MIXING FOR 16MM OPTICAL SOUND
Mixing in order to make a 16mm optical sound print implies a number of choices, that go along the specificities of that medium. As the possibilities of digital audio are way more important, failing to respect the limits of optical 16mm will mean surprises when listening to the finished film.
* The project has to be at 24 fps, as this is the standard sound projection speed.
* Exporting the mix has to be done in mono, as 16mm optical sound is necessarily mono. Center the panning pots of the mix by setting them at 0, as in the image below.
* The dynamic range — the difference in volume between the strongest sounds and the lightest ones — has to be limited to a maximum of 30 dB. This means that if the strongest sounds come near 0 dB on the peak meter, the lightest parts are not below -30 dB. Everything under that level will be lost. Beware, because the capacity of digital audio is more like 80 dB or 100 dB of dynamic range!
• Usually, voices are set between -3dB and -6db.
• 16mm optical sound reproduction yields serious losses in the high frequencies, a bit like old analog telephones, and it can be felt on voices, especially on womens’. Fortunately, this limitation can be overcome by filtering low frequencies to respect what is called the tonal balance. As there are losses at one end of the spectrum, one cuts the other end to bring the sound back in balance. Old school mixers know the 400 000 rule, that corresponds to the limits of human hearing : 20 x 20000 Hz. As 16mm optical tracks cut frequencies over something like 7000 Hz, one cuts down frequencies under 60 Hz too, as (400 000 / 7000 = 57…)
• It is therefore a good idea to simulate what happens during the transfer to optical and the printing, by creating a filter such as this one:
One works and mixes with such a filter, and naturally removes it for the final export of the mix! It is only meant for simulating what happens in the later stages.
• One also has to be able to synchronise the image and sound negatives and therefore one adds a beep at the beginning of the mix, that corresponds to a specific frame in the countdown that precedes the film. On a standard Academy leader, the beep is set two seconds before the actual first frame of the film, meaning 00:59:58:00 if the film starts at 01:00:00:00. As it is very close to the beginning of the film and not a necessity to share it with an audience, there are also other options, like exactly 10 seconds before the first frame.
• It’s also a good idea to place an end beep, for example 10 sec. after the last image in order to check the exact length of the sound negative. Some sound cameras used for transfers have a peculiar sense of sync…
• Last but not least : if you need more than one film reel. Sound camera magazines can usually take about 300m, so about 30 minutes of duration at 24 fps. If the film is longer, you need to do the sound transfer (and negative cutting) in more than one reel, each one having its academy leader, and sync beep(s). Usually, reels are made according to the norm of 16mm, that is no more than 20 minutes (about 610m in 35mm).
But there is a trick : one should add to the end of the mix of the #N reel a few seconds (more than 26 images in 16mm) of the beginning of the #N+1 reel to anticipate the editing of the reels with no silence.
That’s because on a film print, sound and image are physically slightly off sync to allow for the projector to show an image at one point on the film path and play the sound relative to this image with a device that is farther along the path. Once the reels are assembled for projection the sound of the 26 first images of reel #N+1 will be read from the end of #N reel.