Well after some bad weather I finally got a break and some good
weather arrived. So I decided that it was a good day to try out how
JQAM v2 worked with a 1Km distance.
When creating JQAM most of
the time I would just loopback the soundcard with a piece of wire, so
the distance i was transmitting in effect was no distance at all. For a
real life test I just sent the signals from one end of the house to the
other via either wide band FM or NBFM at about 88MHz and 476MHz
respectively. All very well in theory but how would it behave traveling
a longer distance, through a built up area, through hills, and bouncing
off hills. Would it even work at all?
If you've been reading
what i've done before you'd know that I've done this test before using
an older version of my somewhere. It was an OK test but streaming media
dropped out a far bit. With this in mind I went about making JQAM v2
more robust, an hence its existence.
This 1Km distance for me is
always a bit tricky to do, I'm surrounded with hills and houses and get
no direct line of site at all. I need at least a watt to get any signal
at all. My 476MHz transmitters are 1/2 a watt and are very iffy doing
the distance. I needed something else. I had to make something that
would do the job.
I have bits 'n pieces of RF stuff around, so
first I got a RF modulator to work, its PLL'ed. I fed that into a RF
amp/buffer to try and boost the signal a tad an stop the darn feedback
into the oscillator happening and stopping the oscillator, this went
from about 5mW to 125mW. Then into a 2SC1971, this went from 125mW to
4W. Then through a LPF to make the signal pretty. Below is a picture of
the transmitter, you can see it's quite a mess :).
Next
I needed an aerial. In the garage I found an old car aerial, some
pieces of pipe, and a hose pipe clamp. In the house I got some string
and one of the grills from the oven. Climbing on the roof and putting
it all together I got the following (It's all held together with string
btw)
I
used UltraVNC so I didn't have to go home every time I wanted to change
the transmitter's settings. When I got the the other house I turned
JQAM on, QAM16 gamma 0.54 fc 4800 fec 20%. The receiver was just a
common FM radio pluged into the soundcard. Started JQAM on the
receiver's end, and low and behold it locked no problem. So then I up'd
the anti and switched to QAM64, no probs. So then I up'd the carrier
frequency all the way to 11000Hz, no probs. It was quite amazing how
well it worked, it worked just as well as if I was in the same house as
the transmitter.
At this point I was using about 22KHz of
bandwidth. My settings were interleaving=500, fec=20%, fc=11000Hz,
gamma=0.54, QAM64. This meant I was transmitting 20370 symbols a
second, equivalent to 122220 ones and zeros a second. After taking into
account of fec and tcm this works out to be a usable 81.5kb/s, witch is
about 2 times as fast as a dialup modem can download, pretty good.
Next
it was time to see how streaming media worked with the new version of
JQAM. I started streaming TV from the capture card at about 70kb/s for
video and 10kb/s for voice. This is quite watchable, in fact I sat
there and watched TV for about half an hour before heading off home. I
didn't get any dropouts while watching TV.
The
test really went prefect, I couldn't have hoped for better. I think its
so cool that its totally possible to use standard radios for
transmitting video at a reasonable rate over what I consider long
distances. That you don't have to modify radios to have perfect flat
response with no dispersion to do this is just the icing on the cake.
