Psk-31

Nalle Puh

Hi,

I'm new here. Short pres: Amateur radio operator, electronics hobbyist, previously used "Picaxe"-chip.

I have searched the forum for PSK-31 stuff, no luck.

Are there any of you out there who are experimenting with Phase Shift Keying (PSK) or PSK-31?
aintel.bi.ehu.es/psk31.html

I think the propeller would make a really nice portable PSK-31 terminal. All you need is a keyboard and an small monitor. And of course a portable shortwave radio.
There are many PC applications for PSK-31, but then you have to bring a battery-eating and heavy laptop on the trail.

I myself have not the skill to implement the PSK-routines, so I was hoping someone here is playing around with PSK.

./Erik

Beau Schwabe

Nalle Puh,

I have not implemented anything in code yet, but I have outlined a possible method similar to PSK using 4PWK (4 phase Pulse Width Keying) which would basically
increase your throughput by a factor of 2.5 times.

In the attached example I am using a baud rate of 4800 to get an effective throughput of almost 12k baud.· Worst case scenario is about 11.2k baud, while the best case
scenario is about 12.7k baud.· By lowering the Carrier Baud to 4800 , transmission errors can greatly be reduced.· With an average throughput of 2.5 times that of what
you would get with standard RS232, there is enough overhead to implement crude CRC checking to further reduce transmission errors.


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Nalle Puh

Tnx Beau,

Interesting reading.
The modulation-modes have different design-goals. The primary goal for PSK-31 is narrow bandwith. With extreme narrow bandwith we can use all the transmitter's power to carry information. Compare to human voice, where there are many frequencies which is not necessary to carry the information. Our transmitters waste valuble effect on these frequencies.

There are other modes such as QPSK-31, which use 4 phases instead of 2 phases in PSK-31. If I remeber the figures right, QPSK requires 3db better S/N-ratio but it can transfer more data per time. The designer of the mode chosed to use the extra data for error checking (which PSK-31 has none of). So the actual bitrate is about the same for the both modes.
QPSK-31 is seldom used, PSK-31 is the major mode.

A bit clearence maby: PSK-31 is NOT designed to transfer files or binary data. It's only intended for man-man communication in text. It's like MSN over shortwave radio.

./Erik

Beau Schwabe

Nalle Puh,

When I was working out the 4PWK method mentioned above I originally was looking at 4PSK and 8PSK and realized I could substitute the Phase delay with Pulse widths instead. The problem I saw with
PSK from the Propeller was generating Sine waves with minimal external components ... manipulating the Phase of the sine waves would not be an issue. PWK allows a "digital" version, keeping the
external component count down to a minimum. PWK could have a much lower bandwidth depending on the quality of transmission medium. The example above is just that and could probably be narrowed
down to something MUCH less than a 600Hz bandwidth. ...likely something similar to the 31 Hz bandwidth of the QPSK-31. That said, PSK is more analog in nature where as PWK is more digital. The Propeller
speaks "digital" so at some point the PSK signal will need to be converted into a digital signal. It is likely that the code required for either method will end up being very similar at the end of the day.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

lcyepiz

You can see at:

http://forums.parallax.com/showthread.php?p=873388

The modulator of PSK31 is working, now the target is the decode.



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