Film-Tech Forum ARCHIVE: Reverse scan and slit loss

This is topic Reverse scan and slit loss in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Fabian Schreyer (Member # 5120) on 07-26-2016, 06:32 PM:

I was wondering the other day if you need any slit-loss compensation when using a perfectly aligned reverse-scan soundhead. (For example Kinoton)

As I imagine, due to the magnification the slit should be small enough that the cutoff frequency is never reached. At least not in the audible spectrum.

Any thoughts on this?

Posted by Steve Guttag (Member # 268) on 07-27-2016, 05:57 AM:

You are seeking the widest frequency response...then yes, even with the Kinoton RSSD, you can go a little wider with slit loss correction.

The big trick is getting good test film and having a good enough preamp to make it worth your effort. Much of the Dolby test film has questionable HF response. I tended to use NTAV's NT-Pink test film...which was good. Older CAT69 (B&W) was good save for a couple of batches.

But if you are really trying to get flat out to 16KHz and beyond, slit loss correction, even on reverse scan is the only way.

Posted by Fabian Schreyer (Member # 5120) on 07-27-2016, 06:39 PM:

Thanks for your input!

Seems like I have to test it out. I hope I will manage to find some NT audio test films some day.

Until then I'll have to stick to my CAT69P. I have prints of different age, but none of them is B&W.

Posted by Sam D. Chavez (Member # 1841) on 07-27-2016, 07:57 PM:

I agree with Steve and in particular about the less than stellar quality of the 69P. Thus, I tend not to use the slit loss with red readers unless I know the quality of the alignment film. If you do use the slit loss, you should apply the same amount of correction to both channels as in counting turns on the trim pot or slider. If in doubt go with the channel that requires less EQ and duplicate it on the other channel setting.

Nothing much useful above 12 k in any case.

I use my dwindling supply of B & W 69 for my tuneups.

Posted by Fabian Schreyer (Member # 5120) on 07-28-2016, 05:21 PM:

quote: Sam D. Chavez
Nothing much useful above 12 k in any case.

Can you explain what you mean by this? Does it mean that the linear frequency response above 12k is not that important for good sound?

Posted by Sam D. Chavez (Member # 1841) on 07-28-2016, 06:22 PM:

HF above 10Khz did not make it onto the print from the negative or the recordist did not try too hard as what ends up there is noise and scratches along with the audio. Dolby SR helped make it tolerable to good but analog optical was only ever so good, esp. distortion which could run to 3% typically.

Hardly pristine in any case.

Posted by Fabian Schreyer (Member # 5120) on 07-28-2016, 07:40 PM:

So it is kind of "If you have some reliable pink-noise film, adjust the slc and you have perfect results. If not, just go without it and the result will still be fine"?

Posted by Sam D. Chavez (Member # 1841) on 07-28-2016, 08:32 PM:

That is my opinion for everyday use. You do need to make sure the slit loss controls are wound all the way down as there's no telling after all this time where they are set.

Posted by Fabian Schreyer (Member # 5120) on 07-29-2016, 06:02 AM:

Thanks for your thoughts on this!

Posted by Randy Stankey (Member # 64) on 07-29-2016, 07:11 PM:

I had to retrofit an entire theater when they decided to get rid of all the Jax Lites and convert all the projectors to reverse scan LEDs.

My instructions were that slit loss correction wasn't "needed" but I was still supposed to use the frequency analyzer to check it anyway. The reason I was told was basically what Sam said. After years of being adjusted every which way, there's no telling what they were set at.

Since you are already going to have your scope hooked up to do the work in the first place, it only takes two extra minutes to hook up the analyzer and check the HF setting.

That control exists. Even though it ends up being set all the way at the minimum, 99% of the time, the prudent person should verify that all is working the way that it should be. Right?

Only a dummy would pass it up when it takes such a short time to check it.

Posted by Louis Bornwasser (Member # 3063) on 07-29-2016, 07:38 PM:

Actually there is quite a bit of energy above 12k. Whether or not anyone can hear it is another question. The smaller the cinema and the better it is the more you can tell. Big houses cannot get the hf energyery far back. Who has large cinemas anymore?

Also check phase across the entire spectrum using pink noise. L-R on proper analyzer.

Not turning up the hf? I would fire them.

Posted by Louis Bornwasser (Member # 3063) on 07-29-2016, 07:38 PM:

Posted by Sam D. Chavez (Member # 1841) on 07-30-2016, 01:42 PM:

My main point on this topic was the lack of reliable test film. That being the case, the best practice is first do no harm, so minimum slit loss is best.

Posted by Tom Ruff (Member # 6153) on 08-01-2016, 02:32 AM:

I was sitting here at the studio reading this post and had a thought. We have a SR-D/DTS camera here and still shoot a lot of optical negatives. If there is a need for black track pink noise or level set tone, we could set up a discussion thread and see what everyone needs and maybe use up some of our short ends.