Of Stripes and Recording ...

[Brian Fretwell]

70mm sound, especially the 30fps TodAO, was extremely good, even surprising some Dolby engineers when doing a comparion at a talk at the BFT many years asgo. I think it was about the time that Dolby Digital was laounched.

[Martin Davey]

I think that talk was at the Momi theatre. I do remember hearing and seeing the 30fps opening to Oklahoma. The smoother picture was also impressive as well as the sound on the faded copy that was shown. The strange thing is that the 'America' sequence from West Side Story was also shown in the same talk, in conventional 70mm, and seemed very unusually, like a step down!

[Lee Mannering]

Yes Osi. Kodak mag stripe was always the best.
Once upon a 1970's time my expert local dealer got hold of around 800ft black Kodak film for me. I have around 50ft left now of that pre stripe.

[John Taylor]

Optical on super 8 was never going to be a priority as has been well documented. The majority clearly made for the airlines where most prints were destroyed after a short life once they were finished with. I read an old Derann article about this subject and the managing director of the company talks about super 8 optical prints. They were generally printed on cheaper Eastman stocks due to their short life but also, unlike 16mm films, the 8mm prints never went through the final polishing stage which resulted in more hiss and bumps on the soundtrack. It's a sound track "that will do" for its purpose on the airlines.
As Lee said, the Kodak pre stripe was always the best on super 8, and despite all the points on here regarding quality, sounds heads and pressure pads, I think most collectors will be more than happy with the sound they get from their projectors whichever way film goes through the heads, amplified with good a good analogue amplifier you wouldn't have a clue that the technology our films are projected was 40 years plus of age.

Equally, the 16mm prints I own from the 80/90s all have soundtracks as good, if not better than anything on super 8, at the end of the day, if it sounds good to you and your audience, then it's fine.

I'm not into all this jargon about frequency, thickness of bases and all the rest of it, I cant say I've ever even thought about it. Once the heads wear, get shot and buy new, worn heads not only sound rubbish, they also scratch your films, how the film is held against the head and other points on here are irrelevant unless someone's planning to make a new modern day projector with new designs. The hobby is stuck with whats available. On the other hand, 16mm sound with zero contact for the sound production is much easier.

Back to the start of this thread by Mr Osi, most of the problem with early films, (presumably the 70s from companies like Mountain etc) wasn't with the sound stripe, it was the poor quality they put out back in the day from crappy source material which most collectors seemed to accept and a lot of companies pushed out, there was no other choice I guess.

[Brian Fretwell]

I think that talk was at the Momi theatre. I do remember hearing and seeing the 30fps opening to Oklahoma. The smoother picture was also impressive as well as the sound on the faded copy that was shown. The strange thing is that the 'America' sequence from West Side Story was also shown in the same talk, in conventional 70mm, and seemed very unusually, like a step down!

That was the one, they were worried that the projectors may have hed trouble running at 30fps as they hadn't done so for a long time.

[Lee McCaffrey]

Higher frame rate equals higher perceived image quality, but also means the audio quality/fidelity is higher as the relative tape speed is faster. Going back to super 8mm one second of film at 24fps is 4 inches in length, so the speed at the soundheads is 4 inches per second, over double the tape speed of the audio cassette. Quality would also be improved by having dedicated playback heads (with a narrower head gap) but most projectors used a dual purpose record/play head with a compromised head gap.

[Tim Gillett]

The difference betweel tape and film that meant all projectors had pressure pads is the thickness of the base which gives a lower flexibility. Tape will mould itself to the heads with less pressure so heads should last longere.

This may be true to an extent but as Leo and I were discussing, I suspect the reason consumer sound projectors used the plastic "pressers" was, understandably, to avoid the extra costs of adding to these projectors a full professional film back tension system upstream of the sound head - which seems to have been installed in pro cinema projectors as well as production equipment. It would be interesting to view the production equipment which actually recorded the sound tracks to the pre striped 16/35/70mm release prints destined for cinema projection back in those days.

The problem as I see it with the "pressers" is that they were by professional standards, crude and haphazard. With the plastic pressers there is a tendency to create the opposite of the desired situation. Instead of the convex curve on the head face, with the gap sitting at the apex, we can end up with a concave curve which we dont want. Also the point of contact of the "presser" with the head gaps is hidden. We cant view it directly. In some Eumig 8mm projectors Eumig at least built in a little brass adjusting screw which allowed moving the "pressers" left or right a little while listening to the effect on the audio, which was at least something. But also the "pressers" press onto the head by mere spring pressure. There is no realistic way to adjust that pressure as either the head wears down, or the pressers themselves wear down. It works - after a fashion...

Whether striped film or magnetic tape, it's so much better to apply back tension to the film itself to achieve reliable stripe to head contact. With back tension properly set up, the maximum stripe pressure is exactly at the apex of the curve, where the head gap is situated. This is the optimal situation. True this will naturally and eventually tend to flatten the shape of the head's profile, which is a primary reason we relap a head, to restore that original convex contour. See link below to JRF Magnetic Sciences web page. Also diagram from JRF Magnetic Sciences.

