This is topic silabance (sp?) in Dolby A playback systems in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Josh Jones (Member # 447) on 01-12-2002, 03:23 PM:
I have noticed a rather strange occurance when we run dolby "a" type films. the dialog seems to have an aweful lot of silabance or splatter on s's and other vowels. is this just the nature of the Dolby A system or is something misaligned?Simplex 35/70, reverse scan, CP500 w/ 222SR/A card and EV stage speakers.
Josh
Posted by Gordon McLeod (Member # 33) on 01-12-2002, 06:28 PM:
Check the hi freq slit loss and also the dolby level
Posted by John Walsh (Member # 168) on 01-13-2002, 12:03 AM:
Could be the diaphram in a HF speaker, no? It happens only in "A"?
Posted by Steve Guttag (Member # 268) on 01-13-2002, 12:34 AM:
Dolby-A doesn't or shouldn't have any characteristic "S". The EV speaker is probably a bit twitchy on the high-end. Also, if the print was SR encoded and played in A it would be a bit spitty. This all presumes that everything is calibrated/aligned properly to begin with.Steve
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"Old projectionists never die, they just changeover!"
Posted by Josh Jones (Member # 447) on 01-13-2002, 01:07 AM:
we are running an old print of "Batman" 1986 or so. I only really noticed it last night when I watched the movie. put a little too much FG on thought, nice white circles around the dirt. aside from that, SR plays back nice and clear without any problem.Josh
Posted by Ray Derrick (Member # 1001) on 01-13-2002, 01:57 AM:
JoshSibilance is not characteristic of A-type NR (if the print is also A-type as you say). Do you have any spare A-Type NR cards? You may have a faulty one. If there is a fault it will usually be only in one channel which will cause the sibilance to constantly shoot across to the left or right speaker. Sit in the middle of the front row and you will easily hear this if it is happening.
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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
Posted by Craig Hanham (Member # 1010) on 01-13-2002, 01:50 PM:
Your sibilence problem is most likely to be your EV speakers.
What model are they? What is the frequency respone like for HF (from manufacturer specs)
EV does have a bit of a rep for noisy high freq for some of their units.
If you can find time try different speakers, also look at the accoustic masking of your auditorium.
Posted by Josh Jones (Member # 447) on 01-13-2002, 08:34 PM:
I really doubt it is the speakers, SR and digital prints playback nicely. or could it be just that with the better bandwith of reverse scan, that would normally be covered up using a forward scan system. after all reverse scan wasnt around until the early 90's right? and like I said the print was struck in the 80'sJosh
Posted by Steve Guttag (Member # 268) on 01-13-2002, 09:07 PM:
Is it just that print? I have heard similar problems with the original BATMAN. Another thing I heard on BATMAN was an occasional crackling like it was overmodulated and possibly some application spatter (but that would seem more print related.Steve
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"Old projectionists never die, they just changeover!"
Posted by Josh Jones (Member # 447) on 01-13-2002, 09:20 PM:
"scrooged" also had the silibance to it. the exact processor we use is a CP500-70 meaning it has two more dolby a cards for 70MM, so in effect 6 channels of NR processing. preamps are Ultra Stereo and Teccon headsJosh
Posted by Paul G. Thompson (Member # 655) on 01-13-2002, 11:50 PM:
I have to agree with Steve Guttag. When I watched the movie "Dazed and Confused" and that other POS you were running, I thought the sound was great! What I have heard in "Scrooged" that you speak of was something I heard being emitted from the HF drivers, but since the other movies sounded great, I just contributed it to the age of the print. It was so minor I didn't even think it was worth mentioning.
The only thing I noted was too much of a volume change when the system defaulted to SR "A", plus the dynamic range seemed to suffer a little more than what I was accustomed to hearing in other theaters equipped with reverse scan.
I think your system does need slight amount of tweeking. It might be worth the effort to call that to the attention of "TJ" the next time he is in town.
Paul
Posted by Ray Derrick (Member # 1001) on 01-14-2002, 01:34 AM:
Sibilance is normal with human speech, varying in degree from one individual to another. But "sibilance distortion" is caused by distortion of upper mid-frequency sounds, usually around the 6kHz region. It causes a kind if slurring or spattering on "S" and "F" sounds. So Josh, are you talking about sibilance distortion or the sound just being excessively bright?I agree with what others have said regarding EV HF systems. They have a natural (and significant) response peak at around 6kHz which is normally equalised out by the cinema technician. If the driver becomes faulty, this natural peak can turn really nasty, although as Josh says, this would normally show up on all formats.
