lauren@vortex.UUCP (Lauren Weinstein) (02/07/84)
Any further discussions on this topic should be sent to me via
direct mail; this topic is getting too specialized for this newsgroup.
I'm going to address David's points one by one.
(D:) is "David", (L:) is me.
{allegra, decvax, ihnp4, seismo, harpo}!vortex!lauren
----
D: Are you sure that you want to use the vertical blanking interval with
cheap satellite receivers? Whose video signal are you going to use ? And
$ 1000 dish/TVRO systems are notorious for differential gain/differential
phase errors, IF phase anomalies, df/dt errors (with resulting cycle slipping
in PLL receivers), horrible DC restoration in the vertical interval, and
so on.
Lauren: Yes, David. I'm sure. I'm approaching a variety of satellite
programmers regarding their intervals, including non-commercials of
various sorts. Several have expressed interest. They are not interested
in direct commercial use of the VBI, but consider applications such
as I'm suggesting to be worth consideration. The required equipment for
insertion at the uplink is relatively inexpensive and off-the-shelf.
Most are willing to give up 4 lines, some only 2 (still a lot of data).
Most are using their interval for NOTHING right now, not even insertion
testing or VIR.
Cheaper receivers do have more phase problems, but testing of
such systems with running teletext services via various transponders
indicated few problems. An extra level of confidence can be obtained
through the use of high-level ECC codes in the data stream, which are
highly practical given the high bandwidth of the channel. However,
empirical evidence is that these are only sometimes needed, even with
extremely inexpensive satellite equipment and/or horrid cable equipment.
D: In addition, who is going to give up four lines in the VBI. The easy
way to go is
L: Several major groups are interested. It's mainly a matter of
logistics and planning more than anything else. See above.
D: 1) An aural subcarrier (say 7.2 mHz) with 56 kb/s synchronous or
less -- the implementation is trivial; use a 13 foot dish and receiver
with threshold extension;
L: That's just fine, Dave, so long as you're willing to force everyone
to buy a dish. That is NOT the idea here. Most people will receive
the data via local cable companies -- only those not served will
need the cheap (6-9 foot will do in most locations) satellite equipment
for direct reception. Aural subcarriers can only be demodulated at the
downlink and would force everyone to have a dish and the fairly
expensive (and not always "off-the-shelf" equipment for converting that
subcarrier back into usable data. Not practical. Most sites will
NOT want or need to have their own earth stations.
D: 2) A SCPC satellite, with much lower line charges, running digitally.
In this case, the IF bandwidth can be drastically narrowed, saving money
on dish (and permitting a simple receiver/modem.) The cost for VBI stripping
equipment is nontrivial.....
L: Are you going to pay for the uplink charges, Dave? It appears likely
that we will be able to negotiate (with luck) extremely low rates
for the vertical interval of an existing video service. Commercial
data services are tariffed assuming that you're making money from
your operation such that you're willing to pay for a "premium"
transmission system. The Usenet system is NOT commercial and we do
not have that kind of money to throw around. The whole idea is to
buy some time on a carrier who is NOT engaged in commercial data
transmission operations. In any case, your concept once again
requires a dish at all locations.
VBI stripping equipment is extremely simple. A multitude of LSI chip
sets which do all the real work are already available from various
sources, and the only other equipment needed is a simple video demod.
My personal favorite technique is to take a cheap Sears Closed
Captioning box and wire the VBI PC board into that. I've gotten
excellent results, even over cruddy cable systems. Our own system
will use ECC and such to assure even better performance.
D: Performance of $ 1000 and below satellite receiving systems is
unacceptable for plain old video. I don't know what price Lauren
was quoting, but even at the wholesale, 10-99 unit price, the
equipment really stinks.
L: The new precision one-piece fiberglass dishes are proving extremely
useful even in weak satellite footprint areas such as Southern
California. With such a dish, a 100 degree LNA, and an inexpensive
receiver, excellent teletext test results have already been achieved
in various areas around the country. Some areas might need slightly
larger dishes or cooler LNA's, but not by much. Most sites will just
use cable and won't need the dish at all. This is all based
on empirical evidence, NOT speculation.
D: SCPC or 12 gc is the way to go........
L: Uh huh. You send the money for the uplink costs, downlinks
and processing equipment for all sites and we'll talk then.
I won't be holding my breath.
D: --David ( Last of the Analog ) Anthony
akgua!mcnc!(urp,unc-c)!dya
L: --Lauren--