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When I sold my test loops recently, I found that I had a unique test film that is very handy and important for setting up optical sound, whether it is the old split (bilateral) solar cell or one of the new readers.
I forget the exact part number, (Dolby Cat 97, Thanks Ryan!) but is a pale blue stock with a "negative" (track modulation is black instead of clear) burst tone (It sounded like 1khz) recorded as alternating between left and right channels.
The goal was to adjust the lateral position of the solar cell or reader so that when viewed with a 2 channel oscilloscope, you saw the burst on one channel as the other showed a flat line with very little or no burst signal present. It gave a very clear visual indication of how much, if any, crosstalk was happening.
The only trick in using it was to make sure when you cut it to make a loop you had the blank modulation flatline on both sides of the splice so you never saw both bursts as the splice passed.
To use it:
I forget the exact part number, (Dolby Cat 97, Thanks Ryan!) but is a pale blue stock with a "negative" (track modulation is black instead of clear) burst tone (It sounded like 1khz) recorded as alternating between left and right channels.
The goal was to adjust the lateral position of the solar cell or reader so that when viewed with a 2 channel oscilloscope, you saw the burst on one channel as the other showed a flat line with very little or no burst signal present. It gave a very clear visual indication of how much, if any, crosstalk was happening.
The only trick in using it was to make sure when you cut it to make a loop you had the blank modulation flatline on both sides of the splice so you never saw both bursts as the splice passed.
To use it:
- Run Buzz Track AND set Dolby tone (CAT 69T) before using this test.
- Prepare a loop as per above, making it big enough to thread easily (I did both smaller loops for soundhead only and a full size to cover the entire film path)
- Mark the loop for proper orientation to make sure you have proper focus on the tracks (not too critical but you get better results if the emulsion is facing the correct side)
- Connect a dual channel oscilloscope to the optical test points in the sound processor (set input voltage for a full division on the trace, and your timing to show a stable waveform. DO NOT use "ch 2 sync" for timing.)
- Start the projector and allow the soundhead to stabilize.
- Observe the waveforms and you should see the channels alternate flatline to burst, with both channels always showing the opposite in vertical alignment AND little to no burst visible during the flatline timing
- If any modulation (visible burst) shows on either channel, SLOWLY shift the lateral position of the cell or reader to eliminate it. (Note that most readers can completely eliminate crosstalk, but many solar cells can only minimize it), so adjust to equal and minimal crosstalk.
- If a solar cell setup cannot reduce to a minimal amount of equal crosstalk, make sure the cell is positioned as close as possible to, BUT NOT TOUCHING, the film. If the position is correct, but crosstalk is excessive, replace the solar cell.
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