Digital ATV with DVB-S
bob at stratton.net
Tue Mar 6 16:54:36 CST 2012
I seem to remember someone recently was asking about thoughts on approaches digital ATV. It looks like DVB-S is making its way to the ham bands.
I was reading one of the satellite (as in TVRO) boards I follow fairly closely, and found that one of the posters has been tinkering with some ATV modulator designs that European hams have built to transmit DVB-S, a discovery which delighted me.
I was thinking about starting with DVB-IP and working up from there but this may change my mind.
W0FMS has an overview of the modulation method on his blog at http://w0fms.blogspot.com
"The BATC's solution of using an inexpensive PIC and the FTDI serial interface is a maybe slightly kludgy but awfully clever solution of inexpensively and simply pumping data to theQPSK modulator chip. Coming up with an exact kludge like that kept me from this project for 8+ years. The only disadvantage to it is that they can only do Symbol Rates (SR) up to about 6250Ksym/s.. but for ham use at this time.. that is more than enough! It's an awesome start!
The modulation used is DVB-S, which is the older digital standard used by most of the world for satellite transmissions. I've been playing with LEGAL Free-To-Air satellite for many years. The majority of what is left unencrypted on C-band and Ku-band FSS satellite is receivable on an inexpensive (from $30 used DVB-S to $130 new DVB-S2 high definition PVR type units) set type box and/or PC receiver card. This same receiver can take in 950-2150 MHz signals as an IF (with a converter and/or a LNA in front of the IF) in Amateur use.
The disadvantages to DVB-S for ham radio is that it is QPSK with Convolutional Encodingand Reed-Solomon FEC.. Meaning that it's about 3 dB "weaker" than the more "modern" BCH/LPDC codes used in DVB-S2 and that it's QPSK -- optimized for weak signals, but pretty weak when it comes to handling multipath.
Current Analog NTSC AM TV doesn't handle multipath well either ("ghosting").. so those who already do ATV are using Yagi beams other directional antennas for their front-to-back and front-to-side directivity more so than the secondary benefit of gain. Experiments with DVB-S for terrestrial Amateur TV/Data links show that the same antennas work to fix many of those issues. Hams will likely use beams at these power levels anyway.
One great advantage to DVB-S for ham use-- and overwhelming one in my opinion is that the bandwidth and data rates, even the video and audio coding the the MPEG-2 Transport streams are pretty much completely up to the link user. DVB-T in Europe and ATSC in the US is only setup for 6/8 MHz channels and IMHO there is no reason for hams to use this much bandwidth in 2011 for ATV.
Experiments by the BATC and others show that the digital signal is much more usable and stable than equivalent bandwidth analog ATV and it just gets better with reduced bit rates."
A web site about one of the BATC prototypes, the DigiLite, is at: http://www.g8ajn.tv/dlindex.html
More information about the Tacos