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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
Posted by Paul G. Thompson (Member # 655) on 01-14-2002, 03:13 AM:
Correct me if I am wrong, but I think the wider bandwidth the reverse scan can handle, the more it is going to "see" that a standard optical lens cannot. I would think that reverse scan would be very capable of reproducing harmonics to produce the sound Josh discribes.Paul
Posted by Josh Jones (Member # 447) on 01-14-2002, 07:42 AM:
yes, that splattering is what I am talking about. it only occurs in dolby A.
Posted by Steve Guttag (Member # 268) on 01-14-2002, 09:16 AM:
Also, what test film did you use to set up your "A-Chain"? Dolby's Cat. 69P (color film stock) is HF deficient and will cause one to boost the HF response too far if set up the traditional way (boost HF trim for flat response).Steve
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"Old projectionists never die, they just changeover!"
Posted by Ray Derrick (Member # 1001) on 01-14-2002, 09:38 PM:
Paul, the slightly narrower slit in current reverse scan systems extends the frequency response (on average) by only an extra third of an octave above forward scan exciter systems. This extra energy in the 16kHz region is not normally associated with sibilance distortion. Furthermore, the SR playback system has an in-built de-emphasis curve which boosts signals at 16kHz by about 10dB. So if the problem were related to the wider response of the reverse scan system, it should get worse when Josh switches to SR. I tend to think it may be a problem in the A-type decoders on Cat222 card.------------------
Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
Posted by John Pytlak (Member # 331) on 01-16-2002, 08:28 AM:
Sibilant distortion is often due to cross-modulation distortion. Here is a link to the Kodak publication on procedures that should be used by labs to control the exposure of analog soundtracks:
http://www.kodak.com/US/en/motion/support/h44/h442.shtml ------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
Posted by Josh Jones (Member # 447) on 01-16-2002, 09:21 AM:
Steve,I didnt align the A chain, our tech did when he was up here last, I wasnt around at the time.
Josh
Posted by Ray Derrick (Member # 1001) on 01-16-2002, 10:40 PM:
John said "Sibilant distortion is often due to cross-modulation distortion". In the world of audio, there are several distinctly different types of sibilance distortion.Conventional sibilance distortion is usually caused either by selective clipping of upper mid-range signals (harmonic distortion) which produces new higher frequency harmonics or by non-linear dynamic action which often occurs in compressor-limiters used in studios, resulting in excessive emphasis of low energy speech sounds like "s". This latter phenomenon is commonly referred to as "excessive sibilance" rather than sibilance distortion.
In optical film soundtracks the type of sibilance distorion caused by cross modulation distortion results in the generation of new lower frequency signals, not dissimilar to the effects of aliasing in analog to digital conversion. However the audible effect of these three types of sibilance distortion is noticeably different.
John, on the subject of cross modulation distortion, do you know what the "carrier" frequency is that is normally used for the cross modulation test signal used by labs? The article you provided a link to mentions that 400Hz is used as the amplitude modulating frequency but not what the higher "modulated" frequency is. I thought is was around the 4.5kHz mark.
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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
Posted by Frank Angel (Member # 248) on 01-17-2002, 12:55 AM:
Could it be your 500 logic is not kicking the Cat 222 to A decoding? It might be locked in SR mode and decoding the A track incorrectly. As much as Dolby doesn't like to use the word incompatiblity when talking about these two types of NR, pretty noticable audio anomolies, especially in the high frequence area of the spectrum, will become evident if an A print is played back with SR NR, and vice-a-versa.
Posted by John Pytlak (Member # 331) on 01-17-2002, 08:17 AM:
Ray Derrick asked: "John, on the subject of cross modulation distortion, do you know what the "carrier" frequency is that is normally used for the cross modulation test signal used by labs? The article you provided a link to mentions that 400Hz is used as the amplitude modulating frequency but not what the higher "modulated" frequency is."The original SMPTE paper by Baker and Robinson suggested 9000 Hz and 400 Hz. as the frequencies:
J. O. Baker and D. H. Robinson, "Modulated High-Frequency Recording as a Means of Determining Conditions for Optimal Processing," Jour. SMPE, 30:3-17, January 1938
SMPTE Recommended Practice RP 104 states:
"3.1.2 Section 2: A high-frequency sinusoidal
signal amplitude modulated to (80 ± 5)% of full
modulation (2.0 dB ± 0.5 dB below full modula-tion)
at 400 Hz ± 10 Hz. The peak amplitude of
this modulated signal will be (80 ± 5)% of that
required for full modulation of the audio track (or
2.0 dB ± 0.5 dB below full modulation). The high
frequency will be 6000, 7000, 8000, or 9000 Hz
± 100 Hz for formats 16-mm and larger. The
modulation high frequency will be 4000 Hz ± 100
Hz for formats 8-mm and smaller."
