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Author Topic: Format 42 Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 02-09-2008 06:19 PM                        This is probably a stupid question, but what is the purpose of having two discrete subwoofer channels in format 42 (and 43)? I'm curious as to the reasoning behind the need for this. Actually, since I've raised the issue already, what is the purpose of having a discrete subwoofer channel at all in any sound format? Why not just let the cinema processor deal with the subwoofer crossover issue just as is done for optical sound? Is there some reason why a user-defined crossover frequency is a bad thing?  |  IP: Logged Ron Funderburg Jedi Master Film Handler Posts: 814 From: Chickasha, Oklahoma, USA Registered: Nov 2007 posted 02-09-2008 07:51 PM                        the director and sound editor determine when a sub-wolfer is to be used to fulfill the two purposes for having a sub-wolfer in the theater. 1st the sub-wolfer moves air so that you feel the sound and second so you hear the sound they intend. A sub-wolfer in a theater is not there to have sound outside the frequency it is designed to handle pumped in to like some trunk mounted unit in a car playing all the sound though it no matter if it is right sound or not. The reason for having more than one sub-wolfer is to move more air with less effort never taxing the units! You almost always need two or more subs! The sub-wolfer function being handled by the processor in optical sound is because there is really no other way to do it with just two discrete channels. You set your base threshold and every thing that frequency or below is pumped to the sub. And the sub-wolfer really isn't a discrete channel even in digital it is the point one channel of 6.1 and is piggy backed on the surrounds in DTS  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 02-09-2008 08:57 PM                     I don't understand how rons anwser fits scotts question on format 42 and 43 on analogue formats it realy is not a sub woofer channel but as its name implies Bass Extension It started out as many stage speakers didn't go down all that low in response and there purpose was to extend there response below 100Hz which is why they originally had a selectable filter that was set to match the cutoff point of the stage channels In optical the sub recieves the filter sum of Lt and Rt before the NR circuit and it is passed through a downward expander, vca, filter and eq circuit For format 42 and 43 the low frequency was recorded on Le and Re and summed and passed through the same expander and filter to the sub The digital sub input doesn't pass through the downward expander and usually uses a different cutoff frequency From so old notes it was envisoned that the low frequency sections of the LE and Re speakers could agument sufficiently the bass reruirements  |  IP: Logged Ron Funderburg Jedi Master Film Handler Posts: 814 From: Chickasha, Oklahoma, USA Registered: Nov 2007 posted 02-09-2008 09:04 PM                        Gordon you are probable right I concentrated on the second part and not the first part sorry!  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 02-09-2008 09:46 PM                        Thanks, Gordon. So, the two low-frequency tracks are there for compatibility with existing Todd-AO systems (pre-CP200), rather than because of any real need to have two discrete subwoofer channels? That makes perfect sense to me.  |  IP: Logged Michael Coate Phenomenal Film Handler Posts: 1904 From: Los Angeles, California Registered: Feb 2001 posted 02-09-2008 09:56 PM                     Isn't a "sub-wolfer" one who devours a submarine sandwich?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 02-09-2008 10:04 PM                     On the cp100 with out the SA5 adapter for format 42 one unplugged the eq cards for Le and Re and plugged in a filter card The CP100 didn't have a sub out The Eprad Super StarScope was done the same way  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 02-09-2008 10:17 PM                     Gord got it mostly right and Ron merely spouted what he thinks he knows. As Gordon said...it was/is called Bass extension or Bass enhancement. The first incranations of the BE was in 70mm theatres that had 5-stage channels. There was NO subwoofer requirement. The idea was to improve the bass response however and often using the same speaker types as used by the remaining stage channels. That is, the Altec A4 (the most popular, by far, for 70mm theatres. The A4 (as well as the A2 or A1) can go down to 40Hz pretty comfortably, depending on vintage if the bass-wings are installed. By doubling the track (since they weren't going to use it for a discrete dialog/music) one effectively doubles the power handing and potential bass enhancement of the system. As Gord, noted, it does not need Dolby noise reduction, but uses downward expansion. It also goes as high in frequency as...anyone...anyone...Bueller?...anyone? 