> Am I wrong to thing that the direct tv signal path is more direct (no
pun intended) than a cable company. Do cable companies compress the
signal more or less than direct tv? IIRC direct tv is an mpeg 2
stream. Can anyone shed some light on the signal path from a cable
network to direct tv vs. a cable channel?
I say:
I guess no one else is going to go for this so I will try. I do not
work for DirecTV nor a cable company (yet!) but I have a basic
understanding of satellite delivery. To compare cable delivery with
satellite delivery, let's start at the source of each program and
follow the path to your home in each case: Most programs originate
from a network master control facility which feeds a satellite uplink
site. In many cases these two are one and the same, but not always.
Nowadays, the connection between the network master control and the
uplink is a high-quality digital one, either by HD-SDI or ASI through a
fiber-optic terrestrial link or microwave link or directly. The feed
at this point is uncompressed and not likely to be impaired in any way.
But once the signal reaches the uplink facility, it must be compressed
along with other feeds in order to be uplinked to the cable network
satellite. Though this transmission is usually much less compressed
than the signal you ultimately receive at home, it still is compressed
so that many different feeds can fit onto the satellite transponder
(many transponders carry as many as 10 feeds or more at once). There
are several formats for uplink compression. MPEG2 is one of them.
Others are proprietary and encrypted.
At this point, both cable and satellite-delivered systems see the same
quality signal. Both receive their programs from the same C-band
(usually) satellites. However, the many feeds available for
distribution may differ in various ways from one another. Each cable
network or group of networks has its own standard of quality and just
as often, each may have mare than one master control site and/or uplink
facility which may differ slightly in performance. Despite this, all
delivery systems use the same sources for their programs: usually a
group of C-band satellites.
From here on however, satellite and cable delivery systems differ in
how they transport the signal to your home. In the case of your local
cable system, the cable company downlinks the feed,
decrypts/demodulates it, then recompresses and remodulates it onto your
cable pathway. Many cable companies have downlinks in one location and
headends in another, requiring some kind of terrestrial link between
them. In these cases, this terrestrial link is usually a wideband one
that carries many feeds at once, often already modulated for your cable
pathway. The grouped feeds may be combined with other feeds at the
headend and then fed on to your home. All of this is done without any
of the feeds being re-modulated again. That is, once the feeds are
modulated initially (digitally or as analog), they remain so right to
your home. If they did demod-remod repeatedly along the cable path,
you'd see impairments in the picture and sound, especially if they did
this multiple times. Instead, the satellite reception is downlinked,
decrypted/demodulated and remodulated ONCE, I think entirely digitally
(although I'm not certain of that, but it's likely). This remodulation
is the most likely point where degradation can take place. Since cable
systems vary, results here can vary greatly from one system to another.
They can even vary from channel to channel WITHIN a system if the
process is done inconsistently between the various channels.
After all the feeds are combined at the headend, other problems can
arise along the way to your home, but usually these are bandwidth
transmission problems that affect many channels, not individual ones.
DirecTV and DISH Network satellite delivery work by retransmitting the
cable satellite network feed at their master control sites. I recall
that DirecTV's master control/uplink site is in Denver. I'm not sure
where DISH Network's site is located. They take program feeds from
various satellite downlinks and (like cable), decrypt/demodulate them,
then recompress and remodulate them into a grouped feed for uplink to
their own satellites. Unlike cable, satellite delivery has no wired
infrastructure between the uplink site and your home. So the problems
associated with a physically wired link between a cable's headend and
your home are eliminated. But of course, satellite delivery has its
own set of limitations which usually affect all channels equally.
Conclusion: Differences between delivery systems will affect many (if
not all) channels equally, NOT individually. The amount of data
reduction (compression) done to the incoming feeds is similar for both
satellite and cable delivery, though their formats may differ. If you
see a difference between one channel and another on the SAME delivery
system, it can probably be attributed to either errors in the
conversion of the cable program to the system's own transmission format
or more likely outright differences between the originating feeds
themselves.
Since there are many different program sources and many different
delivery systems, this analysis cannot pinpoint an exact reason for you
to see a particular problem with any single channel.
Dennis Degan, Video Editor-Consultant-Knowledge Bank
NBC Today Show, New York
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