Todd AO 12Khz

[Mikael Christensen]

Hope it is the right forum to post this :-)

Can anyone explain how the "12Khz" worked on the effect channels ?

[Vern Dias]

Todd-AO in it's 70mm format did not use any 12KHz keying tones.

It was introduced with CinemaScope on magnetic prints and also used on magoptical prints. The keying tone was used to unmute the surround channel when it was active and carrying an actual audio content.

This was done because the mag stripe for the surround channel was much narrower the the mag stripes for the front channels and had a much poorer signal to noise ratio and was prone to extraneous pops and noise.

Your confusion likely arises from the 35mm Cinestage prints of Around The World in 80 Days. This release used a one off system of a Perspectasound integrater on the 4th surround channel to steer the audio to the left, right, and rear surrounds, and it also used the keying tone for muting when the track did not contain any effects audio.

[Mikael Christensen]

The keying tone was used to unmute the surround channel when it was active and carrying an actual audio content.

What was the freqency on the "keying tone". Haveing this unit I'm trying to get up and running:

bofa_057.jpg

[Vern Dias]

As stated above, 12KHz is the frequency. It's a sine wave.

The likelihood of getting an old tube based device like you show here to work is next to zero, because all the capacitors have likely aged to the point where they will need to be replaced. They may even be hand picked, custom values or tolerances. There will also be a R C (maybe even an L C) notch filter in the path to knock down the 12 KHz tone level as I (a much younger me) could always hear the 12 KHz tone in the auditorium announcing the fact that the surround track was about to be active.

You will need to use a signal generator and an oscilloscope to trouble shoot this. It looks like those jumpers on the black block at the top are used to select different value capacitors to achieve the proper tuning to 12 KHz.

In a pinch, you might get by with simply bypassing the surround keyer if the mag tracks on the print are in good shape. You will have to screen the print to be sure.

If it's a one off showing, you could create a cue sheet and assign a person to ride the surround channel gain control but you will still need a 12KHz notch filter which you could do with a separate equalizer box.

Be careful, that B+ rail voltage can give you a painful shock.

[Steve Guttag]

I'm sure it was adjustable for numerous reasons, not the least of which would be the speed of the projector may not be a precise 24fps (async motors, loading, voltage variances...etc.). I am not familiar with all of the various surround switch devices...mostly Dolby's CAT93...which was miserable at filtering out the 12KHz tone, as Vern noted...it was always audible. Better systems not only notched the 12KHz done but also low-passed the entire track as it didn't haven't anything above 8KHz worth hearing anyway (and surrounds of the day were, typically, just 8"-12" drivers in a sloped front box.

The last cinema product that I recall having 12KHz processing was Panastereo's CSP4600. I believe it used a very steep notch filter plus LPF on it but I never ran 12KHz material one one.

They got the idea backwards...they should have used the tone to turn the surround track off! Had they done that, you'd never hear the tone (except, maybe right when it came on until the unit had time to react and shut the channel off).

[Gordon McLeod]

Many of the older preamps had a tunable coil to adjust the filter for error of projector speed

[Vern Dias]

My experience goes back to the original Simplex XL tube based systems originally installed for "The Robe" circa 1954. As Steve mentioned above, the filters were tunable to account for projector speed variances.

[Mark Gulbrandsen]

I've never had any Todd-AO prints in my collection that had a surround control track. My print number of Oklahoma was # 28, so that more or less goes back to the first or second year, yes, it was beat red. Now, even though I've seen a Smell-O-Vision machine, I am unsure how that stinker was cued into a running movie. Control tracks? Noches on the film? Anyone know?

[Ryan Gallagher]

I've never had any Todd-AO prints in my collection that had a surround control track. My print number of Oklahoma was # 28, so that more or less goes back to the first or second year, yes, it was beat red. Now, even though I've seen a Smell-O-Vision machine, I am unsure how that stinker was cued into a running movie. Control tracks? Noches on the film? Anyone know?

Dunno how they did it then. But now they use the audio track and AI. Because AI. LOL (to probably less success than the original).
https://www.youtube.com/watch?v=Z54jc75GyVM

The only way that ever would have been an enhancement is if you had an olfactory artist on the same skill level as a folio artist perform their craft live in real time, and some way to clear the previous smells from the room instantly.

[Mark Gulbrandsen]

Apparently, the one in the Cinestage basement fed the smells into the main air handler system. Then, once that smell was over, the air handler vented that air outdoors to clear things for the next odor. The machine was a carousel device with the smells inside special glass tubes that were rotated into position. Then at the right moment, the odor would be injected into the air handler. I also remember vaguely a system.called Odorama. Not sure if it was one and the same system or not?

[Mark Gulbrandsen]

Here's an article with a picture of the carousel holding the vials of smell. The inventor has an odd name...