anyone else noticed dolby's calibrated cat 69t's test film suffers from rather highish (4.0-4.6) failure rates? We're talking after pain stakingly aligning a dolby reader on a kinoton FP-30, and on a CP650 running i *think* it was 1.1.7 firmware.

I've seen prints running 20 mins after the alignments with error rates in the 1s and 2's.

Mark Hathaway
Atlab Image and Sound Technology
Melbourne, Australia
 

Dolby's test films have been rather unreliable for some time and hardly the model of Digital Correctness.

If you get batches of Cat. 69T, you will find that the level does vary somewhat and it doesn't agree with the older B&W versions. The HF response on Cat. 69P is truely pathetic. If you tune your sound head with it, watch out on the slit loss...if you set Cat. 69P for flat, you will in fact have boosted the HF too much. If you still have any, check it against some B&W Cat. 69 (or even Ultra Stereo's TT-1).

We have taken to getting 1000' rolls and measuring them against reference films to derive an offset (in 1000' lengths, you sometimes even get a QC sheet to compare against).

In any event, yes 4s and 5s on Dolby Cat. 69T or P are more common than not. For what it's worth, I have found that the current Kinoton readers (with the new super sensitive lens...4/02 and later) to out focus all other readers I've tested, penthouse readers by Dolby included! I was quite surprised.

Steve

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"Old projectionists never die, they just changeover!"
 

So who is correct? The SMPTE or Dolby? We decided that the SMPTE is the culprit because it is the buzz track that seems to vary, not the Cat97.

Also the latest version of Cat69P (which from memory is dated 99) is flatter at the top end than the '97 (or is it 94?) version. It is a shame that Dolby did not follow the long time practice for magnetic test tapes for studios whereby a test tape (film) is either absolutely flat, or if not, is supplied with a correction chart to tell you where and by how much it varies from flat. Otherwise how on earth do you know what is flat and what is not?

In our work developing reverse scan readers for another company we took the approach that the Dolby pink noise signal master was most likely flat when it was passed on to the printing lab, so any HF rolloff is due to limitations in the printing process and thus the latest version of Cat69 at any point in time should represent the same rolloff as feature films processed at that same point in time (albeit at that same lab). It is not a very satisfactory nor scientific basis for determining whether a reader is flat or not, but it is all we have.

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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

Are you saying that I'm going to get wrong results by using the test films?

Why then should anybody use them?

Is Dolby aware of the problem? If so, why do they keep releasing bad test films?

I just paid a lot of money for new rolls from Dolby.

Oh...and to top everything off...my new tech from Miami says that somebody at Dolby told him (off the record) to set levels at 80dBC if we want to continue using 7 as the fader reference...or else play the movies at 6.

It's like nothing is standard anymore.

What test films are the folks at THX using? (I know, I know...they're a dying breed...)

Ken...your theory has some merit but it isn't the case. When on is setting up an A-Chain, they are going to focus and set slit loss on the same piece of test film. As such, any variences in the thickness of the film will be negated. Where the older style Kinoton head wins out is that once set, the focus will be spot on regardless of release print thickness. However, I've found that Kinoton's small drum (again, this is the style previous to their current reverse scan reader) is a nightmare to keep from having flutter...they having it spinning too fast; ANY problem with the bearings (drum or roller) shows up VERY fast. Use the Dolby Jiffy test film's channel sweeps...it will show up painfully well. On the current readers, I haven't had this problem.

Ray, I was leaving Cat. 97 out of it . The Current SMPTE Buzz track (P35-BT) is shot in a pin-registered camera. In fact, it is the same camera that shoots 35-PA/IQ. That camera is and target are consistantly checked for accuracy. Who checks Cat. 97? If it is the same people that check Cat. 69 then the answer is nobody. Therefore, I sided with the Buzz Track as being the one that tells the true tale. Have you found that the current style Buzz track (the one that has the name on each frame and has the "ticking" noise on the track to vary from batch to batch? Have you put it under a microscope to compare it to the standard to see if it agrees? How about Cat. 97, ever measured it?

How about Dolby Cat. 566...uniform illumination film, another film that will let you read the edges of the soundtrack. This is one of the most handy of test films and yet most people don't use it or have ever heard of it. It will let you set up your exciter lamps or LEDs...infact it will show up the problems of using non-adjustable LED mounts. Ever get a new soundhead (factory aligned) and found that when setting Dolby level, one was notably higher than the other? Run some Cat. 566 and the reason will be found. By properly setting the lateral position of the LED, you will then find the Dolby level is same on both channels. component engineering's readers beat everyone on that one.

