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


Interface Unit: Features, Applications, and Circuit Description

Interface Adds Versatility. Under special conditions, the Data Converter can accept frequency-modulated (FM) signals directly from a frequency-mode transducer. However, for maximum versatility, an Interface Unit is required (see Overview Part 2 for a general description and block diagram of the overall Infrasonicon channel). As shown in Figure 1, the Interface Unit is housed in an enclosure sturdy enough for field use. The schematic diagram is in Figure 2. (Note: figure and component numbers refer to this page only.) The Interface Unit's functions are:
  1. Route appropriate DC power to the transducer. The all-CMOS design permits a range of external DC supply voltages.

  2. Receive the FM signal from either a frequency-mode transducer or an audio recorder. Schmitt input circuitry "squares up" waveforms filtered by long cable runs or audio recording.

  3. Provide a standby switch to manually control signal duration.

  4. Scale (down-convert) the carrier frequency to the Data Converter's nominal operating point (1 KHz). One can select up to seven cascaded divide-by-two stages, supporting transducer carrier frequencies up to 128 KHz. Using at least one divide-by-two stage assures carrier waveform symmetry; the Data Converter's demodulator (a phase-locked loop) requires a 50-percent duty cycle for best performance.

  5. Provide compatible outputs for Data Converter, audio recorder, and headphones. Audio recording facilitates field work, allows multiple data conversions on one recorded signal, and adds a layer of data backup. Occasional headphone monitoring helps to confirm proper system operation.

  6. Count total input cycles. A resetable binary register overflows at 232, which requires more than nine hours at 128 KHz. This enables exact (albeit manual) calculation of carrier frequencies and audio record/playback check-sums.

Figure 1: Photos of Interface Unit

Figure 1.
Photos of Interface Unit. Left: view of front panel, with "plug-in four-place binary display module" in the foreground. Center: view of interior with top cover removed. Right: view of rear panel; note that the frequency division header's shorting plug is in the 25 position.


Interface Construction.
Designed for medium-low frequencies, the Interface Unit requires no special layout or construction method. I used point-to-point wiring on a 4.3 X 3.1-inch perfboard. Figure 1 (center) shows the assembled board and its enclosure, with views of the front and rear panels on the left and right. Epoxy-cemented flat to the bottom of the board, dual-row female headers poke through notches cut into the front and rear panels. The long header accessible in the front allows binary read-out of the count register, and the header in the rear is for setting the frequency division ("conversion factor;" see below).

Figure 2 is a schematic diagram of the interface unit. Via power jack J1, the DC supply can be (for example) a 6-V battery when using the Water Theremin or a 5-V regulated supply with the Optical Probe. Diode D1 protects the ICs if supply polarity is reversed; the power supply should have a fuse-protected output for this strategy to work reliably (if a forward-biased D1 fails before the fuse, it may no longer protect the Interface Unit against reversed power supply polarity).

Input Circuits. Female XLR jack J2 is the input for a transducer (or "probe"); DC to power the transducer is supplied via an RF choke (L1) at pin 3, pin 2 receives the FM signal, and pin 1 is common (ground). RCA jack J3 is the input for audio recorder playback. Each of these input jacks drives a separate input of U1A, a CD4093B Schmitt-trigger NAND gate. The two alternative signals should not be plugged in at the same time. The "input select" switch (SW1) simply holds U1A's pin-6 input high when a transducer is used. The probe input (U1A pin 5) is always pulled up by R10. De-bounced by R11, R12, and C4, run/standby switch SW2 enables signal flow via gate U1B.

The audio playback input (J3) accepts a portable recorder's headphone output. Load resistor R6 stands-in for a headphone impedance; C2 AC-couples the signal to U1A. Bias trim-pot R8 is adjusted for best waveform symmetry measured at the interface's outputs (with N = 0 at J4; see below). The recorder's playback amplitude must comfortably exceed U1A's hysteresis voltage (VH; the difference between trigger thresholds). The "high" and "low" CMOS Schmitt input thresholds are symmetrical about one-half the power supply voltage (see http://www.fairchildsemi.com/an/AN/AN-140.pdf). With a 5-V supply, the typical VH is 0.9 V, but it can range from 0.3 to 1.6 V (see http://focus.ti.com/lit/ds/symlink/cd4093b.pdf). Gates with a low VH are preferred; the particular RCA-made CD4093B that I chose measured VH = 0.6 V with a 5-V supply. Fairly consistent record and playback amplitudes on the audio recorder are desired between different recording sessions; I found it helpful to mark my settings on the recorders.

Figure 2: Schematic Diagram of Interface Unit

Figure 2. Schematic diagram of Interface Unit.

Frequency Division and Counting. The Interface Unit has a 31-stage ripple counter (U3, U4, and U5) which dominates the lower half of the schematic (Figure 2). Momentary switch SW3 resets the entire register. The counter ICs have built-in output buffers, so the external world (via header J8) can't disrupt the count. Buffer-wired AND gates (U2B, U2C, and U2D) serve this function at the input of each counter IC.

The counter's buffered input and first seven outputs (those of U3) feed one row of header J4, which pokes through the rear panel. Here, a shorting plug selects the interface unit's frequency divisor by octaves from 20 (no division) to 27 (division by 128). The target frequency is normally 1 KHz. The required divisor depends on the transducer's operating point (its "carrier" or average frequency). For an audio-recorded signal, 20 (no division) is selected because the carrier was previously scaled to near the targeted 1 KHz while recording.

Header J8 (four 8-position units arrayed end-to-end) brings out the counter's input and all 31 outputs for access through the front panel. To read a total count, I used the epoxy-potted "four-place binary display module" seen in the foreground of Figure 1's left photo. As diagrammed in the lower right corner of Figure 2, it has four LEDs (D3-D6) and series resistors (R2-R5) tied to a male header. With the interface in standby mode, this module is plugged into different positions along J8 and the binary tally written down on a peice of paper for manual conversion to a base-10 integer. (LED on = 1; off = 0. Readers may envision a more user-friendly counter display, of course.) The total count equals the carrier frequency times the number of seconds the Interface Unit was in "run" mode since its most recent counter reset. Thus, knowing the run time (using a stop watch) and the total count, the exact carrier frequency can be calculated. Also, one can compare counts for record versus playback as a "check-sum" for audio recording errors; the counts should be equal after compensating for frequency scaling during recording.

Outputs of Interface. Buffer-wired "spare" gates U2A, U1C, and U1D drive the Interface Unit's three output jacks. For the output to the Data Converter's demodulator, U2A couples directly to RCA jack J5. Via 3.5-mm phone jack J6, U1C drives both channels of stereo ear-buds; R14 limits current and C5 provides AC coupling. Constant earphone monitoring is not necessary (and would be somewhat nerve-wracking). It's used for a quick system check; one can learn to hear certain problems with the FM signal.

RCA jack J7 is the output to a portable audio recorder. I tried two different recorders for this project: A GE model 3-5025 monaural cassette deck and an Aiwa model AM-F70 digital minidisc deck. Each needed a different AC-coupled passive network on U1D's output to limit amplitude and bandwidth (Figure 2, inset on right; the cassette deck's input is designed for a high-impedance, low-amplitude "remote mic," while on the minidisc deck I used a lower impedance, higher amplitude "line input"). I preferred the minidisc deck since it can record longer signals (more than 2.5 hours in "mono"), and unlike cassettes, it did not introduce audible wow and flutter. You can hear time-compressed cassette wow and flutter in Example 6 on the Sound Example Page.
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Overview Part 2 presents a general discussion of the overall Infrasonicon channel, including a block diagram that includes the Interface Unit and Data Converter.


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