Film-Tech Forum ARCHIVE: CP65 random buzz sound

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Posted by Bernie Anderson Jr (Member # 410) on 12-23-2010, 05:50 PM:

Hi guys,

I'm running a CP65 (with the Cat 300 cards installed) and a CE Reverse Scan ASR-30 reader in a RCA 9050.

I'm getting a weird random buzz that comes and goes and seems to happens when there is dialog or effect present in louder sequences. Its kind of hard to explain. Its not all the time. It seems to be present in all channels, particularly in the center. It almost sounds like a ground hum at some times and its comes and goes fairly quick with in a second. It isn't reading the sprockets of the film nor DTS time code.

I checked the A chain and slit loss, checked and re-seated all the optical cards.

On Digital (DTS), it sounds absolutely fine, only bad in analog. Switching inputs from Proj 1 to Proj 2 makes no difference, still present. I checked all connections. The Reverse scan is isolated from the projector and is "floating" and the Reverse scan is quiet when there is no film running through it, so its not a ground. Makes no difference if the 300 cards are in or not (It has the SR/A Cat 222 card too). It only seems to happen on high gain parts (almost like the cell input is too hot and is clipping, but its not).It's present in both A and SR but is worse in SR. Haven't tried it in mono yet. And it happens on different film stock.

Could it be my A-chain alignment? When the reader was in the theatre, I don't remember having this problem.

All I know is its in the A chain stage of the processor. Any suggestions?

Thanks

Bernie

Posted by Steve Guttag (Member # 268) on 12-23-2010, 06:14 PM:

Have you tried running Cat 566 and verifying that your illumination is even? This could give you the clipping like sounds on loud passages (and likely a shift in the stereo balance too).

To use Cat 566, you will need an analyzer that will let you display "A+B" so that left and right channels are mixed on the display.

Adjust the LED for the MAXIMUM level overall (vertically) and the most even amongst the 6 frequencies (laterally). It is not uncommon for the middle two frequencies to be the highest but you should see all six with the far left and far right being about the same level.

-Steve

Posted by Bernie Anderson Jr (Member # 410) on 12-24-2010, 12:06 AM:

I think I have some of that test film at work. Is there another workaround if you don't have the film?I think it's in my alignment too

Posted by Sam D. Chavez (Member # 1841) on 12-24-2010, 01:49 AM:

It is possible the Dolby Level meters on the 222 card are not metering correctly. This might exlpain the clipping.

I would confirm with a voltmeter that Dolby level at the output test points of the preamp is even between left and right channels at 300 mVolts running Pink noise test film.

Posted by James Falloon (Member # 1933) on 12-24-2010, 04:56 AM:

Stray light getting into the soundhead? Cover up the window and turn off the projection room lights, see what happens.

AC ripple on LED? Pretty unlikely but one more thing to check and eliminate.

Posted by Pete Naples (Member # 761) on 12-24-2010, 05:08 AM:

LED knackered, drive current max'd out, pre-amp max'd out? SO there's no headroom left in the front end?

I seem to recall the CP65 is 'deafer' than CP55. Or maybe my memory is malfunctioning, again.

Posted by Tony Bandiera Jr (Member # 2365) on 12-24-2010, 12:57 PM:

quote: Pete Naples
I seem to recall the CP65 is 'deafer' than CP55.

What???

Bernie, possible issues include:

Bad optical preamp card;
Defective reader (My most likely suspect);
Bad lateral LED alignment as Steve pointed out;
Bad CAT 222SR/A card (another strong possibility as they are crap);
remote possibility of bad CAT 249 power supply reg card.

If this came on suddenly then I would try the reader and CAT 222 card first.

Sam, when you have both the 222SR/A and CAT 300's installed, which cards handle the optical SR? I had thought that with a 222SR/A it would still do optical SR, but with a 222A the 300's did optical SR...

Posted by Steve Guttag (Member # 268) on 12-24-2010, 01:25 PM:

Tony,

The Cat 300 or 350 take precedence over the Cat 222SR/A (thankfully).

-Steve

Posted by Bernie Anderson Jr (Member # 410) on 12-24-2010, 02:55 PM:

And it does it even with the Cat 300s installed.

