Bandwidth Questions

Archived from groups: alt.tv.tech.hdtv (More info?)

Hi,

I've got an MDP-130 connected to Comcast's digital cable service in the
Dallas area, and trying to come up to speed on the technical aspects.

The way I understand it, ATSC describes a per-channel data rate of 19.8
Mb/s. I'm assuming that if the broadcaster is carrying a 1080i signal on a
channel, there's not much (if any) bandwidth for subchannel signals. If he
wants to make room for more subchannels, he caps the signal at, say, 720p or
480p. But however he configures it, an ATSC channel can't exceed 19.8
Mb/s... right? :-)

From what I can tell, I should expect to see a 19.8Mb/s data rate
(~9GB/hour); connected to my local cable provider (Comcast), I'm seeing more
than double that rate; a 9GB .tp file holds a little under 30 minutes. I'm
guessing that the difference is due to QAM versus ATSC -- my friends who
have the same card and who are going OTA report a data rate closer to
19.8Mb/s... but it'd be nice to know what's going on. (I'm not
complaining -- it sure looks like I'm seeing a better bandwidth than I would
be with OTA, but at these rates, I'm going to be eating up some serious
drive space!)

Can anyone throw any corrections or additions to these assumptions?

Also, do you know of any good .tp/.ts utilities out there? One nice utility
to have would to be able to split the transport stream into separate
subchannel streams (without rendering).

And -- finally, and TIA for your patience! -- can you give me a glimpse into
your workflow? The goal for me is to build a DVD from a transport stream
source, and I'm really curious as to do this with as little rendering as
possible (I believe that depends on the MPEG2 compression settings by the
broadcaster, and that suggests another nice-to-have utility, to be able to
spec out the MPEG2 settings for a particular subchannel, so I can know in
advance whether I need to recompress when going to DVD.)

Thanks for reading this -- no more wordy posts, I promise!

Apocalypso Now

Archived from groups: alt.tv.tech.hdtv (More info?)

In article <ga6dncfR7u_IX-XfRVn-3Q@comcast.com>,
"Apocalypso Now" <apocalypso.now@exegesis.org> writes:
> Hi,
>
> I've got an MDP-130 connected to Comcast's digital cable service in the
> Dallas area, and trying to come up to speed on the technical aspects.
>
> The way I understand it, ATSC describes a per-channel data rate of 19.8
> Mb/s. I'm assuming that if the broadcaster is carrying a 1080i signal on a
> channel, there's not much (if any) bandwidth for subchannel signals. If he
> wants to make room for more subchannels, he caps the signal at, say, 720p or
> 480p. But however he configures it, an ATSC channel can't exceed 19.8
> Mb/s... right? :-)
>
Actually, 1080i can look reasonbly good with less than 19.8Mbps esp if the
source material is 24p. Given a 24p source, there is ALOT of redundancy
that the ATSC MPEG encoding can take advantage of. This is probably one
reason why MPEG artifacts can be relatively more common on 60i football
games, but on 24p, the quality is often very good. So, 15-16Mbps might
do reasonably well for 24p (film) source material.

>>
> From what I can tell, I should expect to see a 19.8Mb/s data rate
> (~9GB/hour); connected to my local cable provider (Comcast), I'm seeing more
> than double that rate; a 9GB .tp file holds a little under 30 minutes. I'm
> guessing that the difference is due to QAM versus ATSC -- my friends who
> have the same card and who are going OTA report a data rate closer to
> 19.8Mb/s... but it'd be nice to know what's going on. (I'm not
> complaining -- it sure looks like I'm seeing a better bandwidth than I would
> be with OTA, but at these rates, I'm going to be eating up some serious
> drive space!)
>
The terrestrial version of ATSC for a 6MHz bandwidth supports about 1/2
of the payload relative to the cable version. I suspect that the cable
version of QAM has approx similar capabilities of the cable version of
ATSC, so it would be plausible that it has a 38Mbps payload capability.

