35mm optical sound

[Javier de Blas]

Hi, newbie on 35mm screening here. So feel free to tell me how wrong I am doing things.

Let me explain to you my problem. I have a kinoton FP30 with reverse scan optical sound (it also has the dolby digital reader, but I don't use it for now). What I'm doing right now to get sound from it is getting the positive part of the balanced output (made a simple rca apadter) and connect it directly to my tx-sr875. Even though it does not sound bad at all, I noticed that I had to boost the sound like crazy to get a decent volume. In that moment I realized that the level output by the projector is way below line level.

Right now I don't want to get myself into buying a proper dolby processor since I would like to do that when I decide to upgrade to digital also buying all the proper necessary equipment like the test films from dolby, a sound pressure measuring device, etc.

Do you guys know of any way I can boost a bit the current stereo signal to feed it properly to my onkyo receiver?

Thanks in advance for any recommendations.

[Tj Hopland]

Optical is really close to microphone level so if you can't get a proper cinema preamp a mic preamp is the next best thing perhaps in the form of a mini sound board?

Most of the cinema processors had a way or could be modified to get 'pre outs' so you can get good results using the cinema pre amp and noise reduction but then route that Lt Rt now line level signal to your AVR. Even something as old as a CP50 could get you decent sound that way if you can find one really cheap. USL JS (not JSD) processors had terminals and links on the backplane so those are also easy to use as preamps. I would not spend much on any of that but if you can find it cheap it would get you going while you wait for the right deal on something like a 500 or 650.

[Javier de Blas]

Thanks a lot Tj Hopland, actually there's a CP45 for sale right now for just 50€. That processor would work just as preamp? Avoiding surround surround processing, just the straight left and right for the AVR.
I'm reading the manual and I think it would do what I want in dolby stereo A mode.
Please tell me if I'm right.

[Steve Guttag]

The CP45 has a suitable preamp (and noise reduction) in it. A mic preamp, as TJ noted also will have enough gain. Optical readers are based around solar cells that are at their best as current sources so a typical optical preamp will be a low-impedance source. You may find it sounds better on a mic preamp with a loading resistor across the input (somewhere in the 100-330Ω range. But if you are willing to get the CP45, you'll have what you need right there.

[Javier de Blas]

The CP45 has a suitable preamp (and noise reduction) in it. A mic preamp, as TJ noted also will have enough gain. Optical readers are based around solar cells that are at their best as current sources so a typical optical preamp will be a low-impedance source. You may find it sounds better on a mic preamp with a loading resistor across the input (somewhere in the 100-330Ω range. But if you are willing to get the CP45, you'll have what you need right there.

I'm getting the cp45 but I don't have an oscilloscope and after reading the installation manual I see that it's quite important for setting up the preamp level. I don't know if setting up all the cp45s configurations by ear it's going to be a good idea.
My plan it's to delete any kind of eq from the processor leaving all options at death 0 since I don't have a way to measure if I'm doing the right thing. Getting the 5.1 output from the processor and feed it to my AVR.
The same goes to any other levels or delay inside the processor, everything set to 0 to make as few changes as possible.
As I said my biggest fear is to actually set preamp level right since I don't even have a test film. My first idea is to a get a mono film (witch I do own a couple of) and play with the preamps levels until both front speakers sound at the same level in dolby stereo type A. Again, I don't know if it's a better idea to just set the processor to dolby stereo type A always and only send stereo to the AVR to process it or actually use the surround decoding of the processor itself.

This is going to be fun

[Marco Giustini]

To just set the level you don’t need a scope. You need a Dolby cat 69T and look at some LED’s in the 45.
a full A-chain will require a scope and multiple test film but maybe you can assume your reader is correctly aligned for now.

[Marcel Birgelen]

If you want to play stuff that's in Dolby A or SR, you should put in a proper processor.

If you have a proper 3 or 5 or even 5.1 channel setup, I'd do decoding on the CP45 and use the 5.1 analog in on the AVR, if it has any of those. It will be interesting to see how the AVR will handle the output of the CP45.

[Javier de Blas]

Reading the manual more properly I've seen that the only real thing I need to adjust the decoder preamp is a dolby mono tono (cat.no.69t) for the level of the preamp and the pink noise film (cat.no.69p) to adjust the high frequency boost. I'll need an oscilloscope for the second part but I guess any old thing will do.
Do you guys know anywhere I could find those test films? I've looked around ebay but couldn't find them. I know I'm getting a bit off budget for installing a lousy CP45 but at the end I will use that same test film on the cp500 when I buy it, so it's a future proven purchase.
Hope to here from you soon

[Javier de Blas]

Sorry, I was completely wrong, I don't need and oscilloscope, I need an RTA, sorry, got confused there.

[Marcel Birgelen]

Dolby used to sell those 35mm test reels, but I'm not sure they're still available via that route.

Alternatively, you could check Elia Orselli's webshop and check if he's willing to ship it to you. He's also a member on this forum, so you could try a PM. Or maybe saying his name three times will awake him and have him chime in on this very topic.

Older oscilloscopes are generally better than newer ones, as they have less delay. On the other hand, most modern digital oscilloscopes do have an FFT function built-in, saving you the money for an extra RTA... Having 35mm film as a hobby, is pretty expensive, but at least, at this day and age, it's achievable, as long as the equipment is still around.

[Josh Jones]

In the interim, you can use the phono inputs on your receiver. The phono input will have a large bass boost due to the RIAA equalization , but you can compensate for that by rolling the bass off. At least the level will be close to correct.

[Tj Hopland]

I have never tried to tap into a CP45 to just use the preamp / nr before. May be a little more difficult since the majority of the system is on one board. Previous designs tended to be several cards so even without detailed diagrams it wasn't too difficult to find what you were looking for on the backplane and make it work how you wanted it. At least the NR card is separate and was used in others like the 55/65 and 500 so maybe looking for the cp55 SRA5 upgrade manual which would show which are the audio pins on the card and you could then find those in the 45?

Does your AVR have a 5.1/7.1 input? If so and since this isn't likely your final solution maybe just use the 45 as is? IF you have the inputs it seems like it will be less work and the matrix decoder in the 45 was pretty good. It was really if you were dealing with Cat 150's earlier than an E or some of the lesser brands were the matrix was kinda lacking.

On the oscilloscope front what you needed for optical was pretty forgiving. I have not tried an alignment on any of the lower cost modern stuff but I would think most of it should work. It was SRD where things got a little more difficult especially before we had the WINDRAS app to aid in things. Focus was one thing you could not get out of the digital scopes at the time but you could get voltage and position out of them. Position was the big thing WINDRAS could not do accurately so you still needed a scope that could at least get you that much.

[Josh Jones]

Some of the modern digital scopes have FFT which would be usable for focus, as you can look at the video in the frequency domain. Kinda like optical stereo, tune for best HF response.

[Marin Zorica]

Decent sound card is all you need to do A chain. You can use some rta software like TruRTA and scope software, there is one very old on download section in forum. Than, just folow same steps as you would do with hardware RTA. For example my first hand interface when doing simple measurements is focusrite scarlet 2i2 which is USB and goes up to 192K sample rate......or you have behringer umc202 for 50EUR and similar characteristic, only lower headroom, but quite enought for set up soundhead.

[Javier de Blas]

At the end I purchased a cp500. I'll keep you updated in this thread about the journey. On the other hand, I need the digital cable to connect the digital camera from my kinoton fp30D to the cp500. Do you guys know of anyone selling them?