HP 8591C User Manual page 259

Test Descriptions
Visual Carrier Frequency and Visual-to-Aural Frequency Difference Test Description
The aural carrier needs to be within 4.5 MHz (+/-0.4 MHz) of the visual carrier
to be detected as the aural carrier. If the aural carrier cannot be detected, the
visual-to-aural frequency difference measurement cannot be done and a '-))
will appear in the system measurement test data for the channel.
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