DD
Dennis Degan wrote:
>
>
>
> On Jan 8, 2010, at 2:30 PM, John Moore wrote:
>
> > 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
>
>
--
David Dawkins
780-905-9121
dawk2@shaw.ca
[Non-text portions of this message have been removed]
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