8mm Forum: Balance beteween Tracks 1 and 2

This is topic Balance beteween Tracks 1 and 2 in forum 8mm Forum at 8mm Forum.

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Posted by Maurizio Di Cintio (Member # 144) on August 09, 2005, 03:40 PM:

I have several stereo machines, including a Eumig 926, a Bauer T 610, a Sankyo S 800, a Goko RM 8008 and, recently acquired, an Elmo GS 1200. Apart from the last one, they all feature some discrepancy between track one and track two, as regards tone quality (upper freq. range) and/or output level. BTW Sankyo and Goko only differentiate frm each other for the output so they are the easiest ones to use when I need accurate stereo recording.

On the Elmo I can achieve a perfectly balanced output, for both tone and level, but if I play a soundtrack recorded on this machine on the Goko or on the Sankyo, channel 2 is consistently higher in output level. And viceversa....

Of course the two tracks cannot have identical performances, so somewhere it is necessary to compensate in the REC/PB chain, be it during recording (boosted actual level on Track 2 by means of Rec level trimmer), or during reproduction (increased gain of signal from sound head).

So the question I have always been turning in my mind since my first stereo sound machine [Confused]

is: is there any kind of general standard norms for this issues (like, for example, the RIAA eq. curve commonly used in vinil records), or not? Differently said: am I to compensate during recording or during playback? And what about compatibility with other projectors?
Thanks to all those who will supply an answer.

Posted by Kevin Faulkner (Member # 6) on August 09, 2005, 04:58 PM:

This is a difficult one to answer as there are many factors. The GS does have slightly more gain on track 2 during recording and playback to compensate for the smaller width track. The tollerances of the way the heads are aligned on the machine will also come into play. Any slight missalignment will be far more noticeable on track 2 due its is narrower width. On the majority of machines the head widths are the same for the 2 tracks and are slightly wider than the main track. This then means hopefully that both stripes will be fully covered by both heads even if the slitting is slightly off centre etc.
I dont think there was ever a recording standad for all this except for the std that Kodak set at the outset of super 8 sound with respect to the widths of the stripes. Dont forget that the balance stripe was not put into the equation to record onto. It was there to make sure the film was flat in the gate of the projector for best sharpness etc. The projetcor manufacturers very much did their own thing to make sure they got the best out of the two stripes.

Kev.

Posted by Jan Bister (Member # 332) on August 09, 2005, 09:01 PM:

I would think that any differences between the tracks are already taken care of in the recording/playback chain so that you should not have to do any balance adjustments after the fact. It should be transparent to the user, meaning if you record the same thing on both tracks, playback should come out the same too (same VU meter display and same volume on both tracks).
What kind of difference are we talking about - is this in volume only, or in sound quality as well? How many dB of a difference is there between the left and right channels?

Posted by Rick Skowronek (Member # 385) on August 10, 2005, 10:48 AM:

Just another answer to your equalization question, there are basically two standards that were established. One for records, which you mentioned - RIAA, and the other is the NAB (National Association of Broadcasters) for magnetic media. These standards were obviously chosen so that media recorded/cut could be guaranteed reasonable success, all other things being equal, to playing on any other machine.

I can't believe Kodak mucked with that standard so probably used it verbatim in their specs for film. I'm sure also that most projector manufacturers used pretty much available tape heads and technology to get sound on and off the film. The NAB equalization curve they used was more than likely the one for 3 3/4 inch per second tape recorders. That's pretty close to the speed at 18 fps and almost dead-on at 24 fps. By the way, the equalization is done in both record and playback circuits. In a tape recorder you purchase a standard NAB equalization and alignment tape and fully setup the playback circuit to provide 0 db out across the frequency response of the tape unit. With full 1/4 track heads at 3 3/4 inch per second that provides about a average of 13k to 15k Hz of response. I'm sure the same is done when setting up and aligning a projector. The last stage is to record the same information on the machine and setup the record equalization, alignment and record bias so that, when played back on that same machine, it closely matches the output of the standards tape you used to setup the playback circuits.

With the smaller tracks on the film, I'm sure the pre-emphasis and de-emphasis on the record and playback respectively were jacked up somewhat at the higher frequencies just to get better highs out of the film track. That said, if there is a standards "film" for sound, it should theoretically be possible to realign the playback circuits to be roughly equal in both output level and general frequency response.

A last side note, things like circuit degradation, head wear and normal head alignment shifts over time have a dramatic effect on the above. If you have a great prerecorded film with sound tracks that do equally well on your GS, you may wish to try putting it in one of the others that isn't equal and try slight azimuth head alignment of the machine. This is, of course after a good head cleaning and degaussing. The balance track, as Kevin said, is even more susceptible to mis-alignment than the normal track.

If alignment doesn't help, and if you have a good manual for the machine, playback equalization and level adjustment is probably in order. I wouldn't touch the record circuitry since you have to go back and forth so much (record and playback head are one in the same) it can really mess up your record capabilities.

Hope this was close to what you were asking about.

Rick

Posted by Maurizio Di Cintio (Member # 144) on August 10, 2005, 11:42 AM:

THank you very much guys. It was! [Smile]

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