On Apr 5, 2010, at 4/5 7:33 AM, lstavenhagen wrote: > Still with PSK31, trying it out with cocoaModem, I find I can't keep > the > buffer full. I can get ahead on simple plain language but throw some > numbers > and punctuation in there and the buffer catches up pretty quick. 45 > baud > RTTY is able to keep up with me no problem.
One thing to keep in mind is that PSK31 (also with DominoEX and Olivia) uses Varicode which encodes lower case characters into fewer bits than upper case characters. Not only will you be sending text faster by using lower case, but there is also a corresponding lower character error rate (for the same SNR) than when using upper case characters. Something to keep in mind when you engage in a pileup or a digital mode contest :-). With BPSK31 Varicode, average English text (mostly lower case) takes about 6-1/2 bits per character, yielding about 5 characters per second, or just about 48 WPM if an average "word" is 5 characters plus a space. Standard Baudot RTTY is 60 WPM. So BPSK31 is a little bit slower than Baudot RTTY even when you type mostly lower case in PSK31. PSK63 will feel (and is) quite a bit faster than RTTY, but also has larger character error rate than PSK31 for the same SNR, and if you like sending fast exchanges, PSK125 is yet faster (and even less error free). While the "PSK31," "PSK63" and "PSK125" nomenclature may appear weird, their speeds are really just a factor of two from one another. The symbol rate for PSK31 is 31.25/second, for PSK63 it is 62.5/second and for PSK125 it is 125/second. The "31", "63" and "125" are just rounded from 31.25, 62.5 and 125. (The symbol rate is equivalent to the bit rate in BPSK, and equivalent to the dibit rate for QPSK). I suspect that most programs simply re-decimate the input signal to get a BPSK63 demodulator from a BPSK31 demodulator; that is how cocoaModem implements PSK63 and PSK125. You just need to change the input bandpass filter. Why such a weird number as 31.25 baud? It has to do with ease of implementation when using 8000 samples/second or 16000 samples/second with a sound card. 31.25 baud is simply 1/(32 millisecond). Similar to the reason why Baudot RTTY uses 45.45 baud and not 45.0 baud. The "45.45" comes from 1/(22 millisecond). 73 Chen, W7AY ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
