Film-Tech Forum ARCHIVE: CP50 Alignment questions

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Posted by Sam Hunter (Member # 1119) on 03-12-2003, 10:11 PM:

My CP50 project is getting down to the nitty gritty and I came across some things I am not sure of.
1st off, adjusting the equalization on the CAT64. The manual states that you turn down all CAT64 input controls to minimum. I have not found where you adjust them any more after that. Also, there are four adjustments on the front of the CAT64 marked HF, GAIN and the other set is marked INPUT, OUTPUT. I haven’t found any reference to GAIN, HF so what is it used for and should it even be touched?
2nd, I do not have a CAT85 Pink Noise card so could I use a PC based pink Noise Generator instead and just clip lead it into each input and would it need to be terminated?
3rd, I am considering purchasing a sound pressure meter from Radio shack that has an output on it that I think will serve as a MIC as well.
Also, my screening room is around 20x20 so would there be any special considerations in setting the response and surround delays?
Thanks!

Posted by Steve Kraus (Member # 476) on 03-12-2003, 10:39 PM:

The Cat64 contains some vestigial circuitry that are not used in the CP50 (or any CP) inplementation. HF and Gain are part of a head EQ circuit (like the slit loss adjustment on the pre-amp card).

You might be able to use PC pink noise. I don't know about termination needs. The RS SPL meter is good for setting levels but don't expect a flat response from the mic when connected to an RTA. If you are really desperate and have access to a properly tuned theatre you could take your setup there while someone runs pink noise and trace what odd graph you see displayed (knowing that it represents flat response) then go back home and try to match it. Very crude.

Posted by Mark Gulbrandsen (Member # 72) on 03-12-2003, 10:55 PM:

Sam,
Yes, you can use the PC based PN generator as long as it is a good broadband pink noise. There are many inferior generators out there.....
Mark

Posted by Steve Kraus (Member # 476) on 03-12-2003, 10:59 PM:

Or simply download .wav file of pink noise.

Posted by Manny Knowles (Member # 1171) on 03-12-2003, 11:01 PM:

Something to consider about trying to use the mic from the Radio Shack meter with an RTA...

You'd need to know if that output is before or after the built-in A-C weighting.

Also, I'd think you ought to have a mic that is matched to your RTA unit.

Posted by Mark Gulbrandsen (Member # 72) on 03-13-2003, 08:55 AM:

Or simply download .wav file of pink noise."
____________________________________________________________

If one is doing important EQ work I would consider downloading an unknown WAV file for this use to be reckless and meaningless unless the PN is first tested for its quality. Especially from something like a heavily compressed wav file. Its doubtful that even the best WAV file decoding could properly reconstruct a really high quality PN signal which is a random signal in and of itself. It may sound like PN, but I guarantee you that its not really. Most audio PC cards are pretty lousy too. If one has to use a pre-recorded PN source at least use one from an SACD, or DVD audio disk that is capable of giving decent results. I fear that even PN from a regular CD is not even good enough. It would however be better than using a WAV file source.....see this link......
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2515156091&category=23788

Again, like it was mentioned in another previous thread, supposed high quality PN such as everyone thought was in the Sony Processor can turn out to be not so good and you can end up with meaningless eq curves and having wasted alot of time.

While many use the Cat 85 cards for Dolby EQ the PN out of the C.E. card is actually quite a bit better. Both are acceptable but I've always used the C.E. card at Dolbys reccomendation. I would definately reccomend that Sam aquire the C.E. card. Its a handy tool and he seems to want to take his hobby into the next higher realm anyway...If one has gone this far the rest of it is worth doing the right way!!
Mark @ CLACO

Posted by Christopher Seo (Member # 7) on 03-13-2003, 11:05 AM:

Sam,

I think the input controls on the Cat. 64s stay at minimum (with the "Head Equalizer" controls unused) and you only adjust the output controls, per section 7.2.5.e-g on page 7.12.

Posted by Darryl Spicer (Member # 711) on 03-13-2003, 11:17 AM:

I wouldn't use the RS preasure meter for anything but a rough reading of the preasure levels. I couldn't find the instructions for it but if memory serves that output is for re calibration of the preasure meter. Not for sophisticated measurement of EQ cards.

