1 Sep
2017
1 Sep
'17
8:57 a.m.
Y'know, there is some relevance in a significantly asynchronous circuit
with this stuff.
Put the High speed TX on the top of the mountain and have it push high
speed frames in broadcast or multicast mode whilst the outlying stations
listen to the stream and write down the packets relevant to them. The
outlying stations could then be polled for traffic and would respond at a
lower speed on another frequency. Whilst the net effect would be as though
the whole network were working at (say) 3mbps at least it would be reliable
and sove issues like hidden transmitter etc.
This has been thought of before I'm sure.
On Fri, Sep 1, 2017 at 5:32 AM, Ron Economos <w6rz(a)comcast.net> wrote:
Well aware of digital ATV, I'm the developer of
the digital television
component of GNU Radio. The DVB-T2 DSP code was developed by myself and
Charles G4GUO. The DATVExpress transmitter board (that G4GUO is one of the
developers on) could be used for this project along with many other
transmit capable SDR's. However, the Ettus B2x0 is known to have the best
phase noise performance on microwave frequencies.
The cheap DVB-T dongles can work, but the latency of DVB-T is not
controllable due to a fixed "PHY" frame size. In DVB-T2, the frame size can
be made very small and this works wonders for latency. Currently, I'm using
a 3.458 millisecond frame size out of a maximum of 250 milliseconds.
However, short frame sizes sacrifice throughput for latency.
The DVB-T2 receiver I'm using is not too expensive, only $69.
http://www.hauppauge.com/site/webstore2/webstore_pctv292e.html
The transmit functionality is accomplished by having an instance of Pcap
running in the GNU Radio ULE block. All the IP packets sent to the normally
receive only dvb0_0 interface are captured by Pcap before they go to the
bit bucket.
Ron W6RZ
On 09/01/2017 01:40 AM, Rob Janssen wrote:
Very interesting indeed!
You are probably aware that there is some activity on Digital ATV and the
boards made
for that purpose (and/or the cheap dongles they use) could be used for
this as well.
We have a Digital ATV repeater here that is currently under some
reconstruction, and I
have suggested putting some IP broadcast on the multiplex. Indeed, as you
note, a
Linux-based DVB receiver can easily put the demodulated and extracted IP
traffic on its
LAN interface. More than a decade ago I did some "experiment" logging
the downlink of
satellite-internet on a Dreambox and it just used standard features of
its software.
But your test of using this full-duplex (rather than just unidirectional
as it was envisioned)
surely is innovative.
It could be useful here as well, indeed as you indicate to make links on
amateur bands
for which there is no commercial WiFi equipment. It is not
straightforward to use that
equipment with transverters, and a steady fullduplex link would certainly
be easier.
Some people are also working on a 70cm digital access with smaller
bandwidth, there is
little detail on what modulation they use, it would be interesting to see
if using DVB-T2
in halfduplex more is feasible. Probably not, due to high turnaround
delays.
Rob
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