[CALUG] Digital TV Tuner card

Dave Dodge dododge at dododge.net
Tue Sep 20 15:54:06 CDT 2005

On Tue, Sep 20, 2005 at 03:08:12PM -0400, Danyelle Gragsone wrote:
> sometimes when I watch TV.. you will see a little caption that comes up 
> and says " offered in HDTV where available." Are they full of it? What about 
> the HD services? Do they really work?

Yes, "where available".

Each over-the-air broadcaster essentially has a second, UHF channel
where they send out their digital feed.  For example WMPT in Annapolis
broadcasts analog on channel 22, and digital on channel 42.

The digital channel can actually be split up into multiple
subchannels, each carrying a different program.  There are many
possible formats and resolutions for the digital video in the
subchannels.  480i (SD) is the digital equivalent of "normal" TV; 720p
and 1080i are widescreen high-definition formats; and there are things
like widescreen 480p in the middle.  The station decides what format
and bitrates they want to allocate to each subchannel.  HD requires
more bits than SD, so if they choose to include an HD feed it limits
the number and quality of SD programs they can carry at the same time.
I think MPT sends out four SD programs during the day, then switches
to one SD and one HD program in the evening.

For various reasons cable/satellite companies are usually not allowed
to bypass the local station and pick up a network feed from another
city.  So the first issue is that in order for you to get the HD
version of a program, the local broadcaster has to decide to carry it.
I believe most of the local network affiliates do carry HD feeds.

If you're within range of the station's transmitter, you can pick up
their digital feed with a UHF antenna and a digital tuner.

To get these over cable, you then have to rely on the cable company
carrying them.  Recall that this is a _second_ channel, _and_ it eats
up a lot of bits.  So the cable company needs to decide whether it's
worth giving up space for 3+ SD channels in order to carry the HD
channel (in addition to the normal SD feed from the same station).
They might also compress the data further, lowering the quality.  For
the major network affiliates, Comcast probably does carry their HD
programming in its digital feed; for the smaller stations, perhaps
not.  I don't see UPN on the list that was posted earlier today.

Another issue is that when the cable company negotiates the rights to
carry the digital feed from a station, there could be battles over the
price.  The station may ask for a lot more money or concessions, to
help recoup the cost of their digital gear.  This was definitely a
problem a few years ago when things were just getting started.

If you're interested in what HDTV picture quality is like, here's a


                                                  -Dave Dodge

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