Film-Tech Forum ARCHIVE: Todd-AO question

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Posted by Michael Schaffer (Member # 1204) on 08-12-2002, 10:34 PM:

On a German forum I read that the Todd-AO format also used the Academy roll-off on the soundtracks. Is that true? What was the specified frequency response?
Michael

Posted by Steve Kraus (Member # 476) on 08-13-2002, 07:20 AM:

That doesn't sound correct.

Posted by Mark Gulbrandsen (Member # 72) on 08-13-2002, 07:35 AM:

Most of the roll off afforded by the Todd-AO system would have been afforded by the speakers themselves. Altecs of the era typically made it to 10khz and then suffered drastic roll off. Films from that era that I've run sound a little bass heavy sometimes but that may also be a result of wear and tear on the mag tracks themselves. Then again, it may have also been added in at the re-recording stage as a little built in low frequency eq for Altecs that were used at the time, and because most mag booth playback preamps only offered high frequency eq.
There had to be a recording and playback curve based on what the capabilities of the sound equipment of the time were, but I've never seen it published anyplace. Perhaps in Audio Cyclopedia.......
Gord has run alot of films made in this era, perhaps he'll chime in......
Mark

Posted by John Pytlak (Member # 331) on 08-13-2002, 10:03 AM:

I would suspect that the 70mm Todd-AO equalization was similar to that originally used for 35mm CinemaScope (except for any compensation for the higher transport speed), as documented by Altec:
http://www.widescreenmuseum.com/widescreen/53stereo.htm
http://www.widescreenmuseum.com/widescreen/cinestage.htm
http://www.widescreenmuseum.com/widescreen/80page1.htm

Does anyone else have data or publications that document what was used?

------------------
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
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Posted by Michael Schaffer (Member # 1204) on 08-13-2002, 01:46 PM:

As I wrote to Patrick in answer to his question about training opportunities, "read the manuals"...
My discussion partner on the German forum just pointed me to the CP100 manual. On p.63 of the pdf-version available here it says
(section 8.7, opration instructions, magnetic soundtracks)
"4. If the soundtrack is not Dolby-encoded, it must be a conventional Academy print. Release the Dolby NR button (red light goes out). Fixed accurate Academy magnetic filters are automatically switched into the circuit. However, the house equalizers are retained to give accurate reproduction."
Still, I am not entirely convinced that that automatically means those filters are identical with the well-known Academy standards for optical soundtracks. Maybe the Academy set new standards for magnetic soundtracks in the 50s? Maybe Dolby didn`t care in the mid-70s to implement filters for all the different, then already historical formats? At that time, were any movies still produced for the full 5-screen-channel format?
Michael

Posted by John Pytlak (Member # 331) on 08-13-2002, 02:19 PM:

AFAIK, there were no SMPTE standards for 70mm magnetic recording prior to the mid 1970's. First 70mm audio standard that I know of was PH22.185 in 1973 that specified the track dimensions, reproducing speed, and picture-sound separation.

Posted by Steve Guttag (Member # 268) on 08-13-2002, 05:52 PM:

This is an area Ray Derrick (Panastereo) and I have been exploring. Thus far, we have not found a "published specification" but have measured things like the CP-200's Mag academy curve vs the optical one (afforded by the Cat. 109 meter/filter cards).

The need, as I see it (not necessarily Ray) comes from the fact that the modern cinema processor is tuned to the ISO 2969 curve. This is a response that the historical systems did not have. If a film is mixed to an old speaker in an old room, without EQ (or sound "warping" units), it will not sound normal in a modern theatre. This is putting aside all of the differences due to the reverberation and acoustics of the theatre.

The idea of the "academy" filter in processors like the CP-100 and CP-200 is to superimpose an old theatre response on top of the new tuning in such a manner that is will be the ideal, average, response.

As Mark accurately points out, in the 50's-60's the reponse of theatre was pretty much a funtion of the reponse of the speaker as it interacted with the room. Such is not the case with an "EQed" system, starting with the CP-100 onwards.

Within the past year, I installed a Panastereo CSP-1200/4600 system. As delivered, the magnetic tracks do not receive any filtering since none are specified anywhere. One of the test films we ran was Ben-Hur in 70mm 6-track (panned dialgue and everything). To my ears, it was way too bright sounding. You could hear everytime a track was cut in and out. The hiss level and hum level would change. Certainly, this was not to be heard. Applying a bit of Bass and Treble cut would move it into acceptable. Having run many conventional films on a CP-200 and not having the same sort of problems (including Ben-Hur), I investigated the academy curve issue for magnetic films. Thus far, there are none that we have found. Certainly, until some definitive documentation states to the contrary (and I want documentation that all of the studios followed it) there is some room for interpretation and opinion here.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Steve Kraus (Member # 476) on 08-13-2002, 07:00 PM:

It would be instructive to see the RTA graph of pink noise going through the 100 or 200's magnetic chain (sans any NR or house EQ of course). How about the 65? Would a modern 70mm mag track be presumed to be mixed for uncompensated playback?