John

Archived from groups: alt.tv.tech.hdtv (More info?)

Del Mibbler (mibbler@nycap.rr.com) wrote in alt.tv.tech.hdtv:
> The MyHD cards record the entire transport stream for the chosen
> channel, including all subchannels.

The latest software has an option to record only the sub-channel you are
interested in.

> You can select which subchannel
> to watch during playback. That could be useful if there are two shows
> on you want to watch, and if both stations happen to be on the same
> channel. On my cable NBC, ABC and Fox are all on one channel.

OK, for you, this option is a bad thing.

> > The goal for me is to build a DVD from a transport stream
> >source, and I'm really curious as to do this with as little rendering as
> >possible (I believe that depends on the MPEG2 compression settings by the
> >broadcaster, and that suggests another nice-to-have utility, to be able to
> >spec out the MPEG2 settings for a particular subchannel, so I can know in
> >advance whether I need to recompress when going to DVD.)
>
> I know there are utilities for that, but I can't bring myself to do
> that to a good HD program.

The best tool for this is the converter program that comes with the
Fusion HDTV cards. It uses some MPEG tricks so it doesn't do a full
recompress when moving from HD down to DVD resolution. The quality is
excellent (although not HD), and the file size is managable.

Of course, the best thing to do would be to re-compress to an MPEG-4
format, which should allow you to fit two hour-long shows (1:30 without
commercials) on a single-layer DVD while still maintaining the full HD
resolution.

--
Jeff Rife | "Tragedy struck today in Sector Nine as rebel
| terrorists blew up the Death Star, killing
| thousands. The Rebel Alliance, a fringe group
| of anti-Empire fanatics, has claimed
| responsibility for the terrorist act.
| Fortunately, Lord Vader escaped without harm.
| Our hearts go out to the families of the
| victims."
| -- "NewsRadio"

Archived from groups: alt.tv.tech.hdtv (More info?)

Del Mibbler (mibbler@nycap.rr.com) wrote in alt.tv.tech.hdtv:
> Not as bad as my OTA situation, where CBS (HD) shares the channel with
> UPN (SD). Try finding two shows you want to watch opposite each other
> with that pair! Several good ones on CBS, nothing but Enterprise (in
> SD) on UPN.

My local UPN station is owned by Fox, as is the local Fox station. I really
wish they did this same thing, since Fox has 4Mbps free anyway (the network
feed is only 15Mbps). At least then I might get a watchable signal. The
analog UPN is really bad. They do have a digital transmitter, and they
broadcast HD *and* DD5.1, but use only 1.3kW, so I can't get it from 20 miles
away.

Because of this, I download all the episodes of "Enterprise".

--
Jeff Rife | "...the flames began at a prophylactic recycling
| plant, near the edge of the forest..."
|
| -- "WarGames"

Archived from groups: alt.tv.tech.hdtv (More info?)

