Amateur Extra: Variable-Length Coding

Which of the following HF digital modes uses variable-length coding for bandwidth efficiency?

C. MT63
D. PSK31

Now, I guess this answers my question about last weeks Extra question. Mainly why is it important.  It’s important because it does matter in transmission efficiency.  So which mode would be a good and efficient method of sending digital transmissions.  Well, once more lets examine the possibilities.

RTTY?  RTTY or Radio Teletype is more or less and ancient method of sending text via radio.  It actually dates back into the 1930’s.  RTTY uses a fixed-bit encoding.  It’s NOT variable.  So that basically rules it out as an answer.  (See: https://en.wikipedia.org/wiki/Radioteletype)

PACTOR?  PACTOR is a much more recent development, but its primary use is to send large messages, essentially being packet over HF.  It has built-in error correction, and other features.  Unfortunately, PACTOR I uses anywhere from 400 to 2600 Hz bandwidth, so not all that efficient.  (see: https://en.wikipedia.org/wiki/PACTOR)

MT63?  Now we’re getting somewhere in terms of bandwidth efficiency.  But again, has a bandwidth of anywhere from 500 to 2000 Hz. (https://en.wikipedia.org/wiki/MT63)

PSK31?  Hello!  Here’s what you need to know.  A proper PSK31 signal has a bandwidth of just 100 Hz.   That’s all.  Take a look at the image above, there are several PSK signals being seen here.  Yes, its a variable length coding.  See https://en.wikipedia.org/wiki/PSK31

So there’s your answer: D. PSK31


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