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Steve

SMPTE Recommended Practice RP 68 specifies the Buzz Track dimensions with the print Reference Edge (NOT the perforations) as the datum plane. Likewise Standard SMPTE 203 specifies the lateral location scanned area of the analog track relative to the Reference Edge of the print, NOT the perforations. An edge-guided camera/printer would be preferred to a pin-registered camera for this application (principle of cancellation).

However, no one's printer seems up for it. Presuming the film is slit properly and the perforations are made properly and are consistant relative to the reference edge...all should work. It is completely odd that the current method of making Buzz track is to photograph a target frame by frame (the necessary overlap is the thumping people hear in the current version).

My point above though is that with the current Buzz track, it is at least in a measured system, Cat. 97 doesn't seem to be able to make the same claim.

But hey if someone could have a measured test film printer, I'd be all for it.

Steve

It's not at all encouraging when you hear that if you set your system up using this new batch of Dolby test films, you wind up with a super hot high end and a reference level that will make people's ears bleed. You gotta admit, this is pretty scary.

And here I just bought a whole set of new test films and when I opened them I thought, gee these films are not on B&W stock any more....Dolby must have found color stock to be more accurate, maybe with better tolerances than B&W....silly me. Hmmm, I didn't tear any of the stickers off these rolls yet; maybe I can return them and use the money for extra shrink sessions. I guess I'd better get out my splicy old B&W loops. And adding insult to injury, I also discover that I didn't even order a most important test film, the Cat 566. I don't think it was even on the list of test films that Dolby sent me.

Not all of us have test labs where we can test the test films, not that we ever thought that would be necessary. Manny and I, like most of us I would imagine, just follow the Dolby Manual and use the test films Dolby and SMPTE sell to us and we assume that all is well in the Land of Cats. Is NOTHING sacred?

And to reiterate Manny's very pertinent last question: What do the THX honchos use to set those THX certified systems up to spec? Huh, huh?

Also, with all due respect, I think you are wrong regarding the use of this film for lateral alignment. I have not seen any dips between LED elements either in the vertical or the horizontal axis when the LED is correctly positioned 1mm behind the film. If there were such dips you would get significant distortion of the soundtrack, regardless of the lateral alignment, because dips = non-uniform illumination = distortion.

There are five separate tracks on 566, each being a different frequency, and there are roughly 5 LED elements laterally positioned across (behind) the track on each of the two rows. You also have 3 (effectively unused) elements on either side outside the soundtrack on each row. If you used Cat566 to align the system laterally AND if there were dips as you suggest, then you would see the left right levels go through numerous dips and peaks as you moved the system laterally. Which set of peaks would you chose? I for one have not had any such experience. For me, you're idea just doesn't quite gel.

Oh and one final thing. As the 5 tracks on Cat566 are each a different frequency and as most reverse can readers are not all that flat (the CE unit has a hole in the midrange, 1k25 and 1k6 are 4dB down compared with the region from 40Hz to 400Hz and 3dB down compared with the region from 2k5 to 6k3), using cat566 will not necesarily give a true indication of relative level. Think about it for a moment.

First off, I believe you misread my statement (or I miswrote it...I haven't scrolled back to see exactly my wording...)

Cat. 566 was actually developed to help align exciter lamps. There are actually 6 tones, not 5. Three are on the right channel and three are on the left...if you only have 5 your LED is out of alignment!

What I said (or ment) is that the Cat. 566 test film is for setting LED lateral alignment, not film lateral alignment. Once you get the film laterally positioned with respect to the cell (on a reverse scan) you need to get the LED laterally centered on the film. Cat. 566 does this just dandy. Cat. 566 will also show up hot spots in the lenses used for readers. Try it on a slow lens or one of the more modern ones...the middle two frequences will tend to be higher than the others. Whereas one has little control over the lens used by a manufacturer, it is more of an informative thing than anything else.

However, I will reitterate, Using Cat. 566 is the best way to determine LED lateral placement. If the LED doesn't light the whole track area, you can't possibly get everything else right.

So once again on the test films...

Pink noise (Cat. 69) sets the focus/azimuth
Buzz track (P35-BT) sets the film to cell/slit relationship
Cat. 566 sets the LED to film or Exciter to slit relationship
Dolby Tone (Cat. 69) sets the reference level.

For forward scan systems, Cat. 97 sets the cell to film relationship.

Oh and as to keeping your old B&W test film with bunches of splices in it...that doesn't work either! The B&W stuff was printed on acetate, it shrinks! This more of a factor for the Buzz track test film though. The current Buzztrack is photographed onto polyester...more stable

There are some batches of the old Cat. 69 that wasn't too flat in the 3KHz region either.

Rather than getting depressed about the test films, just remember, ignorance was bliss!

Steve

My references in the previous post to "lateral alignment" refer to the LED vs the film, so we are talking about the same thing. Now please re-read my post and tell me how Cat566 can be used to ACCURATELY set lateral alignment of the LED. The LED is roughly twice as wide as the analog soundtrack so there is plenty of leeway. The most important thing is to ensure that the soundtrack is well clear of the ends of the LED light bar. Of course an exciter lamp is a different proposition, but I didn't think we were discussing exciter lamps.