Posted by Gordon McLeod (Member # 33) on 03-13-2003, 02:06 PM:

THose two pots are also used in the alignement of a CP100

Posted by Fred Georges (Member # 505) on 03-13-2003, 03:13 PM:

Here's a link to a mod that will make the Rat Shack meter more accurate & usefull. Eric Wallin's Homepage

Posted by Mark Gulbrandsen (Member # 72) on 03-13-2003, 07:30 PM:

Why not hunt around at Toronto Science and Surplus for a nice used Bruel & Kjer SPL meter like Gord and I did a few years aback.....They were only around 75 bucks if my memory serves me correctly.
Mark @ CLACO

Posted by Steve Kraus (Member # 476) on 03-13-2003, 09:59 PM:

Mark, if we're talking about an uncompressed .wav file it's going to be no different than running a software-based pink noise generator. Either is going to be delivering the same digitized PN to the soundcard's D to A circuitry and subject to the same limitations of that and the analog circuitry on the card. Such a file I downloaded sounded fine and looked fine on the RTA.

I would think that ideally one would want a purely analog PN source but then again surely the current generation of digital CP's generate their PN in software too.

Posted by Sam Hunter (Member # 1119) on 03-13-2003, 10:44 PM:

I have seen circuits in articles showing how to make an PN generator. I will probably use the RS presure meter just for setting and comparing levels and use a good quality mic for my PC based RTA. Again, what effects or peculiarities should I expect in my 20x20 screening room? Should I make the responce brighter due to the smaller area or go by the book?

Posted by Tom Wienholt (Member # 1510) on 03-13-2003, 11:40 PM:

The CP50 manual says to adjust the center Cat64 for 210Mv signal and the left and right Cat64's for a 150Mv signal while dolby tone film is running. Is this that important? If so, where do you get a millivolt meter to make this adjustment?

What if the CP50 has Cat64B's which have no output adjustment on them?

Posted by Aaron Sisemore (Member # 145) on 03-14-2003, 02:08 AM:

quote:
If so, where do you get a millivolt meter to make this adjustment?

Most if not all modern multimeters have a millivolt range on them.

quote:
What if the CP50 has Cat64B's which have no output adjustment on them?

In the case of 64B's I believe you can skip the parts concerning controls that the 64B doesn't have.

-Aaron

Posted by Mark Gulbrandsen (Member # 72) on 03-14-2003, 08:01 AM:

Steve,
The C.E. generator does use digital I.C.s to generate PN as far as I can remember. I'll look at mine and see what chip it does use.....probably a 74LS series or CD series.... The main difference in the quality of PN generators is usually the quality of the filters on them. Thats what makes the C.E. better than the Dolby CAT 85.
Mark

Posted by Sam Hunter (Member # 1119) on 03-14-2003, 12:44 PM:

What great info so far. Thanks guys!
Here is my setup right now, not the 6 channel amp in the bottom. Its an old Dolby Pro-logic processor that the logic chip is out on and I just wired into the amps themselves. Works pretty good, 25 watts per channel but thats plenty for now.

Posted by Gordon McLeod (Member # 33) on 03-14-2003, 12:58 PM:

You can often find the CEL SPL meters on ebay at reasonable prices

Posted by Tom Wienholt (Member # 1510) on 03-14-2003, 05:39 PM:

Most of the voltmeters I have seen in stores only make readings down to 400Mv, but the readings would have to go down to about 100Mv to set up the CP50 according to the manual.

Posted by Chris Markiewicz (Member # 1415) on 03-14-2003, 06:12 PM:

I have found that as well as a digital multimeter, it is good to have a real analog VOM like a Triplett 630PL.

Posted by Michael Schaffer (Member # 1204) on 06-09-2003, 02:25 PM:

quote:
Thats what makes the C.E. better than the Dolby CAT 85.

Can the C.E. pink noise card also be used in a Ultra Stereo processor?

Posted by Sam Hunter (Member # 1119) on 06-09-2003, 03:18 PM:

In the continuing saga I decided to go the cheap and dirty route which worked out pretty well.
First my equalization sounds good enough for me. I have ran several films as well as the cat 251 listen test film and am happy with the response. I did bring up the bass control up on my center channel a bit. As far as auditoroum levels go I went with the following setup and achieved pretty decent results.I first bought me a Sound Level Meter from RAT Shack and then I downloaded a Pink Noise generator program and recorded the output to a cd. I then used my portable CD player as the noise source and was able to get the required 600mv noise at the point where a cat85 would go. I set my fader at the 2:00 position and marked the spot with a "7" and set my levels on each channel. I then set my mono levels as well as the bypass level. After that I set my Non Sync levels to 2/3 that main level and retested. Sounds great to me.
Now if I can just get the screen setup and my lenses to fit.

Posted by Gordon McLeod (Member # 33) on 06-09-2003, 10:54 PM:

Ultra Stereo makes a PN module that uses the same circuit as the CE cat 85 for their processors