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Author Topic: Getting sound out of film projectors. Stephen Furley Film God Posts: 3059 From: Coulsdon, Croydon, England Registered: May 2002 posted 12-02-2002 02:09 PM                     A recent post in another thread about connecting 16mm projectors to amplifiers reminded me of something I have wondered about in the past. Many 16mm projectors provide a line output which can be taken into a normal theatre sound system, but on 35mm it has been the standard practice to take the signal direct from the photo cells and/or magnetic heads, with all the electronics being in the sound rack. Why is this? I would have thought it would be better to provide pre-amps, head/slit equalisation etc. in the projectors themselves, and then take standard 600 Ohm balanced line outputs to the rack. This would avoid taking the very low level signals from the heads through long cables, with the risk of picking up noise from equipment such as rectifiers and light dimmers, and there would be no problems of cells needing a different supply Voltage or magnetic heads being the wrong impedance etc. All this matching would be taken care of within the projector. The only reason I can think of is that when pre-amps consisted of several valves, transformers etc. there was simply not enough room to fit them within the projector sound heads. Is this the reason, and, if not, what is it?  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 12-02-2002 03:05 PM                     Home entertainment and grade school/high school/college classrooms only needed a pukey sound system to make noise. It was not in a commercial enviroment. It was self contained, unlike 35/70mm systems of today.  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 12-02-2002 04:43 PM                     quote: I would have thought it would be better to provide pre-amps, head/slit equalisation etc. in the projectors themselves In the Norelco AAII/Philips DP70 and certain Century and Simplex soundhead models that was exactly what was done, usually with a small solid-state preamp (Century TR3/TR4, later model Simplex SH1007) or with a pair of 6J7 tubes in the AAII/DP70 (optical only-mag was taken care of at the sound rack). -Aaron  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 12-02-2002 04:45 PM                        Also consider that back in the days of tube amplification, it just wasnt possible to squeeze a few octal tubes in the soundheads, Even thought it is possible today, many guys are just stuck in their ways. Josh  |  IP: Logged Will Kutler Phenomenal Film Handler Posts: 1506 From: Tucson, AZ, USA Registered: Feb 2001 posted 12-02-2002 08:15 PM                     I was the projectionist for a 16mm outdoor community venue using a Hell and Bowell Mark Lamp Auto-Thread machine. The set-up was interesting. We used a 9volt guitar transmiter plugged into the projector. This transmitted to a simple outdoor mult-venue mixer/equalizer, an amp, then to the speakers mounted at either side of the screen. Sound level could be adjusted at the projector and at the console. Sound could also be tweaked/equalized to improve on the somewhat used and abused 16mm rental prints from Swank.  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 12-02-2002 10:01 PM                        When Dolby introduced their format in the 70s, it was necessary to provide a complete processor who could handle all the signal processing required. Before Dolby, there was no real standard and they also had to include equalising in the unit to battle the ubiquitious academy curve. To make a difference with their new format, they had to introduce equipment which could take care of all the settings in the A- and B-chains or else it would have been compromised. Since then, all processors have had the capabilities, so it would have been superfluous to introduce new soundheads which would have some of those features.  |  IP: Logged Andrew Duggan Expert Film Handler Posts: 127 From: Albany, NY, USA Registered: Jun 2002 posted 12-03-2002 12:06 AM                        Surprisingly enough, tube technolgy in a lot of commercial applications is starting to make a comeback. I guess it's a durability issue. One way or the other, I wouldn't be totally shocked to hear about a new tube based amplification system coming out in the future. (Investors, take note )  |  IP: Logged Josh Jones Redhat Posts: 1207 From: Plano, TX Registered: Apr 2000 posted 12-03-2002 11:31 AM                        I agree. Tubes are more forgiving in overload conditions than transistors or fets are. There are thousands to tube transmitters round the world broadcasting to the masses, and power tubes are still being manufactured by companies tha world over. A lot of studio equipment uses tubes for their sound more than anything else. Josh  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 12-03-2002 01:03 PM                     Even with the issues of making tubes fit into a soundhead there were many that had the preamps on the front wall or on a cabinet on the back of the mag penthouses  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 12-03-2002 06:44 PM                     Some older soundheads in "valve days" did have a single and sometimes two valves mounted in the soundhead and then a 600ohm line to the amp rack and of these quite a few had gain controls on each machine. This was good with the old photo electric cells as their output level was VERY low and the impedance was very HIGH (in the order of megohms) and IF you dragged the impedance down by using long cable lengths or crappy cable you lost the high frequency response and in those days the tracks were not the best and usually most cinemas were hunting for the best top end they could get. The use of soundheads that did not have a preamp and low impedance outputs almost always dictated that the amp rack was mounted on the front wall between the machines and as short as possible and in many cases the same LENGTHS of coax cable were used to connect the machines to the amp. As circuitry got smarter a rather crude equaliser was often incorporated into the amp to compensate for varying cable lengths so the problem of same length of cable went away. The output impedance of a silicon cell is quite low and its signal output is quite high compared to the tube photo electric cell. So with more signal to spare and quite low impedances involved the cables could be made much longer and virtually any old bit of screened cable would do the job. Thus it became more practical to stick the amp rack in a convenient place and just have a remote fader at the machine/s rather than have this damn big hot rack stuck between the projectors. I have a very strong memory of brushing my elbow onto the top cap of an 807 valve one night and the insulation of this cap clip was a bit suspect so the resulting 400 odd volts shock I copped caused me to fling my arm up. This whacked the monitor speaker and dislodged that from its peg, it then crashed down and smashed two valves in the amp and killed the sound and unfortunately for me made my cursing in the bio box even more audible in the cinema. Needless to say I built up a cover to go over the top of that amp so in future others could not get the same jolt I did. Some old amps had the Non Sync turntable built into the rack on the front wall and any records stood on top of the amp soon distorted with the heat from the tubes in the amp. So as circuitry improved shifting that rack to a rear or side wall was good move for many valve type installations of the past. It also gave better access to the second machine as the amp rack tended to obstruct access to the non op side of No 2. (Often a fair sod when you had to change a belt while a show was on) Lindsay  |  IP: Logged

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