HF Radio Systems & Circuits
Written for the engineer working in radio communications, regardless of frequency, this comprehensive reference book explains the design of high frequency communications systems and equipment.
TABLE OF CONTENTS HF Radio Systems & Circuits
Chapter 7: Speech Processing, Squelch, and Noise Blanking
William E. Sabin
7.1 Speech Processing
The human voice does not utilize the SSB power amplifiers very well. The 15-dB peak-to-average ratio, or 24-dB dynamic range, means that a high power amplifier that handles the peaks well is just "loafing" most of the time. Unless the amplifier and power supply are marginal, much greater power can be transmitted if the deficiencies of speech can be improved. The usual case is that amplifiers that will handle the peaks well are capable of higher average levels than natural speech provides. The central idea is that the weaker components which we will emphasize in this section contribute much to the intelligibility.
Audio processing
The two methods of audio processing are clipping and compression. In the clipper the audio signal, after 10 to 20 dB of additional amplification, is simply sliced off at the required level. It is "memoryless" in that the action at any one time does not affect subsequent actions. The strong syllabic peaks, which occur from 5 to 10 times per second, are thus instantly reduced. The weaker parts which lie between the peaks are clipped much less and are therefore relatively stronger. The resulting truncated wave is then high-/lowpass filtered to remove low-/high-frequency distortion products, and then converted to SSB. The clipper generates significant amounts of audible harmonic and IM distortion, but emphasizes weaker speech elements effectively. Many of these are high-frequency components which are important. The distortion reduces the effectiveness appreciably since it degrades the improvement in intelligibility.
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Copyright Noble Publishing Corporation 1998 under license agreement with Books24x7
