels.
For the Non-Sync, the L/R N/S inputs are connected directly to the L/R inputs of the cat 242 card and to the cat 150 card. By adding diodes to the 243 card for format 60 (line F6) for Surround inhibit and stereo inhibit I should get the same setup as the modified format 03 (Lt/Mono/Rt). (possibly having to add a diode for N/S decode)

I think I’ve got everything right, but for those of you who can follow what I’m doing, please double-check me. The only thing I’m not sure about is the N/S. According to the “CP55 Signal Path Block Diagram” the Center channel doesn’t connect to anything for the non-sync, but one can set up the non-sync to go through the matrix so I can only assume that the aforementioned diagram just doesn’t show it.
Also which lines on the cat 243 card are for “Stereo Decode inhibit”, “Surround inhibit” and “N/S decode”? I think that they are the lines labeled “SPR”, “SURR” and “NSC” respectively.
And, obviously, leave a detailed description/diagram of what I did with each unit for "the next guy."

Thanks
Jonathan

Posted by Barry Floyd (Member # 385) on 05-08-2003, 02:25 PM:

You just described a SMART MOD IIB-DIT Processor almost to the letter!!

SMART MOD IIB-DIT

I just bought a brand new unit about 2 weeks ago from a dealer for $1250.00 US + shipping. From the time I ordered it until the UPS guy dropped it off at the house was less than 2 weeks.

It does everything exactly like you described above.. without cutting traces and adding diodes and stuff. Right out of the box... it will do what the owner wants to accomplish. The manual for this unit is available here on Film-Tech.

Contact me by email if you want to know where I got mine.

Posted by Jon Bartow (Member # 241) on 05-08-2003, 05:13 PM:

Yeah, I know about the Smart unit (I like it very much in fact). Unfortunately the owner of this DI is not willing to shell out the 2k+ for two new processors. He is trying to do this as cheaply as possible. Hence the mods to his spare CP55's. If I had my way I would be installing two of those Smart units and be done with it. Oh well.
Jonathan

Posted by Ken Layton (Member # 133) on 05-08-2003, 06:06 PM:

Why not have the owner sell or trade the CP-55's? It's just too much work to hack up perfectly good CP-55's. The Smart Model 2B-DIT is an excellent drive-in processor and has SR style noise reduction. I'll bet the 55's getting hacked don't have SR cards.

Jack Ondracek is using 3 of the Smart Model 2B-DIT's at the Rodeo 3 drivein and I can say without a doubt it is an excellent and very versatile processor. It has really made the sound there excellent!

Posted by Robert Golding (Member # 746) on 05-09-2003, 02:38 AM:

I've run my drive in for over 25 yrs. I use to have speakers but went to f.m. & a.m. in 1986. I refurbished it in 1998 and went from mono to stereo by converting over to a cp50 with some mondifications. I also put a led. reader in at that time. The improvement over the old system was fantastic. I for one donot understand why anybody would still be using speakes. I reduced my electric bill in the 80's by over a $100 per/month when I turned off my two 100 watt amps. Also I no longer run a.m. as it just didn't have the quality of an f.m. sys. I'll bet your owner is any older gentleman and hasn't really heard just how wonderful the current f.m. systems are. His patron count will go up and his maint. cost will drop dramaticly. I for one donot miss the hrs I've spent on the field repairing speakers. The cost of taking care of the field and speakers will pay for the f.m. systems the first yr.

Posted by Frank Angel (Member # 248) on 05-09-2003, 04:35 AM:

Jon,

How about this. Instead of cutting up CP55s (you can hear guys out there already groaning at the thought), use a small stereo mixer such as the Shure PSM-20 (not absolutely sure of that number here at home, but I can check on it for you later) or any other inexpensive 4 in 2 out mixer with pan pots for each input. The L/C/R/S outs from the CP55 are fed to the mixer's line inputs. The Left and Right input channels are panned to the L and R outputs; the Center and Surround channels are panned center and fed equally to both L and R outputs. This allows you to get proper levels, especially for the surround, which might need to be mixed under the main channels a bit. Mono easily can be grabbed from summing the L/R Headset output.

Or you can use the Sony Virtual Surround Headset unit which is a nice compact processor that will accept L/C/R/S from the CP55 and give you a L/R output but with the surround channel matrixed using Dolby's proprietary Virtual Surround technology. This technology is used primarily for computer speaker sound systems where the person is in a relatively small space in front of two speaker and for headsets where the listener is always in the "sweet spot." The processor creates the aural illusion that sounds are actually coming from behind the listener. Your boss can then claim that he has "The first Drive-in theatre in the world to use Dolby Virtual Surround.....Hear 360o Surround Sound in your car more dramatically than is possible in indoor theatres!" The marketing potential is off the scale.

For a mono sum, you can always use 33k resistors off the L/C/R CP55 outs tied together into a buffering winkie mic amp.

Frank

Posted by Gordon McLeod (Member # 33) on 05-09-2003, 09:23 AM:

Mono summing is not a good idea since it will be subject to all the nastys of the gain riding in the matrix
I would sujest that you locate the output of the nr section and send that directly to the aux input terminals of l&r and the processor output of L&R to the FM transmitter that will provide the NR to LT and RT directly to the transmitter and the output pots on the output card give you a location to set the gain
For the field speakers I would locate the output of the mono section and wire it through the centre aux path

Posted by Jon Bartow (Member # 241) on 06-05-2003, 11:00 PM:

OK after talking with Dolby and going over everything again and then actually trying a couple of different things here is what I did just in case anyone is curious:

Cut output trace from Cat 150 L out and R out (pins 4 and 6)
Jumped cat 150 Lt and Rt ( pins 7 & 10) to cat 242 L & R inputs (pins 27 & 26)
On cat 243 lifted one side of diodes D84, D59 & D30
Sent mono Non-Sync signal to both N/S inputs(L/R)
Mic was tied to L/C/R mag inputs (mic preamp had a level control)

All this means:
Format 03 has been modified to create a mono signal present at the Center channel output.
A straight unmatrixed (but post NR) Lt/Rt signal present at the L/R outputs
Format 22 used for the mic (in all channels)
Format 60 for non-sync (operates same as format 03 unmatrixed L/R and center mono)
All eq cards are active as well as n/s level controls and master output level controls.
For the field speakers we had to cut out most of the extremly low bass (everything under 200Hz) as it was causing the speakers to distort (not that those little 3" speakers could reproduce those freq anyway) and trim the extreme highs a little bit.
Side-effects: Use format 01, field speakers only - no FM, use bypass - FM only no field speakers, oh well you can't have everything.

End result was far better than I was expecting. It actually sounds really good from both the field speakers and from the FM.
Jonathan