250Hz. Clearly not the stuff for subwoofers there. It idea was that one could effectively pan the bass around a bit too. Picture Raiders of the Lost Ark with the big ball rolling across the screen...a panned bass track would add to the illusion on screen. It isn't that bass is non-directional but it is less directional as one goes to lower frequencies. Who here can't tell where the subwoofers are located in a theatre just by listening? I don't know about the rest of you but I sure can and if the bass track goes up to 250Hz, then it is quite directional. The subwoofer really didn't gain in popularity until about 1980 with the CP200 and even then I was still seeing many A4s or even A5s used in the LC/RC spots. Think about the "subwoofers" of the day in the 1970s and early 1980s. There were few real choices out there. The CP200 probably handled the whole bass extension/subwoofer thing the best. You installed what you had including both LC/RC as well as subwoofers. One has the option of "telling" the CP200 that subwoofers were in use and thereby limiting the Le/Re channels to the 100-250Hz range with the subwoofer taking over at 100Hz and below. Some connected their subwoofers to the Le/Re outputs though few if any "subwoofers" can give frequencies up to 250Hz justice. A modification for the Cat160, first brought about by THX, came out that would extend the subwoofer output to 180Hz if one did not have LC/RC channels. When the Cat560 came out, not only that was standardized but the signal that would have sent optical information to the Le/Re channels was killed. OBE was never mixed with Le/Re in mind and as a result, often had the system seem too bassy. Optical bass extension was always somthing that really should be planned for in the recording. I've played some films that clearly were not (1980s) and with the subs on...things got rather muddy. The OBE circuit has a downward expander and will exaggerate the bass if the signal was not recorded on the film compressed. OBE was also always an augmentation to the main channels and not as a dedicated bass channel like in 70mm and now digital. So Scott, it has more with the evolution of the subwoofer in main-stream cinema than anything as to why subs are handled the way they are. For most sound reinforcement, subwoofers merely extend the bass range of the system. Even IMAX uses their subs that way...there is a crossover there to take a signal on one of the 6 discrete channels and send that which is below the crossover point to the subwoofers. There is nothing to stop an exhibitor from putting subwoofers on all of the stage channels and adding crossovers to extend their range...however films are not mixed to that. ANSI/SMPTE 202M pretty well defines the response of cinema stage channels. Steve  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 02-10-2008 03:18 AM                       Scott, are these the "baby booms" you're talking about? Le and Re screen channels? FWIW, with CP-200's, you also have to watch your "P's" and "Q's".  |  IP: Logged Mark Lensenmayer Phenomenal Film Handler Posts: 1605 From: Upper Arlington, OH Registered: Sep 1999 posted 02-10-2008 07:59 AM                     Did this have anything to do with Warner's MEGASOUND process?  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 02-10-2008 08:00 AM                        Somewhere around here I have a copy of the THX modification to the CP-200 for subs......no more "baby boom." (Too many mods on that poor CP-200.) Louis  |  IP: Logged Andres Briano Expert Film Handler Posts: 162 From: Buenos Aires, Argentina Registered: Jan 2008 posted 02-10-2008 09:15 AM                        The source of the subwoofer signal in the Imax format is all 6 channels summed together and not just one. Then it goes through the crossover.  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 02-10-2008 11:20 AM                     Isn't "42" the answer to "life, the universe, and everything", as stated in "Hitchhikers Guide to the Galaxy"?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 02-10-2008 11:58 AM                     Sorry...I didn't mean to imply that only one channel has the bass information in IMAX...though I can see how it could be read that way. Yes, any of the six channels can feed the subwoofer if the signal is below the crossover point. Steve  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 02-10-2008 12:14 PM                        Thanks to all, especially Steve. Tim: I thought that "baby boom" was just another name for format 42. Or is it something different entirely?  |  IP: Logged

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