Film-Tech Forum ARCHIVE: Homebrew sound changeover?

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Posted by Matthew Taylor (Member # 2310) on 08-22-2005, 04:42 PM:

Is it very difficult to make a changeover circuit for just swapping the sound from one projector to another? I imagine just using a switch would cause a 'pop' or some other noise when the sound is switched.

Posted by Jack Ondracek (Member # 1466) on 08-22-2005, 05:40 PM:

There are several ways to switch sound from one machine to another. The best ways depend on what projectors you're running, and what kind of sound electronics are involved and where they're located.

More info, please.

Posted by Bruce Hansen (Member # 281) on 08-22-2005, 05:57 PM:

You may be able to do away with the pop by connecting a resistor to ground to each output line, if the output uses ICs or transistors. If it uses a transformer, then there should be no problem. The pop is caused by the caps. on the output having to "charge" through the input to the amp. If there is another path for the caps. to charge there should not be a pop. If your preamps have LO Z outputs, try 10K resistors, if HI Z try 100K resistors.

There could be a very quick (about 1/4 sec.) interruption in the audio as the switch goes from one set of contacts to the other. If this is a problem, you could get a "make before break" rotatory switch.

Of course, you could get a sound processor that is set-up to do change overs, if you have the bucks.

Posted by David Buckley (Member # 2600) on 08-23-2005, 02:55 AM:

All you need is audio switching, it wont click or pop as long as there are no DC offsets hanging around.

In this old thread is a picture from our old "processor" system, built by persons unknown.

Posted by Dieter Depypere (Member # 3184) on 08-23-2005, 07:47 AM:

We use a relay which tilts a mercury switch. This doesn't cause a "pop" when changing over (The mercury flows slowly to the opposite electrode).

Posted by Marin Zorica (Member # 1716) on 08-23-2005, 04:46 PM:

What you have there?

You probably have two sound head output and one input in cinema amplifier(processor)?

If is it use HEF4066 IC to switch sgnlas from reader and 5-12V voltage to switch inputs os HEF4066.

HEF4066 is quad electronic switch (it has got 4 switch inside it) and you need about 5V on every switch to activate it, so on switch 1 you conect output of reader 1, and on switch 2 conect reader 2 output, outputs of switch conect together and pas thru amplifier (processor) input, when projector 1 activ, take 5v power from lase/exciter lamp suply thru couple of KOHM resistor to swtich 1, and when projecot 2 activ take them laser/exciter lamp power to switch 2.

This will work if you have setup when laser/exciter lamp is only burning during show, not always like in reverse scan.

Another thing i saw cople of times is taht you just put solar cells in paralel thrue 1KH resistors and thing works!

Posted by Gordon McLeod (Member # 33) on 08-23-2005, 06:39 PM:

The 4066 is a single ended device and the audio must be biased up to +6v to pass through it undustorted with the chip running on a +12vdc supply

Posted by Marin Zorica (Member # 1716) on 08-24-2005, 03:02 AM:

Yes i forgot to say it it must be biased, or except biasing you may powerit from simetrical supply +/- 8V and it works!

Posted by Matthew Taylor (Member # 2310) on 08-24-2005, 10:31 AM:

Thanks for your advice. I have used 4066's in other applications so I will probably go for that. It sounds better than just using a relay.

Posted by David Buckley (Member # 2600) on 08-25-2005, 02:51 AM:

Actually, a cmos switch will not sound better than a relay, due to (a) distortion, and (b) crosstalk. Whether either of these are significant, of course, is another matter :-)

Posted by Marin Zorica (Member # 1716) on 08-25-2005, 03:40 AM:

quote:
Actually, a cmos switch will not sound better than a relay, due to (a) distortion, and (b) crosstalk. Whether either of these are significant, of course, is another matter :-)

I already work with CMOS switch and i didn't get any distortion or crosstalk (what kind crosstalk we talking about)??

Only thing that is mater is that be properly biased input signal or power CMOS with simetrical supply.

Posted by Frank Angel (Member # 248) on 08-25-2005, 07:24 AM:

Of course if you are still using exciter lamps, you can do what they did years ago which was simple but effective: just switch power between the two exciter lamps. Use a dropping resistor to keep the filament of the "off" projector hot, but not lit. This minimizes the time it takes for the bulb to go from off to full brightness. This method can be accomplished with nothing but two DPDT switches in a toggle configuration and the two power resistors, one at each projector. If you want indication, add an LED and dropping resistor for it at each machine to tell you if the sound is ON on that projector; then you will need two 3PDT switches or a relay. Either way, relay or switches, nothing is being switched in the audio chain so it is totally silent.

Note: Even with a pre-heat bias, you will still get a very slight drop in sound at the moment of change-over, but it is nearly imperceptible and way better than a pop.

As for the bias pre-heat resistor, you want to bias it with as much voltage as you can just so it barely lights. The best way to determine the resistor value that you need is to use a resistor substitute box and just dial up the voltage until the filament just begins to glow. Read the value and that's what you need, but be sure you get a high power ceramic power resistor (at least 10 watts). These bulbs draw high current. Be sure your substitution box uses at least 5 watts resistors and only leave it on just long enough to see when the filament glows. Leave it on any longer and it might burn out the resistors which in sub boxes are typically only one watt or even less.

