Infrasonicon icon. Click here for home page.INFRASONICON: Time-Compressing Infrasonic Recordings to Discover New Sounds, by Clark Huckaby


Optical Probe: Circuit Description and Applications

The Optical Probe uses a TSL230R programmable light-to-frequency converter IC made by Texas Advanced Optoelectronic Solutions (TAOS; now ams). I purchased this chip from Parallax in 2007 but did not find it in their product pages recently (September, 2013); a data sheet is at http://www.c-stamp.com/Documents/Documentation/CS454000d1.pdf. A photo of the Optical Probe assembly is shown in Figure 5 of the Overview. The "chassis" is a 3.75-inch length of 3/4-inch wide right-angle aluminum stock. At one end, a male XLR jack (J1) is pin-compatible with the Interface Unit, so a standard mic cable can be used for the connection (avoiding excessive length). The other end holds a 0.6 X 2.2-inch perfboard with the TSL230R and other components wired point-to-point.

The schematic is shown below, in Figure 1 (figure and component numbers apply to this page only). The recommended nominal supply voltage (VDD) for the TSL230R (U1) is +5 VDC, so I used this as the Interface Unit's supply voltage. Logic inputs at pins 1 and 2 of U1 set the relative light sensitivity to 1X, 10X, or 100X (in effect an "aperture control" enabling 1, 10, or 100 percent of the device's photodiode array area), or activates a sleep mode. Pins 7 and 8 program on-chip frequency division by 1, 2, 10, or 100. With pull-down resistors R1 through R4, switch array SW1 selects among these operating modes (see tables on the right-hand side of Figure 1); the output duty cycle is 50 percent except at unity frequency division.

Figure 1: Schematic diagram of Optical Probe
Figure 1. Schematic diagram of Optical Probe (left). Tables showing operating modes as determined by SW1 setting (right).

As characterized in Figure 1 of the TSL230 datasheet, output frequency is a positive linear function of light intensity (irradiance in units of power per area illuminated [W/cm2]) across six decades for any given frequency divisor setting. Overall, the device's linear response spans at least 10 decades considering all available sensitivity and frequency division settings. However, as mentioned in the Data Converter description, my recording channel's phase-locked loop (PLL) limits the maximum signal amplitude, or relative light intensity range in this case, to less than one decade (about 200 to 1800 Hz at the PLL input, in frequency terms).
(Note that any frequency analysis system [or demodulator] has a limited dynamic range. For example, frequency measurement requires at least as much time as the reciprocal of the lowest frequency [i.e., the period]. This effectively ties channel bandwidth to amplitude at the low end.)

Many phenomena have periodic light intensity changes that exceed the amplitude range of my Infrasonicon channel. As noted in the Overview Part 3, in such cases the Optical Probe can be "photobiased" (illuminated) with a LED operating at constant current. This is analogous to lowering the contrast of an image projected on a screen by turning on room lights. Normally, when modulation exceeds the 200-1800 Hz range at the Data Converter's frequency input, photobias intensity (LED current) is increased and the resulting increase in FM carrier frequency compensated by increasing the frequency divisor (either at SW1 or in the Interface Unit). When modulation is within the safe range, fine adjustment of the photobias LED may be used to set the DC offset of the demodulated signal.

Examples of recordings that required photobias are presented at http://clarkhuckaby.com/NewVibe/CloseLookVibe.html, where I investigated neon lamp activity in Fender optical tremolo circuits. Figures 2B and 2E on that page are photos of Infrasonicon and Optical Probe implementation in those experiments, and Figures 5 and 6 show recorded neon light pulse shapes (waveforms). Photobiasing was obviously necessary because the neon lamp has a dark phase in each cycle. The tremolo experiments also exemplify how, given its linearity, the Infrasonicon channel can be an analytical tool (oscilloscope) as well as a means to discover new sounds.
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Overview Part 3 has more about the Optical Probe, along with links to example sounds.


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