Hello,Over the past few weeks I've been adding "instrumentation" to the FreeDV GUI program, to help me understand how well it is working over real and simulated channels. It's all checked into SVN BTW, if anyone else wants to play.
One feature simulates an interfering carrier (like a birdie), something I have seen on the air. Attached is a plot of a carrier in the middle of one of the 700C carriers, but about 10dB higher. The upper RH plot is a rolling plot of bit errors for each carrier. You can see one carrier is really messed up, lots of bit errors. The average bit error rate is about 1%, which is where FreeDV 700C starts to become difficult to understand.
These bit errors would not be randomly distributed, but would affect one part of the codec all the time. For example the pitch might be consistently wrong, or part of the speech spectrum.
I found that as long as the interfering carrier is below the FreeDV carrier, the effect on bit error rate is negligible.
Take away: tune away from any interfering carriers that poke above the FreeDV signal carriers. Placing the interfering tones between FreeDV carriers is another possibility, e.g. a 50Hz shift of the tx signal.
Cheers, David
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