I took a look at the LED alignment on the reader. I haven't had a chance to play with it, but it seems that the LED is sitting a little bit below the center of the reader. I think the whole a-chain is off, where one thing being wrong led to another thing being wrong, even though on the scope it looks fine.

Posted by Sam D. Chavez (Member # 1841) on 12-24-2010, 02:55 PM:

Steve is right except the metering is still done by the 222 if I recall correctly.

Would not bet my life on my recollection in this instance.

Posted by Steve Guttag (Member # 268) on 12-24-2010, 03:36 PM:

Your life is secure on this one. The Cat 300/350 does not have the typical meters either. Also, A-type always uses the Cat 222x.

Note...anyone not using Cat 566 when aligning an LED based soundhead is "doing it wrong." Cat 566 will quickly allow one to know if the LED is centered both vertically and horizontally.

-Steve

Posted by Jeremy Weigel (Member # 4208) on 12-24-2010, 08:31 PM:

quote: Steve Guttag
Note...anyone not using Cat 566 when aligning an LED based soundhead is "doing it wrong." Cat 566 will quickly allow one to know if the LED is centered both vertically and horizontally.

Hadn't even heard of this test loop 'til this thread. How do you use it?

Posted by Steve Guttag (Member # 268) on 12-24-2010, 09:19 PM:

Cat 566, originally developed to allow one to precisely set up an exciter lamp, actually has proven to be one of the more useful tools when setting up a reverse scan reader.

The film has 6 tones recorded on it with 6 unique stripes. Thus, three tones are on the left channel and three tones are on the right. Each tone is one octave or so about the next. So, on an analyzer, you will six but three frequencies on each channel and if you sum the two channels, you will have six frequencies displayed. The lowest frequency plays on the left channel and it represents the left edge of the scanning area. The highest frequency is on the right channel and it represents the right edge of the scanning area.

So, when you look at the analyzer, you should see all six frequencies and the lowest and highest frequencies should be at approximately the same level and ideally all six frequencies are at the same level (presuming the film is recorded properly and you have 100% of the scanning area, you lens is even across its field...etc). The realities are, you will see the center two frequencies higher than the remaining four...depending on the lens, it will be more pronounced too.

With this film running, you can set the vertical (all frequencies get louder/softer) as well as the lateral position (outside frequencies get louder/softer). When you maximize all six frequencies, you have your LED optimized in its placement. Believe it or not, it can make a HUGE difference.

This film will let you see the flaw in those designs that do not allow for LED adjustment...particularly on Century soundheads...their machining tolerances varied GREATLY over the years...I have had to file or shim fixed LED mounts to get them to center on the film whilst keeping the lens centered on the cell. In my opinion, Component Engineering beat everyone on on this aspect as their LED mount can overcome poor machining by the soundhead manufacturer. Remember, most of these soundheads were only designed for optical mono. One did not have to be too good to get an mono cell to read the film. For reverse scan and digital, you really want to get things where they belong.

-Steve

Posted by Louis Bornwasser (Member # 3063) on 12-25-2010, 11:05 AM:

Cat 566: a little more accurate than Buzz. If 566 looks good, you will not get dts "buzz." Louis

Posted by Steve Guttag (Member # 268) on 12-25-2010, 05:30 PM:

Buzz and Cat 566 are different tools. Buzz defines the width of the track. Buzz, which is camera original, is going to be more accurate than Cat. 566.

That said, they are interactive. If you don't have the LED positioned correctly, then you will not hear one of the tones (or not hear it as soon as you should). As such, one should alternate between the two until they both agree that the film is centered with respect to the LED (or vice-versa). Cat. 97 (alternating track) has little purpose in the reverse scanning reader other than to verify that the cell is centered with respect to the track (like it did with forward scan). The only reason it would be needed is if one had to accommodate a grossly shifted LED/Film. It can be run as a final check though. Once Cat 566 and Buzz agree that the LED and film are correct, run Cat 97...there should be ZERO cross talk and the two tones (one on each track) should be fully modulated and not clipped. Cat 97 can be quite useful for setting the analog magnification. Those theatres that run older prints that may have shrunk are advised to overscan a little bit. It is very rare that the optical track is actually modulated to 100% so overscanning to 102% or so will normally not bite you anymore than did did when it was being done on forward scanned with slit lenses and cells. The benefits are that one is less likely to pick up noise, the edge of the doped area, guard bands or even the DTS track. It is never advisable to underscan (show more than 100% of the track as the cell will see any of the junk out there)...so Cat 97 can be used to see if you are underscanning (push the film either way and the corresponding tone should start to clip. Buzz track (a more accurate test film) can also be used to check this too. If you are seeing both tones on the O'scope...you are under scanning. Cat 566, like Cat 97 will let you see if you are overscanning with Cat 97 being easier to interpret than Cat 566.

