On Wed, 11 Jun 2008 06:29:58 -0700, Ed Muns wrote: >What do you think the "limiting factor" is?
I suspect that much of it is poor level matching. One thing I can measure with a great deal of accuracy is the audio distortion at the output of a sound card. This is quite important for narrowband modes like PSK, because intermod is another signal that interferes with others on the band! Several years ago, I measured the distortion at the output of my laptop sound card. It was 30 dB down from the carrier at maximum output, but 40 dB down from the carrier if I reduced output the level by 6 dB. That is, my signal was 10 dB cleaner simply by running the sound card 6 dB down from max output! More distortion will be created if the audio input of the radio is overdriven, AND if the RF stage is overdriven. In both cases, reasonable care with level matching solves the problem. As Simon and Joe have noted, noise and distortion in the sound card can degrade decoding, but again, proper level matching can improve that situation a LOT. Good operating practice for audio systems is to maximize the output of the driving stage and reduce the input gain of the receiving stage. That is, the output of the radio should be set so that it is operating just below the level where its distortion begins to rise, and the input gain of the computer should be turned down so that it is not overloaded. Sometimes, especially if the computer has only a mic input, a simple resistive voltage divider (pad) is needed between the radio and the computer. That's two resistors, one in series, another in parallel with the input. Guidelines for choosing values are in the tutorials I cited in a previous post. In summary, the noise and distortion problems Simon has cited can usually be solved, and can nearly always be greatly improved, simply by proper level matching. Of course, that won't be true if the sound card is junk. :) 73, Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
