Hello friends,
During the "DX Headlines" segment on The Mighty KBC, Peter John has lately been sending a bewildering combination of digital modes and SSTV images, all packed in to one shortwave broadcast channel. Most of us are usually not successful in decoding them all, in part because the digital traces are below or above our receivers' passbands. But it's fun to try.
Peter's experiments have encouraged me to try this on Shortwave Radiogram. Actually, I have tried simultaneous digital modes in the early days of VOA Radiogram. Because of the complexity of producing and decoding such content, I have not continued such transmissions on a regular basis.
This week (program 359), we will experiment with transmitting two streams of MFSK32 simultaneously. One stream will be text about the red squirrels at the Yorkshire Arboretum in England. The other stream will be an MFSK32 image of a red squirrel.
You can decode both streams simultaneously by running two instances of Fldigi. On one instance, the RxID should be on (as normal for most listeners). The RSID will switch the mode to MFSK32 (it should be there already) and the audio frequency to somewhere near 1500 Hz. On the other instance of Fldigi, turn the RxID off. Manually switch the mode to MFSK32 (if not there already) and the center audio frequency to 2100 Hz. A tone before the picture is transmitted will help you fine tune to the specific audio frequency. If all goes well, you will see the text and the image decode simultaneously.
The alternative method is to record this edition of Shortwave Radiogram. Decode the text at (or near) 1500 Hz "live" with the RxID on. Later, from your recording, decode the image with the RxID off and the center audio frequency set near 2100 Hz, guided by the tone before the picture is transmitted.
To avoid clipping, mixing products, and audio harmonics, the level of the combined MFSK32 streams was reduced by about 3 dB. The decode of this combined MFSK32 signal might not be as robust as from our usual single MFSK32 stream.
The choice of a 2100 Hz center frequency for the second MFSK32 stream is deliberate. In decoding our MFSK modes, some listeners notice audio harmonics. I don't know if these are actually transmitted, or if they are a phenomenon within receivers. For our usual MFSK32 centered on 1500 Hz, the trace is from about 1250 to 1750 Hz. The second harmonic for that is 2500 to 3500 Hz. The second MFSK32 stream centered on 2100 Hz is from 1850 to 2350 Hz, thus sidestepping any interference from that second harmonic (if it exists in your receiver).
It seems to me that it would be possible to design a shortwave receiver, conventional or software-defined, that could display both text and an image, or two streams of text, resulting from the transmission method we will use this week.
A video of last week's Shortwave Radiogram (program 358) is provided by Scott in Ontario (Wednesday 1330 UTC). The audio archive is maintained by Mark in the UK. An analysis is provided by Roger in Germany.
Here is the lineup for Shortwave Radiogram, program 359, 20-26 June 2024, in MFSK modes as noted:
1:44 MFSK32: Program preview
2:52 MFSK32: Text at 1500 Hz and simultaneous image at 2100 Hz*
7:33 MFSK64: Greener electronics to reduce e-waste
12:25 MFSK64: This week's images*
28:22 MFSK32: Closing announcements
* with image(s)
Please send reception reports to radiogram@verizon.net
And visit http://swradiogram.net
Twitter: @SWRadiogram or https://twitter.com/swradiogram
(visit during the weekend to see listeners’ results)
Facebook group: https://www.facebook.com/groups/567099476753304
Shortwave Radiogram Gateway Wiki: https://wiki.radioreference.com/index.php/Shortwave_Radiogram_Gateway
(Kim Elliott)