Posted by Louis Bornwasser (Member # 3063) on 08-25-2005, 07:57 AM:

If you switch the exciters in this way, you could use a 3-way, (single pole, double throw) and a 4-way (double pole,double throw with jumpers already installed). After you get proper operation in terms of on-off; you then add the resistor from the on lamp to the off lamp. These switches are easy to find, cheap and easy to wire with no soldering, and most important, will probably never wear out. They are made for 15-20 amps Louis

Posted by John Pytlak (Member # 331) on 08-25-2005, 11:16 AM:

The old RCA sound system used at the Grandview Drive-in where I started in projection used the exciter lamp changeover method. Once played two identical trailers in sync and switched back and forth with no perceptable change in loudness, and certainly no "pops". Of course the method depends on the "thermal inertia" of the thick filaments used in those old exciter lamps.

Posted by Darryl Spicer (Member # 711) on 08-25-2005, 11:30 AM:

last changeover booth I ran had both exciter lamps stay lit. However it did use dolby equipment so the automation was changing it on the preamp boards in the processor.

Posted by Tim Reed (Member # 137) on 08-25-2005, 05:58 PM:

quote: John Pytlak
Once played two identical trailers in sync and switched back and forth with no perceptable change in loudness,

Haha! I can believe it. Only you would try something like that, John! [Wink]

Posted by Steve Guttag (Member # 268) on 08-25-2005, 07:57 PM:

I used P35-SL like everyone else [Wink]

Note, if you use Component Engineering's LS40 power supply you can have them send it to you with either the standard "reverse" logic or non-standard "normal" logic. You can then install these using your old exciter lamp wiring to perform LED changeovers...the LEDs will glow-up and glow-down just like your old exciter lamps did for a seamless changeover.

Posted by Marin Zorica (Member # 1716) on 08-26-2005, 02:25 AM:

I saw couple of system who had solar cell connected in parallel and only exciter lamp is actually doing changeover in sound. When exciter lamp burning on projector what is working, the sound is coming from them, so exciter lamp in time can be only burning in one projector.

Long time ago when gasses photocell where in use, they have been attached similar.
Every cell must be about 40V biased, and on cinema amplifier you got two inputs, but they where actually one, because only thing that was separated is bias voltage that can be regulated separately for each cell.
And exciter lamp is actually doing changeover in sound. When exciter lamp burning on projector what is working, the sound is coming from them, so exciter lamp in time can be only burning in one projector.

Posted by David Buckley (Member # 2600) on 08-26-2005, 02:29 AM:

Yeah, now we've got CE LED readers, we're using the power supply mechanism to switch LEDs, and very nicely it works too. Just a simple DPDT switch which you manually flip with the shutter changeover.

Posted by Frank Angel (Member # 248) on 08-27-2005, 09:15 AM:

quote: David Buckley
Just a simple DPDT switch which you manually flip with the shutter changeover.

The nice thing about being able to manually switch sound and picture separately is that if you run arthouse/retrospective and you get those inevitable battered prints from the "YOU SUCK" booths, and they have significant loss of frames at the heads and tails, there will be many times when you will need to play every last frame of sound on the outgoing projector or you will loose the end of a musical phrase or a few critical words. In such instances, if the changeover is wired to switch sound and picture at the same time, you wind up with one of two bad choices; either you loose that last bit of sound and have continuity on the screen or you get the sound and have the screen go black. If the changeover is wired as you describe, i.e., so that you can manually switch picture and sound independently, then you are able, with a deft slight-of-hand, to get both the last frames of sound and perfect picture continuity, since you can switch picture before sound. Of course it goes without saying that we all agree, PERFECT continuity would be more easily acheived if certain moronic half-wits were not allowed to have at the prints with their hacking blades in the first place and ALL the freakin frames that were originally on the prints were still there!

If you are automated, I guess you could still do that but you would need separate cues for picture and sound. Anyone ever set up automation that way? Is there such thing as automation for change-over setups?....not exactly a big calling for that kind of setup, I would guess.

Posted by Mark Hajducki (Member # 1732) on 08-27-2005, 09:44 AM:

quote: Frank Angel
Is there such thing as automation for change-over setups?....not exactly a big calling for that kind of setup, I would guess

I think changover automation was the first type avaliable.

I have also seen automation cues in the middle of films which would suggest changeover automation (most commonly halfway through to allow 6k operation)

Posted by Frank Angel (Member # 248) on 08-27-2005, 01:17 PM:

Yah, but are they setup so that you can program picture to change before sound? Not likely or without some major futzing.

Posted by Louis Bornwasser (Member # 3063) on 08-28-2005, 02:20 PM:

Frank; it is always "audio follows video"; in fact some of these automations did not even have any provision for sound. (You added whatever relay you needed to use the changeover pulse.) Louis

Posted by Steve Guttag (Member # 268) on 08-28-2005, 06:32 PM:

With the computer based automations that do changeovers (Pennywise and Panalogic) it is possible to do independant audio and picture changeovers. However, I doubt many if any are actually installed that way. However, it is certainly possible if that was what was desired.