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Author Topic: Best sound test film for clipping Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-11-2005 10:14 AM                     What is the best type of sound test film to test for clipping on an oscilloscope, ie with the greatest modulation?  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 05-11-2005 11:25 AM                     I would think; none. To get an amp to clip (for testing purposes) you need to input a signal that is higher than the amp is supposed to take. This means using a signal generator where you can increase the input signal and watch the resulting output on a scope. The amp manufacturer should be able to provide enough data to insure your sound system is set up correctly and won't clip. If I had to do this with test film (which I don't recommend) I guess I would use a loop of SMPTE 1kHz test film, then turn the volume up until I saw clipping on the scope. Most manufacturers' test (and their data is based on) using a 1kHz tone. I would not use regular film (with music and dialog) since the sound is too random and brief to see and record. If you can tell us why you want to do this, we might be able to tell you a better way to get the result you need.  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-11-2005 11:42 AM                     Sorry, I should have been more clear. I want to make sure that the solar cell the right distance from the film so no clipping of the signal occurs. I have already set up the lateral position of the cell so crosstalk is about 8:1 signal/noise ratio but I appear to be getting a bit of distortion on high output soundtracks. I wanted to see if it's the cell position or a bad preamp setup.  |  IP: Logged Rick Long Jr Expert Film Handler Posts: 211 From: Toronto, Canada Registered: Jul 2000 posted 05-11-2005 12:19 PM                     I think Dolby's Cat. 97 is 100% modulation, and is the film used to check the cell position, both laterally, and for proximity to the film. (the less crosstalk, the closer you are)  |  IP: Logged Pete Naples Phenomenal Film Handler Posts: 1565 From: Dunfermline, Scotland Registered: Feb 2001 posted 05-11-2005 05:13 PM                     Yup, Cat97 L-R is about all you can get these days, and it is recorded at 100% mod, look for the tops of the on peaks going flat. It is amazingly easy to overdrive a cell!  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-11-2005 05:22 PM                     Ok..well, I've already set up using cat 97 and got minimal crosstalk and zero clipping so I guess the problem is with my preamp setup.  |  IP: Logged Pete Naples Phenomenal Film Handler Posts: 1565 From: Dunfermline, Scotland Registered: Feb 2001 posted 05-11-2005 06:48 PM                     what's the pre-amp?  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 05-11-2005 08:55 PM                       Cat. No. 97 is actually slightly clipped on the outside edges as it is. I'm not sure I understand the term clipping the solar cell. Do you mean like overdriving it with too much light, or do you mean the waveform from the film is too big to fit inside the dimmensions?  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-12-2005 03:47 AM                     Sam, I mean that the cell is perhaps too far forward so the more modulated parts of the track are projected wide of the cell. Peter, I use a cheap B-Tech stereo Mic amp. I have de-coupling capacitors on each channel of the cell and a couple of 680ohm resistors connected from the each channel to the ground.  |  IP: Logged Gilbert Travin Expert Film Handler Posts: 101 From: Villeurbanne / France Registered: Jan 2004 posted 05-12-2005 04:53 AM                        Hi ! the stereo cell must be set very close to the film (1 mm is a good value, you can see CP-50 manual on this site : there is a scheme). For coupling to preamp : it is better to avoid capacitors (capacitors are no reliable components !!! ) but it is depending on your preamp .... The ideal solution is to use transformers as CP-50 do !  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-12-2005 06:09 AM                     be aware that there is a DC component in the output of the solarcel and some small cheap transformers do not like the DC component as it can saturate them Best is a blocking capacitor in serries with the transformer better yet get a used cp50 or similar  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-14-2005 02:16 PM                     I have been trying to diagnose the distortion I am getting on louder parts of the soundtrack. I hooked up my scope to the preamp output and there's definitely no clipping going on. I ran some trailers and watched the scope output. I noticed that on the louder parts of the soundtrack the wave seems to extend further in the negative direction than it does in the top direction. I have tried to represent this in a dodgy diagram; Is this the reason my sound is distorted with loud bits? If so, what can I do about it? BTW, if I run the 7khz test film, the wave is equidistant around the centre line.  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 05-14-2005 04:13 PM                     Having the 7KHz waveform look symmetrical, but not the waveform from a soundtrack, seems almost impossible. A wild guess would be something weird like one half of a class 'b' preamp is bad. Where exactly do you have the scope probe connected? Is the scope input set to DC? Is this visable in both (Lt-Rt) channels? As Gordon noted, a capacitor will remove the DC, and I would think there would be a cap at the input of the power amp. So even if it's not of equal ampiltude at the preamp output, it will be inside the main amp.  |  IP: Logged Matthew Taylor Expert Film Handler Posts: 186 From: Essex, UK Registered: Mar 2004 posted 05-14-2005 04:26 PM                     The scope was set to AC and measured at the output of the preamp. The preamp is really basic and built around a dual op-amp (4558). There are no transistors - only a few resistors and capacitors.  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 05-14-2005 04:55 PM                     As far as I can tell, with the scope set to AC, the resulting waveform would have be symmetrical even if the signal is not. Only by moving the scope's up/down positioning knob could it be made to look offset. It might be interesting to set the scope to DC and see what you get.  |  IP: Logged

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