Dorrough Model 1200

The Dorrough Stereo Signal Test Set Model 1200 is a 2u rack mount instrument, allowing instant dynamic monitoring of audio levels, balance, crosstalk, system gain, signal-to-noise ratios and program "center-channel build-up."  Verifying proper polarity and balance of stereo broadcast lines is a primary application.  Two loudness meters are provided.

"Function", "Range", and "Attenuation" controls are set in tandem with the meters to achieve all measurements.  Left and right channels are input via loop-thru XLR connectors or a parallel barrier strip.  The back-panel also sports a line-level 1/4" stereo output for connection to a scope.  A front mounted amplified, stereo headphone jack is also provided for monitoring.

A typical application instantly reveals Left/Right polarity and Phase Compatibility.  First, measure the system gain and adjust the channels for equal Left/Right readings. Next, a mono signal is fed into both channels.  With the function switch set to "Sum/Diff" adjust the "Range Attenuator" and/or "High/Low" control until a Left meter reading of -6 dB is achieved.  The "Function" control is then set to "Stereo" and then back to "Sum/Diff."  Then simply note the difference of Left/Sum and Right/Diff meters. Same polarity produces a 6 dB increase on the Left/Sum meter and no reading on the Right/Diff meter.  If the readings are reversed, one of the channels has reversed polarity: The Model 1200 helps provides the ultimate in program consistency and quality assurance.