7.11 FM Transmitter Systems Pre-emphasis De-emphasis
European FM transmitter systems use a 50µs pre-emphasis filter (-3dB at 3183Hz) to
amplify the high frequencies before modulation, and the FM receiver has a corresponding
50µs de-emphasis filter to restore a flat frequency response. This is to obtain the best
noise performance from the system (since FM systems have correspondingly higher noise
at higher frequencies) and is possible since programme material generally has a lower
level of high frequency components.
Applying a test signal with a flat frequency response at full amplitude will therefore cause
the FM system to overmodulate at the higher frequencies, while applying a test signal at a
lower level will not test the system at maximum deviation. The solution is to use a test
signal with 50µs pre-emphasis such that the higher frequencies are attenuated. This will
ensure that the tests are performed with full deviation after pre-emphasis at all
frequencies.
Sequence 17 has therefore been provided for testing FM transmitters using 50µs pre-
emphasised tones for measuring crosstalk, distortion and phase (segments A, G and Y).
The LA102 applies a corresponding 50µs de-emphasis on the crosstalk measurements to
obtain true crosstalk levels relative to the level of the tone on the other channel (which is
not measured since the LA102 refers crosstalk measurements to the measured test level −
in case the interfering channel is not connected). Note that the distortion is always
referred to the level of the fundamental of each tone so no correction is needed. This
sequence also uses segment V to measure the test level at 400Hz instead of 1kHz (this is
common practice to ensure that it is not affected significantly by the 50µs pre-emphasis).
Similarly, it is often desirable to normalise the frequency response results to 400Hz and
this is readily achieved by setting configuration J2.
Note that the 50µs pre-emphasis filters used before modulation can also be tested by using
a standard sequence (such as sequence 11) with the 50µs weighting curve available in the
LA101 (on weighting 9). This should not be used for testing the entire system, since the
LA102 will not apply a 50µs correction for the crosstalk measurement, but it can be used
for testing the pre-emphasis filter on its own where a flat frequency response is expected.
Similarly the 50µs de-emphasis filter in an FM receiver can be tested by copying this
weighting curve to a user weighting and inverting it to make it boost the high frequencies,
as is explained in sections 2.26, 2.27 and 3.19).
75µs pre-emphasis (for American use) can be defined as a user weighting using the
weighting editor. The definition is included in the Windows support software.
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7. Applications Advice
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