Not wishing to be argumentative, but my recommendation is to thread up some Cat97 (the slight lateral offset error with this film is not important for this purpose) and use a small right angled mirror to locate the LED precisely centered on twin track lines of the Cat97. It is simple and foolproof. The really critical alignment is, as you say, in the vertical axis and this is best done running Dolby tone. There are two rows of LED elements to ensure that there is a wide, even band of light with sufficient overshoot to ensure that (a) a small azimuth error in the LED does not cause uneven illumination across the slit and (b) that any vertical vibration will not cause any fall off in light hitting the cell.

If you haven't tried using Cat. 566 to set an LED, I suggest you do so and compare the speed and accuracy it can provide as compared to other methods. Personally, I've tried it just about every which way...including Cat. 97. Give my way a try (if you haven't). If you have, we will have to agree to disagree on this one.

Steve

I also did some tests with Cat69P pink noise film. I found a pristine reel of MAY97 film (the really dark pink coloured stuff) and measured the response on a carefully calibrated reader. I then did the same with a loop of not-so-pristine 8/99 stock (the really pale pink coloured stuff) and this is what I found:

Both films showed a similar response up to 12kHz, however the MAY97 film was down -5dB at 16KHz and -9dB at 20kHz compared with the 8/99 stock. Unfortunately I do not have any of the old B&W double-sided Cat69 to check this against, nor anything more recent than 8/99 but it does seem to confirm what we have found previously, that the newer pale pink stock has been substantially improved at the top end over the older dark pink stock. Maybe this is why the radical colour change. Or maybe the dark pink was a reflection of somebody's very red face over the poor HF response.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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As to being able to move the LED about somewhat on D-Tone before fall off, that wouldn't be surprising since D-Tone is at 50% modulation. Cat. 566 uses the whole optical track area.

Do you have any Cat. 566 to play with?

Steve

Gordy, how much variation have you seen across the LED and have you seen "dips" or simply a gradual fall off either side of centre?

I might add that the red LEDs that I have used in testing are almost brand new. It is distinctly possible that as the LED ages, the degree of loss in light output may be different for each of the individual elements. If that is the case, then distortion will rise over the life of the LED, ugh! Have we uncovered yet another reason why red LEDs degrade audio performance? Now is this interesting stuff or what? I am very keen to investigate this further. Stay tuned folks!

BTW if anyone has a reasonable length of unused Cat69 (B&W version) that has been properly stored, I would be very interested in purchasing it (just send me an e-mail).

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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According to SMPTE 40-1997 and SMPTE 203-1998 the total width of the stereo variable area soundtrack on 35mm prints is 1.93mm and the recommended width of the scanning device is 2.13mm (10% greater). (Presummably this 10% overscan is to counteract any sideways weave).

We have measured the width of the internal element region of the LED arrays to be 4.0mm and the centre to centre distance between the outermost LED elements to be 3.53mm. This gives, for an LED centred precisely on the soundtrack, a worst case minimum light bar overscan of 1.4mm, not taking into account the fact that the LED is out of focus which produces a magnification (increase in width) of the light bar and that light is output from the whole area of the element, not just from it's centre. So there should be a margin for error in the lateral positioning of the LED of approx 1mm, either side of exact centre.

Thus it should be possible to move the LED sideways up to 1mm in either direction off-centre, before any imbalance in the relative levels of the two halves of the soundtrack is observed. This has indeed been my experience in the lab on tests with near-new LEDs which are located at the correct distance behind the film.

Therefore, the question remains as to why both of you have observed peaks and dips as the LED is moved laterally, or a definite peak at the precise centre position.

To summarise what I have said previously....

1. If the LED is positioned too close to the film it will result in an uneven intensity across the light bar which will cause peaks and dips. This will cause significant distortion of the soundtrack. Moving the LED laterally you will see the peaks and dips shift with the LED. You might be able to move the lateral position of the LED to neutralise the visible effect on the RTA using Cat566 film, but there is no way to eliminate the effects on distortion. The peaks and dips will still be there, just not visible on the RTA.

2. If the segments of an LED array lose light output at different rates over time (and this has yet to be proven) it would result in older LEDs having an uneven illumination across the light bar. Once again resulting in peaks and dips and therefore, distortion.

3. If some lenses have a "hot spot" at their centre as Steve suggests, this should not cause a corresponding hot spot at the centre of the solar cell or in the soundtrack reaching the solar cell, because light from any given point across the soundtrack travels though all parts of the lens, not just the centre. However an apparent mid-soundtrack peak could be caused by the LED being mounted off-azimuth compared to the cell. It could also be caused if frequency response of the reader concerned is not uniform for all 6 frequency bands of the Cat566 film and is in fact more sensitive to the two bands nearest the centre of the soundtrack. This would create a mid-soundtrack peak. Conversely if the reader is more sensitive to the frequency bands at the edges of the soundtrack, this will cause a mid-soundtrack dip, and so on. This total dependency on the frequency response of the reader being precisely flat (which most are not) is why I am not convinced Cat566 is the best method for lateral alignment of the LED.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935