The rules say that this should be measured in the headend and at the subscriber terminal.
The frequencies at the tap and at the output of the set-top converter — depending on the type of converter being used — may be different. So, is it necessary to measure the aural offset frequencies on all channels? The answer is no! The rules also say that the aural offset frequency is one of the tests that are only required on the test channels.
As a practical matter, we run this test by measuring the frequencies of all channels in the headend, then, in the field, we measure only the test channels — both at the tap and at the output of a set-top converter. For most systems, frequencies measured at the tap will be no different than those in the headend.
The aural offset frequencies measured at the output of a set-top converter will also be the same as those in the headend — except when a baseband type of converter is used. For a baseband converter, the aural offset frequency is essentially constant. In the field, there's no need to go beyond the minimum required tests.