At 01:29 PM 3/7/2006, darrel wrote:
>Today, almost all the DTV signals are at UHF and relatively low power
>compared to the current AM/FM NTSC signals. In a few years when they have to
>give up their analog channels, many will move back to their VHF assignment
>(though much less likely to 2 thru 6 than to 7 thru 13).
Except in Los Angeles.. Around here, most are going to stay on the UHF
allocation, and give up their VHF allocation, apparently.
>Here in Los Angeles, I can usually get most stations (25 miles away on Mt.
>Wilson) with a small UHF Yagi or 4-bay bow tie. Even when it's only SDTV
>rather than HD, it's amazingly better (like DVD vs a tape). Ultimately, they
>can split the available 18 Mbits/second into as many as 16 subchannels
>(though the most I've seen is 6, on the educational channel). Good quality
>HD needs about 75% of the bandwidth. A good SD signal takes maybe 20%. It
>is possible to pick almost any bandwidth, but as it goes down, both detail
>and the ability to follow action scenes suffers. It also has surround sound
>like DVDs.
Nice description of how the signal chain works:
http://www.broadcast.net/~sbe1/8vsb/8vsb.htm
Regardless of how the station decides to divvy up the bandwidth among
program streams, they still transmit at the standard 19.4 Mbps and your
receiver demodulates the full bit rate(i.e. there's no provision for a
station to broadcast at a lower bit rate with correspondingly lower SNR
requirement).
So, once there's cheap consumer gear out there, it seems an easy matter for
hams to use it to carry their own 20 Mbps data stream. Eventually, since
all receivers will do the demod, there will probably be little "wireless
video transmitters" (like the AM ones used in hatcams) available for hacking.
Then we'll start to see a lot more posts on TowerTalk about high gain
antennas for UHF and microwave.
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