DIY 16mm optical sound recorder, v2:
The results.
The 15 second test track that was used to test the device consists of speech, 20Hz-20KHz frequency sweep, music, clicks and tones. LINK to the sound file.
In the dark room, I played the test track in a loop and started the projector for some ~20 seconds. You can hear the resulting film loop in the player below:
The above is an unedited track recorded with ZOOM H6 recorder plugged directly into the speaker out of Eiki SSL projector.
It is lo-fi, has a bit of a flutter at the begining (when the sound drum was gaining momentum) and some background hiss. There are also some strange oscillations in the far end of the frequency sweep that I think originate because LM386 started distorting. But all in all - for a simple DIY device it is awsome <3
When I isolate the recorded frequency sweep and analyse the frequency spectrum I get these results:
The frequency response is almost flat in the range of 50 to 2000 Hz. Then it starts to slowly deteriorate to about 6000 Hz. Then there is a bump at 7000 Hz (the oscillations that can be heard in the recording). Optimistically, I feel that with some corrective mastering of the source sound file, this system could be pushed to have somewhat of a linear response at the range of 50 to 6000 Hz (Eiki projector amplifier is rated at 50 - 7000 Hz, ±4dB).
Closeup shots of the sound track:
Furhter development plans:
- standardise the slit width with precision optical slits
-
develop noise reduction hardware (to turn off the LED when there is no signal/silence)
- test other audio amplifiers
- standardise the circuit board and get PCBs made
- add a dc power adapter option
- design an enclosure and interface for the device
- test source file mastering for flatter response curve
back to the project page.