Film-Tech Forum ARCHIVE: Questions about track assignment in 70mm Dolby

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Posted by Lionel Fouillen (Member # 1475) on 07-01-2003, 04:50 AM:

[Confused]

This has been a source of annoyance for me since I first got interested by it nearly 15 years ago, for all informations I've read always contradicted each other.

What are the frequency range of the Baby Boom and the Surround on Dolby 70mm prints with formats #42 and #43?

With format #43, on which tracks are the Baby Boom and Surround recorded, and since both go together on at least one track, what is the filter frequency to separate them?

Thanks!

Posted by Steve Guttag (Member # 268) on 07-01-2003, 06:33 AM:

The "boom" channels are from 250Hz on down. If you do not have LC and RC, the subwoofer is contrained to 180Hz. If you do have LC and RC then the subwoofer covers from 100Hz on down and LC/RC cover from 100Hz up to 250Hz. If you only have LC and RC but no subwoofer...they cover from 250Hz down to their usable cutoff.

Tracks 2 and 4 (LC and RC respectively) are the "boom" tracks on format 42 and 43 prints.

For format 43. Tracks 2 and 4 have discrete left/right surround information on them starting at 500Hz while having boom information below 250Hz. Note the gap in the responses to ensure boom does not get into the surrounds.

Track 6 has the full-range surround information. For a theatre equipped with format 43, tracks 2 and 4 are used from 500Hz on up to the Ls and Rs (some call them P and Q) channels and Track 6 is used from 500Hz on down. In this manner you have a quasi-stereo surround system where is mono below 500Hz and stereo above 500Hz.

Furthermore, the system is FULLY backwards compatible with older systems. If your theatre can handle format 42, then the stereo surround information is filtered out naturally by the 250Hz low-pass filters on tracks 2 and 4. If your theatre was not aware of any of the new formats (remember Stereo surround on 70mm begain with Superman on a limited run and Apocolypse Now on a wider release so we are still in the '70s prior to even the CP-200 coming out)...one merely need to disconnect the HF sections of LC and RC channels and the frequencies choosen would ensure that the stereo surround information would not come out of the stage channels yet the boom would. Again, remember that when all of this was new, by far the most common speaker in the better (70mm) theatres was the Altec Voice of the Theatre line. They all featured a 500Hz crossover (except the A-8 but it normally wouldn't show up in a 70mm venue due to its little size).

Steve

Posted by Lionel Fouillen (Member # 1475) on 07-01-2003, 08:34 AM:

At last, I now have some accurate knowledge about this!
Thanks a lot Steve.

Posted by Christopher Seo (Member # 7) on 07-01-2003, 06:58 PM:

Steve,

Did the mono/stereo crossover frequency for the surrounds change at some point? I seem to recall Gordon mentioning something about this.

Posted by Steve Guttag (Member # 268) on 07-01-2003, 09:00 PM:

No. Not that I'm aware of.

Steve

Posted by Gordon McLeod (Member # 33) on 07-03-2003, 04:48 PM:

twice I can remember changing the headers for the frequency for two specific prints will have to do some digging