On
the film, after the picture is printed, stripes
of iron oxide are applied onto the release print.
Every stripe represents 1 audio channel. E.g. Todd-AO had 6 channels, so 6 
stripes
were applied to the 70mm print.
Onto these stripes the sound is recorded in real
time. During playback magnetic heads, located on
the projector in an assembly called 'penthouse'
reads the information and sends it to the
amplifiers of each channel.