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Author
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Topic: Stereo vs duo sound, implementation details?
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Ben Zotto
Film Handler
Posts: 33
From: San Francisco, CA, USA
Registered: Sep 2018
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posted August 07, 2019 09:14 PM
Can someone enlighten me as to the technical distinctions between stereo and duo audio on super 8 mag? I think I get the original *why* of the "duo" mode audio, but I'm trying to understand how it actually works. I assume that:
- Mono = sound on main stripe, no recording on balance stripe (or no balance stripe at all)
- Duo = sound on main stripe, additional different sound on balance stripe, recordable independently, but played back simultaneously?
Now, is a stereo print just a duo recording by another name? Ie, are the left and right channels divided into the main and balance stripe? The balance stripe is substantially narrower so it must have inferior audio resolution-- thus it seems odd that you'd have commercial prints where one channel just always has crappier sound quality. Or is a stereo signal encoded in some different, clever manner across both of the stripes?
The existence of projectors that support "only" duo audio (Elmo ST-1200HD?), and others that explicitly support stereo (GS-1200 etc) is a bit confusing to me (I can't figure out if there's a difference beyond what the tracks are "called"), and googling around at explanations of super 8 audio hasn't been as enlightening as I'd want. This page hints at it but doesn't get into how the signals are laid out on the striping.
Can anyone clarify, or feel free to link me to an existing thread if one exists (I looked but didn't find one). Thanks!
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Osi Osgood
Film God
Posts: 10204
From: Mountian Home, ID.
Registered: Jul 2005
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posted August 08, 2019 11:06 AM
Yes, in my own experience, the balance stripe tends to be weaker in sound reproduction and more "wobbly" overall in recording in general.
I've also noticed that the balance stripe also tends to have a LOT MORE noise to the track. I applaud anyone who has had luck with getting the balance stripe to have an audio output that equals the main stripe.
Since the balance stripe has tended to be weaker, you introduce more background "noise" to the overall audio.
This is why, if I am going after one of those big dollar scope feature films on super 8, I'd prefer a mono copy as, paying that much extra for a stereo copy is money wasted.
bear in mind, that is only my personal opinion. ![[Smile]](smile.gif)
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"All these moments will be lost in time, just like ... tears, in the rain. "
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Ben Zotto
Film Handler
Posts: 33
From: San Francisco, CA, USA
Registered: Sep 2018
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posted August 08, 2019 12:20 PM
Thanks all. That was exactly what I was looking for. To recap my understanding:
1. Duo equipment was designed to play back both tracks (or some manually-tweaked combination of them) into a single channel, which then got amplified. You *can* play stereo films via this equipment as long as you know to use the distinct duo "aux" outputs per-track and feed those separately into an amp and speakers, with track 1 (main stripe) as left channel and track 2 (balance stripe) as right channel.
2. Stereo is just main stripe = left and balance = right.
I did a bit of math on the tape resolution for comparison purposes. Using data from the link above: the main stripe is 0.68mm wide and the balance stripe is 0.13mm wide. (Note that this means the right channel in a stereo recording has a priori only about 20% of the resolution of the left channel):
- Super 8 uses 72 frames/foot. Thats 3 seconds of film/foot at 24fps, so the mag tape moves across the head at 4 IPS (inches per second), with 0.68mm and 0.13mm mag material per channel.
- By contrast, a compact cassette tape has a per-channel mag width of 0.95mm per channel but moves at less than half the speed (1 7/8 IPS). That implies that on a stereo film, the left channel may support comparable fidelity to a cassette but the right channel should be substantially inferior to that.
(A old reel-to-reel tape for home use looks like it would run at 3.75 IPS, which is closer to super 8 film speed but was typically far wider tape at ~3mm per track)
Sorry for munging the metric and inches there, tape is usually spec'd in inches and IPS but the super 8 info link above only talks metric on the striping! If anyone has any engineery knowledge of how analog tape "resolution" is actually measured or conceived of more sophisticated than "area of tape under the head per second", I'd be delighted to know.
As with all perceptual things, how good the sound off a film is will be the result of so many factors (recording equipment, playback equipment, quality of mag stripe itself, but also the nature/sonics of the recorded audio and your own ears!).
Osi raises an interesting notion here:
quote:
Since the balance stripe has tended to be weaker, you introduce more background "noise" to the overall audio.
This is why, if I am going after one of those big dollar scope feature films on super 8, I'd prefer a mono copy as, paying that much extra for a stereo copy is money wasted.
If it's true that the balance stripe is poor enough to introduce some warble and noise on, let's say, a big blockbuster movie soundtrack (Terminator?? Star Wars??), then it may make sense to prefer the mono not simply because the stereo doesn't add massive value, but because mixing in any balance stripe audio reduces the overall quality. There's an implicit case here, perhaps not meaningful pragmatically but interesting, that a mono soundtrack is in some sense actually superior on net simply because it doesn't use the balance stripe.
In any case, I appreciate the context from you all. Thanks!
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Maurizio Di Cintio
Jedi Master Film Handler
Posts: 977
From: Ortona, Italy
Registered: Jan 2004
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posted August 11, 2019 09:22 AM
Warbled sound from balance stripe is usually due to two reasons, often occurring simultaneously: stripe area not flat enough (due to sprocket proximity) and sound head with some signs of wear. The two combined factors result in inconsistent head-to-stripe contact i.e. 12 times/sec the contact is perfect and 12 times/sec it's not (just partially because the presser does not have the strength to overcome the film ondulations). This will result in uneven sound reproductions at certain frequencies in the mid-high range. The greater the sound head wear the worse. But with perfectly laid mag tracks (having the right formulation in terms of BIAS, EQ etc) and a projector that has both sound heads and pressers in order, the quality track 2 can deliver is astonishing, considering how narrow it is; indeed it is now my understanding recording speed is more important than track width. True, some projectors (notably Eumig) at times show tonal differences between the two tracks even if the two conditions I mentioned are met 100%. But I tend to think it's a matter of poor factory adjustament in the electronics. Other machines, notably Sankyo 800, Beaulieu Stereo 708 and Elmo GS 1200 deliver sound outputs from each channel that show excellent tonal consistency to each other, with (almost) no noticeable difference. On these machines it pays off to have actual stereo recordings. Or re-recordings with sound taken form a DVD for instance: every aspect of the sound reproduction will improve as opposed to the original recording made at the time the print was struck.
N.B.: not all films have ondulated sprokect areas: when the film was properly washed and dried after processing, that area should be totally flat and remain so if properly stored and wound.
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Maurizio
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