digital modes on KX1

[Vojtěch Bubník]

Hi Eric and Wayne, and other Elecraft fans.

I am working on a digital teletype program for PocketPC. The software could be downloaded from http://pocketdigi.sourceforge.net including sources. I am porting GMFSK by Toni Maninnen to Win32 and optimizing it for fixed point arithmetic - ARMs do not contain floating point unit and emulation is too slow. The software is able to receive and transmit BPSK31 and RTTY over sound card now. I plan to add other digital modes later.

I would like to implement some kind of digital interface, that would enable modulating DDS directly. This would enable KX1 or Small Wonder Labs DSW+ to operate lot of digital modes. CW (HELL) and single tone FM modes (RTTY, MFSK16, Domino, Olivia, FM HELL, Packet) will be modulated perfectly, PSK modes (BPSK31, PSK HELL) need slow AM modulation of the carrier. Multi tone  modes (THROB, MT63, newer TORs) will not be possible at all.

I think that modulation envelope of BPSK31 and PSK HELL could be approximated by keying the transmitter down slightly after envelope zero and releasing slightly before envelope zero. The signal purity will be slightly worse than of CW at 31.25, which is acceptable at QRP levels in my opinion.

My idea is to use the iambic paddle jack as a serial line to drive the DDS by Pocket PC directly. The transceiver would be switched into a slave mode, will receive and execute 8 byte commands for DDS, and will drive PTT. The problem is exact timing. I am afraid it is not possible to ensure exact timing of data packets on serial line under control of multitasking operating system. I see two options. Either to write quite a complex code into the transceiver firmware - something like KISS protocol for Packet Radio, or generate serial data stream by a sound card directly.

The first solution needs two way communication between PocketPC and TRX. TRX needs to buffer some data before it starts transmitting to prevent jitter caused by multitasking OS. The protocol would have to define some kind of flow control.

Other solution would need just one way communication. Sound card in Pocket PC could generate 44100 samples/sec. Because of  antialiasing filter, each sample must be doubled to produce nice squares. That lowers data rate to 22050 baud. This is enough for 22050 / (11 * 8) = 250 DDS commands per second. Not fast enough for Packet Radio - 300Bd, but fast enough for other modes. One sound channel could send commands for DDS, the other could either generate amplitude envelope of PSK signal, or key the transmitter.

list of feasible modes:
PSK31: 31.25, 62.5, 125 - PSK2/PSK4
MFSK16: 15.625 - FSK16
RTTY: 45.45, 50, 75 - FSK2
FELD HELL: 122.5, FM105, FM245
DOMINO: 3.90625, 5.5125, 7.8125, 11.025, 15.625, 22.05, FSK18

How about that? Is there any interest for it? BTW, PocketDigi runs on big windows too. I think that some kind of DDS modulation scheme is implementable on KX1 just by changing firmware. I am building a DDS doughtercard, with which I will experiment. I would like to hear comments and proposals of a sensible communication scheme.

73, Vojtech OK1IAK
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[Leigh L. Klotz Jr WA5ZNU]

Vojtěch,
Please joins us at gmfsk-devel, a Yahoo group for gmfsk development.  
Your project sounds exciting.
Leigh/WA5ZNU
On Mon, 15 May 2006 12:40 pm, Vojtěch Bubník wrote:
I am porting GMFSK by Toni Maninnen to Win32 and optimizing it for fixed
point arithmetic..._______________________________________________
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