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- AUGUST NEWS LETTERISOTRON 20 A WORK HORSE FOR 20 METERS This article will also appear on our web site. SPEECH COMPRESSION/PROCESSING Most radios have one or the other. What is it doing? Does it help? The circuits and testing of these can get quite involved. However, it is good to understand the basics of what it is trying to accomplish. Single side band modulation has a little disadvantage. When using ssb, you have no power output from the transmitter when you are not saying anything. Therefore, the person receiving will hear the noise and interference as if you are not in contact. Keep in mind ssb is only a modulated radio frequency carrier. Much like a CW carrier, but not of a constant power level. If you sent a CW signal at 100 watts and it is received at S7 and the noise level is S6, the signal will be detected clearly. If you drop the power to 25 watts and you are at S3, detection will be poor. SSB is doing a similar thing, except that it is changing power levels with the change in your voice. If your voice peaks are at S7 and your voice lows are at S3, there will be difficulty copying clearly. So how can you make this more affective? Compression. This could be likened to an audio amplifier that has a fast responding Automatic Level Control - ALC. It is like turning up the volume control when your voice modulation hits the softer part of your speech. Of course this is quite fast. This in turn, when modulating a ssb carrier, will produce power out that is higher during the soft part of your voice modulation. You are then raising the average power output of your signal. The compressed audio is not distorted even though the ssb signal may be clipped to a degree. However, it is interesting that the ssb carrier can be clipped to some degree without hearing audio distortion. Therefore, you can drive the ssb carrier a little harder and get higher audio enhancement at the receiver. Most of the manufactures do an excellent job with this. The audio from many ssb transmitters have a very natural quality and can be heard clearly even though the signal strength is just above the noise floor. Can you mess it up? Yep. Pushing the compression too far will result in an audio that can be described as harsh. Increasing further will cause noticeable distortion. This will also result in a wider bandwidth of your signal, sometimes referred to as "Splatter". Then the stations next to you will call you names that your mother did not when you were born. Some radios have a Processor. This is accomplishing a higher average power also, but is done at the carrier level of the transmitter. This technique may have a little more tolerance to clipping than compression, but caution is still advised to avoid splatter. 73, Ralph WD0EJA 08-17 BILAL COMPANY 137 MANCHESTER DR. FLORISSANT, CO. 80816 U.S.A PH/FX: 719/687-0650 OUR EMAIL LIST IS ACQUIRED BY PERSONS WHO HAVE CONTACTED US IN THE PAST. IF YOU DESIRE NOT TO BE CONTACTED PLEASE EMAIL YOUR REQUEST.
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