Lee
The EXTFSK system is basically bit toggling the TXD line by using timing in
software. This means there is considerable timing jitter so that individual
bit times can be too long or too short. This does cause errors at the
decoder. So this method of transmitting RTTY is sub-optimum. It is also
limited to 45.45 baud.
Next there is one other bit of information you need to consider. The best
decoders of RTTY like Fldigi and 2Tone do not decode the signal as FSK. They
treat the RTTY signal as two tones that are oppositely keyed . That is one
tone is keyed on and the other tone is keyed off. In other words the best
decoders treat the RTTY signal as if it is Amplitude Modulated on off keying
of each tone. So the question then becomes what is the optimum method of
transmitting a signal that provides the best match to the decoder?
The answer is to key the tones just as the decoders decodes them that is to
key one tone off and key the other tone on. Fldigi does this and so does
2Tone DOOK. A further benefit of using this form of transmission is the
keying signal is carefully filtered before it modulates the RTTY tones thus
maximising the transmitter's energy that can be used by the decoders tone
filters. A further benefit is this filtering to maximise the use of the
transmit energy means unlike radio based FSK it does not occupy an
unnecessarily large part of the band causing QRM to adjacent users.
So if you want to use LINUX then I suggest you use Fldigi AFSK which is
actually DOOK (Differential On Off Keying). It also means the same
connection between sound card and rig can be used for all the other modes
that Fldigi has.
73 David G3YYD
-----Original Message-----
From: RTTY [mailto:rtty-bounces@contesting.com] On Behalf Of Lee
Sent: 24 April 2016 16:17
To: RTTY Reflector
Subject: [RTTY] USB serial port adapters
Now for my next question: why is FSK RTTY difficult when using USB serial
port adapters? One article I read was saying that 45 baud rtty is too slow
for the adapter while another said that they don't handle 5 bit characters.
>From my research POSIX allows you to specify 5 bit
characters. Anyway, I'm writing a C program under Linux and I have
successfully got an adapter with a PL2303 chip to send FSK RTTY at 45 baud.
I had to write code for a custom bit rate since 45 baud isn't a standard
rate. Anyway, my project is still WIP, though. I've noticed that there's
EXTFSK for MMTTY and I've tried it - works for me - but I want a program
that runs on Linux. If the developers of FLDIGI ever decide to incorporate
FSK into their code or if another developer puts out a native Linux FSK RTTY
program, I'll drop my project. I don't want to build an adapter to convert
audio tones (AFSK) to FSK (pseudo-FSK).
Hopefully nobody thinks I'm being critical of any developers - I just want
to know what's going on and why.
N0SQ
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