[amsat-bb] Re: Digital Slow Scan in space

[Wouter Weggelaar]

Hi All,

SSDV indeed shows you the images "as they come in"
Most High Altitude Balloons use RTTY with relatively slow baudrates.
The balloons are limited to using FSK modems in the ISM bands, because
of legal issues. Therefore, RTTY is the most feasable mode. Tim Zaman
here in the Netherlands has run 1200bd RTTY before on a balloon. see
http://www.timzaman.com/?page_id=1106&lang=en
He used the faster baudrate to send bigger pictures in the same time
and was running two baudrates alternating.

For satellites, you could run a more complex downlink off course, with
less transmission time and FEC.
64QAM might work on bigger sats, but I think it is not feasible for a
one or 2u satellite at the moment. I would place my bet on flying
BPSK/QPSK for now, and more complex things later.

Needless to say that I agree with David about the handshaking and
ACKs, but the balloons don't do that either.
I do not know anything on HF SSTV, but I have seen SSDV working just fine.

Wouter
PA3WEG

On Sat, Sep 8, 2012 at 9:54 AM, Trevor . <m5aka at yahoo.co.uk> wrote:
> --- On Fri, 7/9/12, g0mrf at aol.com <g0mrf at aol.com> wrote:
>> An essential feature would be that whenever a 'packet' of
>> data is received, that data is displayed rather than waiting
>> for the complete image to be received error free.
>> I couldn't find much on DRM encoding but I think it was
>> originally intended for digital voice in a narrow bandwidth
>> so it sounds suitable for low power satellite use.
>
> WinDRM (and DRMDV/FDMDV) were written by Cesco Lanza HB9TLK some info at
> http://n1su.com/windrm/download.html
>
> To achieve a reasonable data rate it's usually run at 64QAM (with full SSB bandwidth). 64 QAM is less robust and requires a higher S/N.
>
> I've no idea what the practical effects of a high Doppler shift would be, although a computerized station with frequent Doppler correction should track it okay.
>
> Is the data rate on High Altitude Balloons about 300 bps ? I've never received any images from them but if 300 bps is correct then the images are either low res or take a long time to send ?
>
> Analog SSTV is still going strong some 6 years after the introduction of Digital SSTV perhaps indicating that existing Digital SSTV approaches do not yet match the performance/robustness/ease of use of analog for HF working.
>
> 73 Trevor M5AKA
>
>
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