Instruction manual

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Out-of-Band

Components

Simple clipping of a symmetrical waveform will invariably generate

odd-order harmonics of the fundamental frequency. The level of, and

signal degradation by, these harmonics will depend in large part on the

depth of the clipping action. Because of its complementary pre- and de-

emphasis, FM broadcasting can forgive a surprising amount of

baseband clipping, insofar as audible distortion is concerned.

Nonetheless, clipping products can clutter that part of the baseband

spectrum reserved for RDS and SCA subcarriers, potentially creating

crosstalk into those services. With one or two dB of clipping, distortion

products will probably be tolerable. Much more clipping than this will

not only compromise subcarrier services, but can interfere with

adjacent stations as well.

Figures 4a through 4d reveal out-of-band components generated by

composite clipping. These “worst case” examples were created by

feeding a stereo generator from a CD player. No overmod-protection

audio processing was employed, stereo generator input gain was

adjusted such that the internal clippers associated with the pre-filter

overshoot compensation circuit were active more than 90% of the time.

The stereo generator output was monitored by a Tektronics 7L5

Spectrum Analyzer which accumulated data for the duration of the 6-

minute rock music selection.

Figure 4a - Clipping OFF Figure 4b - 1dB Clipping