Wohler ATSC-1 Manual Download Page 18

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PHASE INDICATION DESCRIPTION

Wohler Technologies developed its well known Instantaneous Phase Indicator (IPI) to enable operators to see not only
whether or not a source is “in phase”, but also whether the source is stereo or mono. In the original version (four LEDs in
a “diamond” pattern), the green (upper) LED showed the source’s in-phase excursions, while the red (lower) LED showed
out-of-phase ones. Two amber/red bi-color LEDs on either side of the green and red LEDs showed whether a signal was
present (amber) and/or overrange (red) in the respective channel. These were thought necessary because NEITHER the
green nor red LED would light if signal was missing in either channel.

The revised version of the phase indicator retains the instantaneous type display which is so useful for verifying stereo-mono
status, but adds an average “Go-No go” indication of longer-term phase/polarity condition. For assessing the polarity status
of the Left/Right channels in the ATSC-1, a bi-color (red/green) T1-3/4 size LED displays the Average, while smaller (T-
1 size) green and amber LEDs show the instantaneous condition. The Front/Rear channel phase relationship is monitored by
a separate single bi-color (red/green) T1-3/4 size LED displaying only the Average phase condition (without the two smaller
instantaneous phase LEDs). Wohler Technologies’ Website (http://www.wohler.com/ipi.htm) has animated simulations of
both types of phase indicator displays (older 4-LED diamond & newer 3-LED type) as they appear with various types of input
signals. It should be noted that both the original and the new circuits make their “phase” (polarity) determinations on individual
excursions of the input signals. The average indication on the new type is derived by integrating the instantaneous
determinations.

Due to the highly proprietary nature of this circuit, detailed information is not available. This circuit is, however, very reliable,
and is warranted against defects for an additional year (three years compared to the usual two years for standard Wohler products.)

Input Stage and Level Gain Calibration

The “PPM-2” display driver circuit provides a total of 43dB (+ peak) of visual audio level indication via a 20-segment LED
bargraph. The main PCB's input buffer amps pass unbalanced signals through a DIP switch selectable attenuator pad on each
919030 meter driver PCB. A choice of four different reference (gain) calibration levels, -6, 0, +4, and +8 dB for the 0dB
indication is available. See Fig-1c (below) for a chart showing these DIP switch settings. A chart can also be found on a label
on the top cover of the ATSC-1.

Each of the differential outputs from the meter driver's input buffer drives a half-wave rectifier circuit; IC6a for positive input
excursions and 5b for negative ones. The outputs from the rectifier stages are combined and presented to two different filter
circuits that generate both PPM (IC3B) and VU (IC4) characteristic signals.

Figure 1c.- Gain and Ballistics DIP Switch Settings for Meter Driver (919030)

(Continued)

LEVEL METERING DESCRIPTION