-Steve

Posted by Bernie Anderson Jr (Member # 410) on 12-27-2010, 10:32 PM:

UPDATE:

Thanks for all your help. Although, I'm thinking now it has more to do with the condition of the track on that one print and how good the CE reverse scan reads the track. I could reproduce it slightly with the same print on the second machine using a standard Exciter lamp, slit lens and Solar Cell.

Anyway, I took into consideration everyone's suggestions. I first had checked the LED alignment, although I do not have any CAT 566 test film, I could see that the LED was off center. I aligned it as best as I could using a small mirror to see the placement of the LED on the track. I think there is some 566 test film at the theatre, so I'll be very interested in trying it out.

I cleaned up some of the audio wiring.

But using Sam's suggestion, I put a Digital Multimeter at the test points on the Cat 240A and the Cat 150 and set the meter to VAC. With the Dolby Level set using the LED meters, I was at around .425 and at the Cat150 it was .475. I don't know if this is correct.

But in the end, I played all different kinds of SR film and I couldn't reproduce the sound that I was getting with this one print. I only heard it on one other trailer and that too was kind of beat.

Posted by Glen Rich (Member # 1150) on 12-30-2010, 07:53 AM:

Just been sorting out a similar sound issue... which had nothing to do with any failings in the projection or sound equipment, (apart from filtering).
The low frequency buzz was being caused by the switchmode power supply in the computer/server, (these power supplies pump out a lot of electro-magnetic interference at audio frequencies), for the multimedia projectors we use for the ads.

Posted by Richard May (Member # 2627) on 12-30-2010, 08:13 AM:

This sounds pretty much like the problem I was getting when the reverse scan readers first came out and we were installing them everywhere. We were getting the same sound from prints. On quiet scenes, there would be no distortion or buzzing. When there was dialouge or music, there would be this weird buzzing sound that went in perfect timing with the dialouge or music. We couldn't figure out what was happening until one day. 90% of the prints that were doing this were using super platters. In one of the rollers on the brain, the film would slip out and it would ride in such a way that it rubbed the soundtrack. That was what was causing it. In another situation, a different theater had a loose intermittent shoe on a simplex. The shoe bounced back and forth rubbing the soundtrack. Any little scratch or mark on the soundtrack "confuses" the reader and it comes out as that sound.

Posted by Bernie Anderson Jr (Member # 410) on 12-30-2010, 10:31 AM:

That's exactly what it is. And if you look at the track closely on this one print, or any print that did it, would have a scratched sound track. When I played something with little or no wear I wouldn't get that result. But I did learn a lot from this thread, though.

My only question is still, when I use a multiple meter at the test points, I'm getting around .400 volts when the Dolby level is set to the led meters. That's not correct, right?

Posted by Sam D. Chavez (Member # 1841) on 12-30-2010, 10:44 AM:

Assuming you are running Cat. No. 69T Dolby tone test film, it should read .270 to 300 mVolts. This is the official 50% modulation test film.

Multi-meters are all over the place on tiny audio signals.

Posted by Gordon McLeod (Member # 33) on 12-30-2010, 08:04 PM:

Also many cheap meters have issues with reading the true RMS voltage at other than 50/60 Hz

Posted by Bernie Anderson Jr (Member # 410) on 12-30-2010, 08:46 PM:

I'm using a fluke digital multimeter though. Not really a cheap meter

Posted by Jeremy Weigel (Member # 4208) on 12-30-2010, 09:27 PM:

Not cheap, but not necessarily able to read true RMS.

Posted by Bernie Anderson Jr (Member # 410) on 12-31-2010, 09:10 PM:

So what is?