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
Posted by Don Sneed (Member # 955) on 01-17-2002, 12:49 PM:
When I align the "A" Chain on PM calls & new installations,I always use a oscilloscope & a RTA to check & align the focus on the optics, on LED readers I set to a flat response to 16K, on exciter lamps I set the focus to roll off at 12K, this always sounded good to me & my customers, I try to get the perfect response using a pink noise loop with the optic focus, then adjust the Hi. freq. control on the preamp card to get rid of any bumb in the hi-end using the RTA for that....In my traveling a scope & RTA is alway with me & a R2 for the THX houses....I like the RTA better for "A" chain alignments...
Posted by Ray Derrick (Member # 1001) on 01-18-2002, 04:06 AM:
So Don, are you saying that you deliberately limit the response of an exciter lamp optic to 12kHz, even though with a type-X slit lens you should be able to almost get flat to 16kHz? If so why?Also, thanks for the info John.
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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
Posted by Don Sneed (Member # 955) on 01-18-2002, 12:32 PM:
Ray, on Exciter lamps I have never been able to get it out to 16K, now on the LED readers for analog 16K is no problem, but I never could get anything above 12K so I roll off at 12K...but now everything I do is LED readers, I don't service many exciter lamps anymore...
Posted by Josh Jones (Member # 447) on 01-18-2002, 04:32 PM:
really? my RCA 9050 is still exciter based and it is only down 1 Db at 16Khz. sounds real good too. do any of you guys think it has a .5mm slit, or could it be larger, say .75mm?Josh
Posted by Paul G. Thompson (Member # 655) on 01-18-2002, 07:34 PM:
Josh, on page 77 of the PDF file for the CP-55 Installation Manual Part 2, you will get some idea as to where the roll-off starts according to slit size. I could be wrong, but I think it is the raw output of the solar cell. A .00075 inch slit starts its roll-off at about 2.5K, and about at 9k starts to take a major dump from -2db to a very sharp drop to -8db at 12.5khz. That would be a .042875 mil slit. Quite honestly, I didn't think a lense with that dimension was made for the 9050.If memory serves me correctly, you were looking at the output of a microphone pre-amp, and I wonder if you were seeing a HF boost that was actually built into the pre-amp itself.
If you were looking at the output of the Cat 240, chances are the slit loss compensation adjustments are quite active.
Just my thoughts... 
Posted by Ray Derrick (Member # 1001) on 01-18-2002, 08:19 PM:
You are right Paul, the left diagram is the raw output from the solar cell, but a .00075" slit is down 8dB at 16kHz, not 12.5kHz. The slit compensation circuit in the preamp (shown in the right hand diagram) provides a complimentary boost to bring the level up to about -1dB at 16kHz, which is what Josh says he is getting. BTW the slit size in the diagram refers to the height of the slit focused on the film, not the height of the slit inside the lens, which is usually about 5 times larger. To get a response which is truly flat to 16kHz (after slit loss correction) reverse scan readers use a .0005" (.013mm) slit. Once again, this is the height of the slit which is effectively focused on the film, the slit on the surface of the solar cell can be as much as 3.5 times larger. This is because of the magnification of the lens.
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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
Posted by Paul G. Thompson (Member # 655) on 01-18-2002, 08:22 PM:
OOPS! Sorry Ray and Josh. My eyes shifted when I looked at that, and that is how I got the incorrect 12.5khz figure. That is why I hate graphs. Grrrr....
Posted by Ray Derrick (Member # 1001) on 01-18-2002, 08:33 PM:
Don't worry Paul, if the graph wasn't so squashed up horizontally it would be a lot easier to read.------------------
Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
Posted by Ray Derrick (Member # 1001) on 01-21-2002, 07:21 PM:
For anyone who is interested, here is an excellent diagram from the Handbook for Sound Engineers, showing the effects of slit loss on frequency response. The diagram is actually provided in the book for the purpose of demonstrating the effects of "gap loss" in magnetic heads, but the phenomenon is the same for optical slit loss. In fact the "HF" control on Dolby Cat92 mag preamps is really a gap loss compensation control and uses the same equaliser circuit as the slit loss circuit on optical preamps. 
From the diagram it can be seen that a null appears at the frequency whose wavelength is equal to the size of the slit (or head gap) and is repeated at multiples of that frequency. It is quite easy to calculate the wavelength (in inches) of any frequency on 24fps film by dividing 18 (18" per second) by the frequency in Hertz.
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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