Manny, as to your results...what I'm saying is that they are going to vary. As I said earlier, we have taken to buying the test films in larger rolls, measuring it and then making up offset files to align stuff. That way, from year to year, everything is consistant. As to the Cat. 69P, I believe you said you use CP-45s...don't worry your hosed anyway! The Cat. 222SR/A has an overly bright HF to start with, when in doubt on a reverse scan, leave the slit loss all of the way down, you would need very little boost anyway to achive "flat" response.

I would trust the Buzz track film for setting lateral alignment. With reverse scan readers, Cat. 97 is superfulous. At best, it can argue with Buzz track or show that your cell is not centered to the lens (which sometimes does happen but not too often). If you think about it, Buzz track locates the edges of the track, Cat. 97 locates the middle. You are still dealing with the same soundtrack area. Dolby was specifying to scan 102% of track (yes more than the whole thing) to avoid "clipping" the edges...except, it makes it more prone to noise. Thus, you can get a more accurate position by looking for the edges.

As to 80dB for setting level. The CP-650 people decided to "fix" a nagging problem that the reference level just wasn't right after all these decades...thus they fixed it on the CP-650. If you set a CP-650 to 85dBc, you will actually be 2-3dB louder (I forget what the exact number is but that is the ballpark). Now who asked for this? How many of you out there thought the old standard (using the Cat. 85 or the SG-1) thought "every time I set up a processor 'to spec.' people rush out saying it is just too quiet"? So, if you have a booth with mixed processors, you better set your CP-650s lower if you want the faders to match up in level. So go ahead, set up your CP-650 to 80, you still won't be running at fader 7 to please the bulk of the customers.

Steve

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"Old projectionists never die, they just changeover!"
 

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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

On your Buzz track film, was all of it the new style that varied from batch to batch? If it is, I'm most curious (I haven't found that...yet). I have found Cat. 97 to not always read the same though (even more curious since you say you have consistancy there!). Using Buzz track to center the film with respect to the cell, presuming that the Buzz track really does located the edges of the scanning area accurately, by definition, you are centered on the cell in a reverse scan situation. If Cat. 97 differs with Buzz, one or both of the test films is defective.

This is not the case for forward scan where the SLIT is fixed (on most system, Christie, for instance made it movable). In that case Buzz strictly sets the slit to track relationship, Cat. 97 set the Cell to film relationship. In revserse scan, the slit and cell are one assembly. Whereas the slit is affixed to the cell, you can't really move them independantly...the function of Cat. 97. For instance, on a AA-2, Cat 97 only merely confirmed that reverse scan has no cross talk.

Also with the overscan recommendation by Dolby, one could have a non-clipped Cat. 97 yet not be centered. Buzz track would be the more critical setting. If you get a system where the cell to lens relationship is off, Cat. 97 can be useful to recenter the cell. If the slit is off-center to the cell, you are hosed and at best can come up with a compromise.

Personally, I'd prefer if they went for only 100% scan and settle for the potential of a clipped signal at 100% modulation. I feel some of the noise assoicated with reverse scan is the junk at the edge of the applicated area being read by the overscan.

Steve

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"Old projectionists never die, they just changeover!"
 

Whether P35-BT varies on a batch to batch basis in an on-going fashion I do not know. What I do know is that we possess a number of reels of this product purchased at different points in time, all of which bear the P35-BT lettering in the picture frame, and there are at least two widely differing lateral positions in this stock. Now it has long been known in the industry that there was a difference in lateral position between Cat97 and P35-BT, but as to whether P35-BT was originally incorrect has been corrected in more recent batches I don't have that answer, but I will try to find out for sure and let you know.

Also you are overlooking the fact that some reverse scan readers (the cinemeccanica comes to mind) allow you to change the focal length (that little grub screw on the side). Why they provided this adjustment is anyone's guess as it should never need to be touched, but it is there none the less, looks like a focus locking screw and is therefore a trap for young players. Changing the focal length alters the scanning width. It is also possible to alter the lateral position of the cell in most RV units.

More importantly, manufacturers of reverse scan systems need a reliable reference to work to when setting the internal parameters (such as focal length) of their products. Dolby recommends they use P35-BT Buzz track and Cat97. When you consider that these test films are to some degree inaccurate, or at least are in disagreement, it then falls on the manufacturer to determine which copies of these films are correct and which are not. As I said earlier, a test film should be precise, and if that is too difficult, then it should come with correction data.

BTW Dolby's recommended procedure for setting the scanning width of an RV unit is to alter the focal length on buzz track to the point where the cell is reading the whole of both buzz track square wave signals and then readjust the focal length to the point where the square wave signals are 20dB down.

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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

It would seem that based on our various findings, our industry's test films are at best questionable. Your statements about Buzz track calls into question 35-PA as well since it is shot on the same camera.

What this industry needs is a varification procedure to ensure the test film is accurate.