Also unlike the mere spring pressure of stripe to head, using a playback machine with electronically adjustable back tension, the pressure of the stripe onto the gap is completely adjustable. When the head is new, or newly relapped, we can dial in just enough pressure for reliable stripe to gap contact - plus a little more for safety margin. But no more, to avoid unnecessary head wear. Then as the head face inevitably flattens somewhat, and some extra pressure (per square mm) is required for reliable sound quality, it can be added at the turn of a knob. This is what skilled tape operators would do, but only up to a point which they had to decide. Then the head had to be sent out to a relap specialist to restore the original contour. When the new or relapped head returned and was reinstalled, the operator could back off the back tension to the original lower setting, for long service life of the head.

Head relapping (jrfmagnetics.com)

[John Taylor]

Anyone remember the first Elmo ST1200? They used a rubber roller to press the film onto the mono head.
Tim, your knowledge on this is very impressive, do you know what it would typically cost to have a head relapped, bearing in mind not all collectors would have the ability to remove a head to send it off, it would need the removal, the relapping and the replacement and testing?
I think it would be interesting to get a ball point figure as I'm sure a number of owners would definatly be interested.

I guess the other question is, how does one know if the head requires it and that any fault isn't caused by something else?
I know someone who sent their projector off for a service/repair, the sound quality was fine, but he still needed a replacement head due to wear that was likely to cause other problems sooner rather than later.
I think a majority of people within the hobby have probably never had to have heads replaced or relapped but it is interesting to know it can be done. Does anyone know of this service in the UK or the EU?

[Tim Gillett]

John, thanks for the compliment. Summertone (I think his name is Terry) is the only relapping service I know of in the UK. He appears to be very experienced and knowledgeable, more so than myself in this field. His website has lots of good info. He seems to service the professional audio and film sector. Not sure if he deals with consumer projector heads. Here's a link to his website: Summertone - Summertone UK Hope this helps. Cheers Tim.

[Osi Osgood]

Is the loss of good sound reproduction due to more space between the sound heads and the passing film? I know that may sound like an obvious question, but y am on a learning curve. According to that illustration,(and thank you for that), it would appear that the metal is slowly worn away, ( much likes the wind or waves slowly wear away at rock throughout the years), but my question would be, wouldn't this expose more of a slit, a larger slit, so to speak, which could either increase sound reproduction, or cause sound reproduction to become to "broad" to be reproduced properly?

[Tim Gillett]

Is the loss of good sound reproduction due to more space between the sound heads and the passing film? I know that may sound like an obvious question, but y am on a learning curve. According to that illustration,(and thank you for that), it would appear that the metal is slowly worn away, ( much likes the wind or waves slowly wear away at rock throughout the years), but my question would be, wouldn't this expose more of a slit, a larger slit, so to speak, which could either increase sound reproduction, or cause sound reproduction to become to "broad" to be reproduced properly?

Yes any spacing between stripe/tape and the head gap is coincidentally called a "spacing loss". That's the technical term. Just a very small spacing 'loss' can wipe out the treble. That spacing can also be caused by a thin film of dirt which of course can be fixed by cleaning. The illustration is correct for what it's illustrating but it greatly exaggerates the width of the "gap". It's very thin, sometimes less than a micron wide. In relapping it's very important to not accidentally widen that gap. You cant see it on that diagram but the thin gap runs a certain distance down into the head at the same width and then eventually with wear will abruptly 'open out' into a wide, black looking line and the head is now useless. The aim is to use the head of course but also to try and conserve the life left in it. Magnetic heads especially the small ones arent easy to manufacture. Many head types are no longer made as there's no longer the demand.

[John Taylor]

This is a very interesting thread, so once the head gets relapped, how do you set the azimuth of the re lapped head?, what would the the CLA value be after its lapped and also how do you get the head in phase?, is it possible that a cassette player head could be adapted to work with any projector, given what we know?

[Paul Adsett]

What a great thread Osi has started here! This is the kind of intelligent, expert, technical discussion, that makes this forum the best of its type anywhere on the internet. Keep talking guys!

[Steve Lee]

Normally for the proper adjustment of reel to reel tape recorders heads a calibration tape is used in collaboration with a scope. This is the only way of being able to align the azimuth and phase.
These tapes are not cheap as they have to be produced on machines that have been calibrated to industry standards, otherwise there's no point.
I guess the same thing would be need but this time on super 8 film. This is not something that can be produced at home on the average projector. It would need to be professionally produced.
I should imagine back in the day all of the manufacturers had this test film in order to do this job. Where we would find this now, who knows.

[John Taylor]

The thread is good but I've not seen many comments on the drip/paste stripe as yet, the stripe to wear your heads head at double the rate, rough bumpy and often poor in sound quality.

Just to clear a few things up for those who dont know, the CLA value is a surface roughness measurement, (centre line average). Glass is zero and this is where a lapped head should be at or extremely close to, a mirror finish, any harsher and films will be damaged. Hence my question, when a head is relapped, what would the CLA measurement be? It's a very important question for those who can do it or get it done.