8mm Forum: Stereo vs duo sound, implementation details?

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Posted by Ben Zotto (Member # 6619) on 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!

Posted by Paul Adsett (Member # 25) on August 07, 2019, 11:33 PM:

Super 8 stereo is left channel on the main wide track and right channel on the narrow balance track. Yes, the right channel is technically inferior to the left channel because of the narrower stripe width. Stereo super 8 requires a projector with a built in stereo amplifier or a mono projector equipped with separate left and right AUX outputs connected to an external stereo amp.
Duo channel is separate mono recordings on track 1 and 2 and thefacility to automatically fade up and down the sound levels of these tracks, as in commentary over music, or separate languages on each track. I have even used track 2 for the recording of special features directors commentary from DVD's.

Posted by Matthieu van der Sluis (Member # 6040) on August 08, 2019, 08:15 AM:

So with stereo films, the right channel sounds inferior to the left channel?

I did not notice that.

Posted by Joe Taffis (Member # 4) on August 08, 2019, 09:42 AM:

In my experience it all depends on the quality of the stripes. I would not call the balance stripe inferior just because its narrower. I've had dual language features that sounded fine when playing the English recorded balance track alone...

Posted by Steve Klare (Member # 12) on August 08, 2019, 10:04 AM:

Apparently there was room for a full width stripe where the balance stripe is. EVT Magnetics offered this as an option.

Posted by Osi Osgood (Member # 424) on 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]

Posted by Ben Zotto (Member # 6619) on 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!

Posted by Brian Fretwell (Member # 4302) on August 08, 2019, 02:46 PM:

I don't know about warble, but in general the narrower a track is the greater the noise which is why cassettes needed Dolby more than reel to reel. Remember some 1/4 inch tape reel to reels were 2 track to get better sound. (mono and stereo)

Posted by Paul Adsett (Member # 25) on August 08, 2019, 02:47 PM:

A very interesting discussion Ben.
I can only add the obvious comment that the quality of the stripe is probably the single most important factor. I have had absolutely superb results re-recording some films in stereo and getting stereo playback quality equal to cassette tape. Other films have had very poor quality stripe with lots of drop outs etc. I even have one film where the re-recording on track 2 (thin stripe) is vastly better than track 1 (wide stripe).

Posted by Matthieu van der Sluis (Member # 6040) on August 09, 2019, 06:44 AM:

The striping service on this forum seems to give axcellent quallity.
Not sure how wide this magnet stripe is, but customers are very happy with the rerecordings on that.
If not as wide, you still should here some difference.

Posted by Douglas Meltzer (Member # 28) on August 09, 2019, 08:32 AM:

I have re-recorded a number of my films in stereo and I am amazed at how good playback sounds from the balance stripe.

Doug

Posted by Oliver F. R. Feld (Member # 1911) on August 09, 2019, 08:58 AM:

I agree.
But some of the balance stripes (for example the late „black“ Derann stripes) don’t have the „power“ like the main stripes.
In some case I needed to record the right channel approximately 15 % louder to get the same sound level later.
Every stripe is different.

Posted by Maurizio Di Cintio (Member # 144) on 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.

Posted by Ben Zotto (Member # 6619) on August 16, 2019, 02:57 PM:

This has been such a useful discussion, thank you all. I'm intrigued by Maurizio's note that it's speed vs bandwidth that's a more significant factor in sound reproduction. I wish I was more knowledgeable about the physics on that.

@Doug and @Matthieu : it sounds like everyone here has a go-to service they use for re-recording films in stereo. Can someone direct me to that reference? Is it really reliable across multiple reels on features etc?

Posted by Maurizio Di Cintio (Member # 144) on August 16, 2019, 05:27 PM:

Ben, re-recording IS really reliable if some conditions are complied with, the first of which being to use a crystal synced projector tha can run exactly at the speed the film was converted to digital in (24 or 25 fps), throughout its duration.

Doug and other people here can certainly suggest you someone who can do the job for you in the USA.

Posted by Douglas Meltzer (Member # 28) on August 16, 2019, 06:11 PM:

Ben,

I do my own re-recording (sorry, I don't offer this as a service) however Forum members Adam Deierling and Lance Alspaugh do excellent work. If you enter their names in the search function, you'll find their posts and their contact information.

Doug

Posted by Ben Zotto (Member # 6619) on August 28, 2019, 03:06 PM:

Thanks Doug for the pointers!

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