Posted by Gordon McLeod (Member # 33) on 08-13-2002, 08:12 PM:

I tend to disagree since whent the 200 came out it was possible to format full range non NR playback.
I would tend to believe that they were mixed for a flat response including the room. This was the way Mag full coat was handled but they still in the 50's showed a flat response on the tape itself

Posted by Steve Guttag (Member # 268) on 08-13-2002, 10:12 PM:

Gordon,

Being "flat" on the recording is relative...what room did they listen to it in? If the highs seemed a bit wanting, they would have tried to emphasise them...likewise on the bass.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Jeffry L. Johnson (Member # 453) on 08-15-2002, 08:54 AM:

"The Focal Encyclopedia of Film & Television Techniques" ©1969, SBN 8038-2268-5.

SOUND RECORDING: RECORDING SYSTEMS: MAGNETIC FILM (page 694)
"The frequency response of a magnetic channel is maintained flat from 50 to 12,000 Hz (+/- 1/2 dB), so that second, third, fourth and even fifth generation transfers can be made without any major alteration to the frequency content. It is also essential that strict control of azimuth is maintained to facilitate world-wide exchange of recorded material.

"The standard reference film to which all 35mm magnetic recorders are aligned is the RCFM, available from the Society of Motion Picture and Television Engineers. It is an original recording of three 200 mil tracks on fully coated film, containing a series of spot frequencies from 50 to 12,000 Hz. Each film is individually calibrated and correction factors are provided."

SOUND RECORDING: MULTI-TRACK SYSTEMS: RECORDING REQUIREMENTS (page 704)
"Since the final prints delivered to cinemas carry a magnetic stripe track with a frequency response of 50 to 12,000 Hz, high and low pass filters are not required."

SOUND REPRODUCTION IN THE CINEMA: MAGNETIC RECORDING (page 719)
"There are fewer limitation on magnetic recording. At the standard film speed of 18 ips, there would be little difficulty in recording signals having frequencies as high as 20 kHz if this was advantageous. Amplitude distortions are low and the signal-to-noise ratio achieved on prints released to the theatre is in the region of 45-50 dB. As the loudspeakers used in cinemas rarely have any significant output above 10-12 kHz, it is more usual to allow the recorded signal to fall away above this frequency."

Posted by John Pytlak (Member # 331) on 08-15-2002, 09:12 AM:

Again, it's likely that many of the parameters for 70mm mag prints grew out of the experience with 35mm CinemaScope:
http://www.widescreenmuseum.com/widescreen/cinemascope.htm

"5.6 RESPONSE

The electrical frequency characteristic of each channel will eventually be standardized but for the time being it will be satisfactory to state that the overall characteristic from recording through reproduction shall be flat from 50 to 8000 cycles with tolerances as shown in Fig. 9A.

The relative response of the channels is more important than absolute or average response. The tolerances or permissible variations between channels is shown as Fig. 9B. This measurement must be made with a suitable test film because the recording characteristic will have pre-equalization of approximately 6 dB at 60 cycles and 3 dB at 8000 cycles, both of which are useful to reduce hum and noise difficulties. The amplifier system, if measured independently, will have a frequency characteristic with a slope of 6 dB per octave (6 dB decrease in signal for each doubling of frequency) except as this characteristic may be modified to take into account the low frequency pre-equalization and high frequency losses.

FIG. 9a & 9b

The mid-band channel gains must be adjusted to within 1 dB between the lowest and highest channel.

Intimate contact between the magnetic track and the pickup head surface is very important for high frequency reproduction, and any factor, such as distorted film, low film tension or an accumulation of dirt or oxide on the head surface -all of which tend to destroy good contact-must be observed and minimized. Clean the surface with a soft rag or wooden stick. Azimuth should be adjusted for maximum output"

Of course, the differences in track width and transport speed would be factors in arriving at the final specifications.

Posted by Gunter Oehme (Member # 499) on 08-16-2002, 05:54 AM:

I think the CP 200 roll-off filter for conventional 70 mm magnetic reflects the B chain differences between pre dolby cinemas and X curve cinemas.
Since introduction of Dolby the A chain is tuned for linearity and the B chain is equalized for the wide rang X curve. Due to better gap loss correction the A chain frequency range is also extended.

The ANSI/SMPTE 202M standard specifies a N curve for non wide range installations.
Compared to the X curve the N curve rolls off 7 dB at 10 kHz.
The CP 200 build in filter follows the N and X curve difference within +0/-1.5 dB.

Todd-AO magnetic sound in today's cinemas without filter will sound too bright as said by Steve.

Posted by Michael Schaffer (Member # 1204) on 08-16-2002, 07:20 AM:

This sums it up very nicely. It appears that Dolby caused a little confusion here by using the term Academy in a too generalised way. In their context, they seem to use it as generally meaning "pre-Dolby".
Apart from the limitations inherent in the sound formats and reproduction systems of the "pre-Dolby" era, and the typical characteristics described above was it generally uncommon to use equalisers in cinema installations? As quoted by Jeffry, individual films came with specific tuning instructions and their own calibration reels.
Michael

Posted by Gordon McLeod (Member # 33) on 08-16-2002, 10:05 AM:

Well considiering in the Dolby days no two titles had the same eq and level and always required test noise and tone for each title there is still no standard to speak of that has been followed
The only facitlity that was typically close title to title was Universal so I often wondered if they did there own actual track recording on the stripped prints