Korbin Dallas wrote:
> On Wed, 04 May 2005 07:38:49 -0500, Apocalypso Now wrote:
>
>
>>Hi,
>>
>>I've got an MDP-130 connected to Comcast's digital cable service in the
>>Dallas area, and trying to come up to speed on the technical aspects.
>>
>>The way I understand it, ATSC describes a per-channel data rate of 19.8
>>Mb/s. I'm assuming that if the broadcaster is carrying a 1080i signal on a
>>channel, there's not much (if any) bandwidth for subchannel signals. If he
>>wants to make room for more subchannels, he caps the signal at, say, 720p or
>>480p. But however he configures it, an ATSC channel can't exceed 19.8
>>Mb/s... right? :-)
>
>
>
>
>>From what I can tell, I should expect to see a 19.8Mb/s data rate
>>(~9GB/hour); connected to my local cable provider (Comcast), I'm seeing more
>>than double that rate; a 9GB .tp file holds a little under 30 minutes. I'm
>>guessing that the difference is due to QAM versus ATSC -- my friends who
>>have the same card and who are going OTA report a data rate closer to
>>19.8Mb/s... but it'd be nice to know what's going on. (I'm not
>>complaining -- it sure looks like I'm seeing a better bandwidth than I would
>>be with OTA, but at these rates, I'm going to be eating up some serious
>>drive space!)
>>
>>Can anyone throw any corrections or additions to these assumptions?
>>
>>Also, do you know of any good .tp/.ts utilities out there? One nice utility
>>to have would to be able to split the transport stream into separate
>>subchannel streams (without rendering).
>>
>>And -- finally, and TIA for your patience! -- can you give me a glimpse into
>>your workflow? The goal for me is to build a DVD from a transport stream
>>source, and I'm really curious as to do this with as little rendering as
>>possible (I believe that depends on the MPEG2 compression settings by the
>>broadcaster, and that suggests another nice-to-have utility, to be able to
>>spec out the MPEG2 settings for a particular subchannel, so I can know in
>>advance whether I need to recompress when going to DVD.)
>>
>>Thanks for reading this -- no more wordy posts, I promise!
>>
>>Apocalypso Now
>
>
> 19.8Mb/s is the maximum rate, but a 1080i signal will only fill that on
> peaks. Because of compression the actually data bandwidth will change.
> Here in the DFW area its not uncommon to see 1080i with a low bandwidth
> aux channel. (example: 8-1 & 8-2)
>
> The Cable system uses QAM to lower the RF Bandwidth needed to carry a
> 19.8Mb/s channel. So an 19.8 Mb/s signal fits into a 3 Mhz wide RF
> channel. As far as OTA vs Cable there should be no difference in the data
> received, assuming you have a clean OTA signal. If you don't have a decent
> outside antenna you probably don't have a decent OTA signal. People have
> lived with lousy NTSC reception for decades and ATSC cannot correct for
> bad reception.
>
>
>
ATSC can't correct for bad reception by design. It was only designed to
match NTSC and fails at that.

COFDM can correct for bad reception and will employ multipath signals to
increase overall signal strength. Something that LG claims they have
copied somewhat from COFDM in their 5th gen technology.

Bob Miller

Archived from groups: alt.tv.tech.hdtv (More info?)

Richard C. wrote:
> X-No-archive: yes
>
> "Bob Miller" <robmx@earthlink.net> wrote in message
> news:X7qie.2504$Lc1.1795@newsread3.news.pas.earthlink.net...
>
>>
>> COFDM can correct for bad reception and will employ multipath signals
>> to increase overall signal strength. Something that LG claims they
>> have copied somewhat from COFDM in their 5th gen technology.
>>
> =====================
> Then you are saying ATSC CAN do it?

They claim that they use multipath signals to increase overall signal
strength. But they can not do it as well as a COFDM based modulation.
8-VSB can not do mobile for instance and even in a fixed reception
scenario is still easily defeated by both static and dynamic multipath.

What we tested last summer was "good enough" so that a digital
transition would be possible or that someone could have a viable
business plan for using OTA spectrum. Till now no one has such a plan
including current broadcasters hence their almost exclusive dependence
on must carry and their desire for multicast must carry.

But long term OTA broadcasting dies even with a receivers that work as
well as what we tested. OTA will need all the tools they can muster to
survive their competition in delivering bits to fixed devices, they will
need MPEG4 and mobile IMO.

That means that OTA free TV/DTV is still in a death spiral. And the
vultures are hovering. The attacks on OTA and the current broadcast
model continue to mount. Congress seems particularly unafraid of
broadcasters. In a meeting last week Chairman Barton was shocked at the
crowd of people that showed up to back his aggressive plan to turn off
analog. There are a lot of new entities who are and can shell out the
green that are now making their presence felt and who appose the
broadcasters.

Bob Miller