Enhance 35mm optical mono sound?

[Jerry G. Axelsson]

Hello Brad. Thank you for the suggestion. I have already done that some time ago. I am leaning towards using the "Alternate EQ setting on the QSC DCM30D" for optical Mono sound. This seems like an easy idea to test. The question is how effective it is...

[Stefan Scholz]

Indeed this was the soundhead for the FP5-FP56 series, built from the late 1930's till the late 1950's. Here are some leaflets from 1939 and 1950 of the soundhead type 3837; and a leaflet from 1949 of the type 3838. In the 3838 you can see the same parts used in the built-in soundhead of the FP3 projector and how it would evolve in the soundhead of the FP20. My guess the 3838 was made to have a cheaper option available.

type3837year1939.JPG


type3837year1950.JPG
type3838year1949.JPG

3837 is on my FP 5, built 1938. It has 2 photocells for push pull and bilingual pickup. That's what I remember they originally designed the reverse pickup for, easier to do the separation, than on front scan.
Most of the soundheads from the 50's then came with a single cell. The trade in with retro scanning was the loss of light intensity, that required some preamplification over front scan sound heads. And the lamp used at Philips was either 20 Watts (4A/%V) or 10 Watts (1.48A/ 6.5V, used with RF excitation), not providing as much IR as the commonly used European 30W exciter bulbs or US style 45 Watt lamps had.
So the Philips sound pickup had an issue with higher noise floor, than traditional front scan units, which actually didn't matter in the time of Mono tracks.
PEC cells, and photodiodes have a max on IR, something in my early tries as a young student of electronics brought up the problem with pickup linearity. It is the photodiodes current, which behaves linear to the excitation. So this means, only a 0 ohm load impedance will provide linear conversion for the rest of the system.
The problem lowered with smaller IR content on voltage amplifiers with given high impedance. This might coincide with the white LED spectrum.
The best results I could get, were using a current to voltage converter circuit. And then, the light source did not matter that much, as long no optical interferences with the printed color appeared.
Most head amplifier designs were voltage amplifiers on commercially available cinema units.
For me, there is more factors, than just the exciter light source.
It's a combination of recorded frequency response, the type of recording modulator and amp circuitry used, correct exposure and development of sound negatives, gamma matched in printing, and last, but not least, the playback slit width or hf rolloff of the projector system. You can't use "slit loss eq". Something lost is very hard to regain, and has many other issues behind it.

[Sam Chavez]

Fascinating! I did not know there were two pec's originally. I saw later versions with only one tube socket. I don't recall the effective size of the slit but hf response was higher than the typical optics used in American soundheads which started to roll off at 2 kHZ. I ran across an odd artifact with the DP70 slit mask combined with Kelmar dual solar cell. On mono lateral soundtracks you would get cutoff of the sound when the sound image was smaller than the septum in the slit/solar cell. Made an odd gating sound like a switch.

One reason for the higher noice floor besides the slit being narrower than front scan was light loss in the imaging rod. Quite inefficient compared to a true glass fiber optic rod. On the order of 14 dB.

[Emiel de Jong]

3837b.JPG 3837a.JPG

[Emiel de Jong]

PEC cells, and photodiodes have a max on IR, something in my early tries as a young student of electronics brought up the problem with pickup linearity. It is the photodiodes current, which behaves linear to the excitation. So this means, only a 0 ohm load impedance will provide linear conversion for the rest of the system.
The problem lowered with smaller IR content on voltage amplifiers with given high impedance. This might coincide with the white LED spectrum.
The best results I could get, were using a current to voltage converter circuit. And then, the light source did not matter that much, as long no optical interferences with the printed color appeared.

If I understand correctly, you used photo electric cells and photodiodes in these set ups: did you also try solar cells? As far as my experience goes: all older units I encountered, when still in use, were modified with solar cells.

PS very nice you have an actual dual track model, they must be quite rare!

[Stefan Scholz]

Emil, the original setup utilized 2 PEC cells. Underneath the slit, there were collecting lenses, and each half fed into its own PEC cell. In Philips Kinotechnik, there's detailed descriptions by Dr. Jenzen of the electroacoustical and film sound department. I can post the drawings over Easter weekend.
A solar cell is not more, than a giant area photodiode, the same basics apply. In my Philips I used small silicon wafers which are actually commonly called a solar cell.
The basics of energy transduction are the same with diode and solar cell. Linearity with illumination exists in short circuit mode only for the short circuit current. The voltage is non linear to the illumination.
The closer you get to short circuit, the better your linearity, but the lower your usable output voltage.
Low voltage requires higher gain, resulting in higher noise floor.
That's why most input amplifiers were a compromise, derived from microphone input stages. And this compromise also has a lot to do with how the pickup behaves with different tracks. Like Sam pointed out, you're always getting measurable distortion, at best 5 to 3% with optical sound. Which is far below the acoustical transducers (aka speakers) produce. And for the sum, it's the root suare sum that the ear gets to hear. So the worst component defines the sound. A not perfect pickup and amplification can result in significant non linear behavior, with lots of nasty modulation... so do it correctly from the head on, my professor said.

[Emiel de Jong]

Stefan: thanks, that a solar cell and a photodiode are basically the same thing I didn't know. My background is not in electronics... Now, during the last years when red light conversions and their effects on older tracks were discussed, I always heard that the infrared (or lack of that...) is important. You say (if I am understanding correctly...) that this infrared is not so desirable when we are talking pickup linearity. I wonder if most systems in practical use, during the last decades, just don't have a very linear pickup then?

[Tony Bandiera Jr]

And then, when the soundhead discussion seemed to get up steam and interesting points were made regarding light sources, it was Silent all over again ;-)

It was just a short intermission....

Or, a short interruption to change a blown exciter lamp...

[Sam Chavez]

I came in on solar cells although my screening room uses a Simplex mag system with optical mono, with the original photo cells and Simplex preamps in the XL soundhead cabinets. Plays back current tracks very well. The SR encoding on modern films seems to brighten things up a bit. My tests on solar cells even before Dolby Labs showed as Stefan mentions, wants to see a short circuit. Anything above that causes some LF rolloff. Around 200 Ohms across the cell seemed to work quite well as a poor man's High pass circuit. From memory of course.