18 July 2019

KG-STV MSK 1200Bd/600 (via Qatar-OSCAR 100)

KG-STV is a digital SSTV system developed by JJ0OBZ (K.G. from Japan). Unlike the analog SSTV, which scans the image line by line, KG scans the image in blocks of 16x16 pixels, i.e. 15 scan lines each conisting of 20 blocks of 16x16 pixels, that during transmission are compressed and digitally encoded one by one.The image has also one of the formats used in SSTV, which is 320 by 240 pixels. The 300 blocks of the image are transmitted fropm left to right and from top to bottom. The reception can be made at any time during transmission as in analog SSTV [1].
KG-STV is also the nome of the experimental program coded by JJ0OBZ to transfer digital images and short text-based messages [2].

Fig. 1
The program allows to use one of two types of digital modulation: MSK and 4LFSK. In MSK, KSG forward to 1200 Baud (equivalent in this case of MSK, 1200 bps) and the frequency of marks and spaces are 1800 and 1200 Hz respectively (4LFSK: '00 '1200Hz, '01' 1400Hz, '10' 1600Hz, '11' 1800Hz).
The 4LFSK is a version of 4 levels of MSK, and therefore carries twice as many bits, ie with the same 1200 Baud, transmits 2400 bits per second, but requires a channel with less noise. The transmission can be done with Viterbi convolutional code error correction (NASA standard K = 7 convolutional), but in this case the transmission is obviously more time consuming (almost twice as long).

KG uses a synchronization sequence of 63 bits: 

and a scrambling pseudo random sequence of 127 bits (polynomial x^21+x^3+1):

Fig. 2
Fig. 3
Although KG-STV can be seen on amateur SSTV bands (3733, 7173, and 14233 kHz), I tuned it on 10.496,25 GHz (Fig. 1) via AMSAT P4-A transponder on Qatar-OSCAR 100 (Es’hail-2), the first geostationary amateur radio transponder: more precisely I used the Qatar-OSCAR 100 Narrowband WebSDR [3] since I'm not equipeed for satellite receptions :).

10 July 2019

what a blunder :)

some days ago I went on a S4285 modem running on 4271.0 KHz/USB in 600bps/L submode. Bitstream after demodulation showed a 2163-bit length period: clearly a test tape. Once reshaped, an asynchronous transmission appeared and I thought of a strange 6N1 framing!!!
It was thaks to @swlfrance Nicolas (and later AngazU) that I understood my mistake: actually it was a common ITA2 5E1 framing used in this case by French Ny HQ FUJ in Port des Galets, Nouméa.
Anyway, it was the first time I met that framing.

4 July 2019

110A 2400bps modem carrying 1536-bit protocol

Signal recorded on 14600.0 KHz/USB tranks to the KiwiSDR http://collie2.ddns.net:8073/ located in Western Australia. 
The used HF waveform is 188-110A Serial in 2400 bps mode, note the 48 symbols length frames (32+16 UK). ACF value is 200ms that makes 1440-bits/480-symbols: the length of the ACF is due to the short interleaver matrix dimensions for 2400 bps speed (40 rows x 70 columns) as discussed here.
Fig. 1
Once demodulated, we get a stream that has the well-known period of 1536 bits length that can be attributable to the GA-205 multiplexer: don't know if they were using 4 of 12 channels only. Also found the sync characters 9C16 and 9D16 ... but it might be a mere coincidence.  Most likely it's a naval broadcast by the Australian Navy RAN.

Fig. 2
Fig. 3