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Author Topic: Best sound test film for clipping Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 05-14-2005 08:02 PM                        I think the requirement for a fully modulated film has lead us into thinking of overloads (electrical problems.) Based on the drawings of that scope trace, I think it more likely that there is an alignment problem causing one track to not be fully scanned. That would be the most likely reason for symetrical modulation on lower level material and non-symetrical modulation on 90-100% modulated material......just like AM radio does on purpose!! Remember buzz track alignment is only one phase of scanning the track; the light source & the pickup must also be matched. Louis  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-14-2005 11:15 PM                       The waveform you show would look like this if the illumination is uneven and falling off at the edges as front scan systems do. The Cat. 97 is the best test film to use as it is a pure tone and 100% modulation. Thus, you can infer from this what the peaks of real soundtracks will look and sound like. The 7 kHz is only 70% modulation at best. If you put your dual trace scope into XY mode you will see a vertical and horizontal vector representing the Lt and Rt channels. These should be the same length with no bends in the line. Depending on whether your system is front or reverse scan, you can move the light source and film until you get best illumination.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 05-15-2005 11:30 PM                     If you want to verify illumination uniformity, the Cat. 566 test film was designed specifically for this purpose. Sam's technique is useful for maximizing light output to the scanned area of the soundtrack. I suspect that Cat. 97 is probably 97% modulation. Wouldn't 100% modulation create an optical clash situation? Or is clash what accounts for the crosstalk that is invariably encountered with Cat. 97? [ 05-16-2005, 10:21 AM: Message edited by: Manny Knowles ]  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-16-2005 11:32 PM                       Manny, The film is perfect, the crosstalk on front scan systems is caused by light scatter in the front scan projected slit systems 100% modulation is hard to characterize exactly as various companies have pushed the soundtrack envelope past the SMPTE legal boundries (.076) to get more dynamic range. Buzz track is at .084. Our readers are set to .082 as 100% modulation in recognition that the industry is both overmodulating the track past .76 and "cheating the septum" by making it smaller.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 05-17-2005 02:07 AM                     I'm so glad I asked. Thanks, Sam. So, is there danger of clipping and greater crosstalk if one of these overmodulated/underseptumized (?) tracks is played using heads that were designed strictly based on the requirements of the standards?  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-17-2005 11:38 AM                       Manny, You are halfway there. Clipping will take place "earlier" but crosstalk will be the same. On reverse scan readers, it's a matter of the magnification of the soundtrack image onto the solar cell. The idea is to get the whole image of a 100% modulated soundtrack and have the DTS soundtrack and sprocket holes off the edges of the solar cell, sort of like having the undesired part of the picture image spill onto the black masking. Each manufacturer sets the exact magnification based on their judgement on how much tolerance to allow for misprinting. BACP does it somewhat optimistically which means the entire image gets played more accurately, but closer to the edge as it were. So what is it you do In LA as opposed to Bermuda, or wherever you were last?  |  IP: Logged Edward Jurich Master Film Handler Posts: 305 From: Las Vegas USA Registered: Jul 2003 posted 05-17-2005 09:34 PM                     It is not unusual for audio to have higher peaks on one side of the waveform. If you are getting distortion on high volume levels then you might want to check amplifier level settings to be sure you are not over-driving something.  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 05-17-2005 11:30 PM                     Thanks for all of this info. Now I've got to track down the relevant SMPTE standards. quote: Sam D. Chavez So what is it you do In LA as opposed to Bermuda, or wherever you were last? Sam, I'm having a blast! I just completed my BA in Cinema-Television at USC. I'm staying on thru the fall semester because there is one more of Tom Holman's classes that I would like to take. Ironically, it's his "Intro to Film Sound" class, but it has a number of practical demo's that he thinks I will enjoy. So, I'm going to do that and a research project involving independently-owned theatres. I have also been working a number of part-time jobs at the university in the projection dept. (showing films for classes) as well as in the production sound dept. as an equipment manager, scoring engineer and re-recording mixer for some of the student projects. In this upcoming (and final) semester I will also be the teaching assistant for the scoring engineering class. I'll be going back down to The Bahamas for a brief working vacation in about a week.  