Steve

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"Old projectionists never die, they just changeover!"
 

Just one more interesting point on this subject. An LED contains 22 light emitting elements set in two rows of 11. For an LED which is correctly aligned laterally, only about half of the 22 light producing elements are actually directly behind the soundtrack and therefore seen by the solar cell. So there is some room for lateral error in the LED's position before left/right balance is affected.

Also Dolby recommend that the LED should be positioned precisely 1mm behind the film. If it is closer it will be too much in focus and the individual elements will be seen by the solar cell. This results in distortion because the light is not uniform across the soundtrack but has a dip between each light element. Similarly if the LED is mounted too far away it's image will be magnified and so a smaller number of light elements will be seen by the solar cell, and whilst they are out of focus, this greatly increases the chance of the solar cell seeing dips between the light elements.

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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
 

Steve

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"Old projectionists never die, they just changeover!"
 

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).

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
 

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

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"Old projectionists never die, they just changeover!"
 

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

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"Old projectionists never die, they just changeover!"
 

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.

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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
 

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

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"Old projectionists never die, they just changeover!"
 

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

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

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"Old projectionists never die, they just changeover!"
 

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

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

I think I might have had one LED that had what I would call "peaks and dips." It was a defective LED...like part of it wasn't working.

As to the hot spot in the middle...I have yet to find a lens system that did not exhibit this, from my early days in photography up to the present day.

Here is some information to chew on though. Lets say the Cat. 566 film is flawed and it has louder tones recorded in the middle. I have no problem with the idea that it is flawed. After all, we have now ripped into Cat. 69T, 69P and Cat. 97 so why should Cat. 566 be blessed? But ok, for the sake of argument (and I love those) it is flawed....

...How would you explain that moving the just the cell will cause the hot spot to move? The LED to the film relationship hasn't changed, the lens to film hasn't changed, the slit to cell hasn't changed...only the cell to the lens. Either the cell's sensitivity isn't uniform or the lens isn't.

Instead of throwing up your theories and even the science, play with some Cat. 566 and set up a reader system...compare your results to using Cat. 97 (or other methods). And I mean REALLY play with it...see how it reacts to an LED that is off on it's azimuth, move the LED laterally and vertically, move the cell about. Then take two fresh systems, set up one my way, one the way you have been doing and compare the speed and accuracy of each. If done properly, both systems should agree with each other if both are valid.

In my opinion, Cat. 566 tells you what you need to know (LED position, rotation) in a visually easy to read sort of way. Now a difference I may have is that I use the ABACUS ARTA-600 scope attachment for my RTA so the 6 bands show even the slightest change, unlike LED based analyzers...Also the R2 can be used though there is a delay between action and response.

When using Component Engineering systems (which I use for the vast majority of my retrofits and all Century/Simplex 35 installations) you want to ensure you don't "crowd" the digital LED's space so making sure you are as far inboard without compromising the analog scaning is very important. Cat. 566 lets me do this easily.

Steve

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"Old projectionists never die, they just changeover!"

Moving the cell off-centre, even by a tiny amount will cause one side of the soundtrack to fall off the end of the cell, so I don't quite follow what you are getting at. But I will take your advice and do some comprehensive tests under lab conditions with Cat566 and we will see what comes up.

BTW I prefer to stick with the science, hokus-pokus and witchcraft are not within my area of expertise. Naah... just kidding, of course they are.

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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
 

Lets say the MIDDLE bands on Cat. 566 are higher than the outer ones...When you move the cell with respect to the lens, the high bands move with the movement of the cell.

Lets call the bands looking from left to right 1, 2, 3, 4, 5, 6 (6 bands total)

Now lets say that band 4 is 1 dB higher than the rest. Now I shift the cell with respect to the film and I find that band 3 is 1dB higher than the rest (ignoring what happens on band 6 for the moment)...why would the high band shift if it wasn't a lens hot spot issue?

Also, the slit/cell is wider than the soundtrack. Both Kelmar and Component Engineering (an I would guess the rest) have to mask off the ends of their slits. CE hand paints them, Kelmar has a mask they set up to have them painted....depending on how well the mask was set up or how well the guy/gal dabbed the slit, is how much over you get.

Steve

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"Old projectionists never die, they just changeover!"
 

AFAIK the cells used by CE and Kelmar are made for Dolby by the one company and are the correct width for the soundtrack at a specific focal length. I have not seen any with the ends masked but I have not pulled too many apart so I take your word for it that at least some have been treated in this way. This would mean that the focal length of the lens/tube assembly was made too short for the width of the cell, either deliberately for reasons of mechanical restraints in a particular projector or by accident. It would also result in the height of the slit being greater than necessary, resulting in reduced HF capability.

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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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"Old projectionists never die, they just changeover!"