Wednesday, September 09, 2009

Screen Australia

I enjoy the cinema as well as my home theatre, therefore the purpose of this page is to have a look at the inside of the projection booth [bio box] where the film is actually projected from. I would like to thank the staff at Screen Australia for allowing me to capture these images and explain some of the technology to me.

The Projector
This is a BAUR B14, 35mm film projector. 35mm film runs at 24 frames per second or about 472mm of film per second. Each frame has 4 perfs [sprocket holes that pull the film through the projector]. Given that there is 60 seconds in a minute and an average film runs for 2 hours, an average film has over 3.2KM of film for that 2 hour movie.

The SOUND Head

The Sound Head Reader
This is the device that reads the data off the film strip. This system is Dolby Digital which stores the data between the sprocket holes on the Dolby SR [analogue or pro logic] sound track. Should the Dolby Digital track fail [damaged film stock etc], the sound system defaults to the SR track as a back up. The DTS system uses 2 CD ROM followers and a SMPTE time code [on the film strip] to sync the sound to the picture.

The Dolby Processor

Just like in the home, the cinema processor decodes the data stream and converts it to the 5.1 channels of sound. This device also controls the master volume for playback. In a real cinema, the master volume is set to 7.0 which is the 00dB reference level or 85dB per channel. Cinema systems are calibrated to +85dB using -20dBFS tones. In Home Theatre we use +75dB using -30dBFS tones. In the end, both systems play back at 105dB per channel.

Another key difference between cinema playback and home or consumer playback is the level of the surrounds. In the cinema, the screen channel LCRs are calibrated to +85dB and the sum of the surrounds is also +85dB or +82dB per side.

This is considered too hard for the home, so studios make this 3dB reduction in the film soundtrack itself, so we simply use +75dB [using -30dBFS tones] for all channels. The end result is that the film soundtrack can now be played at the same levels at home and will sound identical to the cinema version of the film.

The Anamorphic Lens
This is an ISCO 2x horizontal expansion anamorphic lens. All CinemaScope films require a lens like this one to restore the geometry of the image on screen. For those of us going Scope at home, a 1.33x stretch lens is required.


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