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-18-2005 03:21 AM                     Sam, When I put the scope in X-Y mode and put the probe on the preamp output, all I see is a single horizontal line. I don't know whether this is a problem with my scope or whether I'm doing something wrong. I assume that if I balance the outputs of left and right by adjusting the light source while using the 7kz film that will at least stop some of the underscan. If there is still distortion then the light source probably needs to be moved back from the film to make the slit wider - right? I am only referring to the 7Kz film because I have given back the crosstalk film that I borrowed so I don't have access to that anymore.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-18-2005 10:23 AM                       Matthew, You need two probes, one on the left channel and on the right channel to get true X/Y. If you have a mono system it wont give but one signal of course and you will only have one line. The length of the line will be an indication of light output. With an LED reader, swivel the LED bracket up and down for strongest signal.  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-18-2005 03:56 PM                     I had a look at the width light slit on the 7khz film and it seems that the slit is not wide enough to completely cover the soundtracks. Is it normally possible to adjust the lens to make the slit wider? Would this be the cause of the distortion?  |  IP: Logged Pete Naples Phenomenal Film Handler Posts: 1565 From: Dunfermline, Scotland Registered: Feb 2001 posted 05-18-2005 04:26 PM                     Some slit lenses are adjustable, earlier Cinemecannica and Fedi spring to mind. The procedure I use is to set everything up centrally using Buzz Track and Cat 97 Left-Right. Open up the slit width until it's over scanning on both sides, then narrow it until the buzz track just stop singing at you. Underscanning could well cause distortion on busy parts of the track, to all intents and purposes the signal is being optically clipped.  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-18-2005 04:43 PM                     The funny thing is, I cannot see any clipping on the scope. The peaks are nice and round. My projector is a Prevost which I believe have very similar sound pickups to cinemacannica. I am starting to think I have a dodgy solar cell which is not helping me with my alignment problems. It seems that one of the cell's outputs is greater than the other. I set up the cell with cat 97 and then adjusted the lens alignment until the scope showed equal amplitude on both channels. But the light slit looks way over to one side when projected on the cell, even though the output is equal. Also, without any film threaded, the dc output of the cell is different on each channel even if the slit is adjusted so it's equally covering both sides of the cell. Where can I get a solar cell for a Prevost P70 and how much can I expect to pay?  |  IP: Logged Pete Naples Phenomenal Film Handler Posts: 1565 From: Dunfermline, Scotland Registered: Feb 2001 posted 05-18-2005 05:11 PM                     quote: Matthew Taylor The peaks are nice and round The peaks of Cat 97 shown on the scope shouldn't be round they should be fairly sharp, look at the illustrations in the early Dolby manuals (CP50) for guidance. IIRC those are actual oscillographs, not drawings. Back to your predicament. It's possible that the lens is full of oil and dirt, but I'm a little concerned by your use of Cat 97 to adjust the lens position. You should use buzz track to adjust the laterl relationship between the lens and the film path, then use Cat 97 to adjust the cell relative to the film path. The two adjustments do tend to play off against each other, so you check and adjust each one until you can make no further improvement. Then you set the scanning width accurately. It's not unconceivable that the cell is damaged, they are fragile and do actually wear in use, especially if static is prevalent.If you really need I'm pretty sure we can supply a cell and block for a Prevost, but I've no idea at what cost.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-18-2005 07:50 PM                       I am not certain whether the Prevost lens slit width is adjustable, but it could account for the problem you are having. The only problem I have wth Pete's procedure is that the Cinemeccanica's lens "iris" closes down unevenly giving you the impression that the lens is over the center of the soundtrack when its not. The best way I have found to get optimum adjustment of the the front scan Cinemeccanica readers is to splice a loop of half Buzz and half 97. This way you can keep working back and forth until slt width and cell postition are both optimized. I do make a reader for the Prevost P55, P60, P70, P 90 as long as you have your original Prevost film guard and solar cell holder. I also make a reader for older P93's as well. Commercial message aside, the cell should be pushed as close behind the film as practical.  |  IP: Logged

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