a question about optical sound
Posted 07 November 2007 - 07:50 PM
Reaching back in memories into the time of optical soundtracks, I remember usually visiting two theaters, one of them had a very soft and dark sound that was always breathing, and another theater (also mono), which had a very crisp sound, but not detailed (like digital soundtracks), but simply felt somehow "sharpened".
It reminded me of those exciters they sometimes use on radio stations.
It seemed like every little pop and crack was exaggerated
So I'm wondering, was there any kind of harmonic excitation or similar technique used in optical playback systems which was suppose to bring life to an otherwise blurry sound?
If not, what exactly was I hearing, is it some kind of artefact of optical recording of sound?
Posted 08 November 2007 - 09:26 AM
Posted 08 November 2007 - 10:06 AM
Assuming other prints sound okay in a given movie house, artificial sounding optical sound is probably because the "A" part of the sound chain (A chain is the part of the projector playback that's directly involved with picking up the soundtrack and the preamps, EQ, etc. involved) isn't tuned up properly - or - the wrong playback format is being used, perhaps Dolby A on a Dolby SR print or Dolby playback on an old non-Dolby print, etc. If the movie house itself sounds bad on everything that's because a theatre house sound system reproduction calibration system like Dolby's or THX hasn't been done (called B chain calibration in the business). Most houses are now but there's probably a few cheapskate holdout theatre owners who won't pay for proper systems and system tuneup.
You are correct in guessing some sort of radio station like processing, that's what Dolby SR and A are but the difference is radio station processing is single ended, it only works on prerecorded sound that is produced without respect to playback processing. Dolby is an elaborate system where the optical sound track recording is processed through Dolby gear that complements the playback processing. That system is called "companding". Highs are boosted in recording, then de-emphasized in playback, etc. Dolby A is fixed format, the processed frequency bands are non-changing. Dolby SR (SR = Spectral Recording) is much more elaborate with more frequency bands that change their bandwidth and center frequencies dynamically.
I own Dolby A and SR playback for my 35mm projector (A Simplex SP with a Dolby CP-200/SRA5 system). It's not a simple process to properly calibrate it. You need at least three different test films, pink sound generators, expensive audio test gear, oscilloscopes, etc. to really do it right. I do own equipment like that and as a result have been getting technical support from Dolby San Francisco as if I owned a commercial theatre. I've really grown to love the gang at Dolby - they've been super to me.