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The
U-verse system uses Windows Media streaming (a.k.a. "VC-1")
to deliver live audio/video to the U-verse subscriber's Set Top Box.
Unlike normal television stations, U-verse aggregates all Public,
Education, and Government (PEG)
channels in a list that subscribers access by manually entering
channel 99. This is followed by a "please wait" message for a
few seconds while the Set Top Box builds a list of available PEG
channels. The list of PEG channels are all of those available
in the serving state or region. Some people believe this is a good
thing because it allows subscribers to view many PEG channels, not just
those of the local town or community. Some people think it
is a bad thing because a viewer must proactively select the desired
channel, hence the PEG station may lose "channel surfing" viewers.
Technically, it is a trade-off. There simply would not be enough
capacity to give a full numbered channel to every PEG station, and
if U-verse followed the analog
cable-TV model then only the local
PEG stations would be available and subscribers would not be able to
see the PEG channels from other towns. To provision a PEG
station, AT&T installs a "T1 line"
at the station facility. In
some cases, they only install it to
what they believe is their
"Demark"...which may
be in some unusual location. Then it is
up to you to run a wire to your station. A "T1 line"
is actually provisioned via a DSL technology
and provides exactly 1.536 Mbps of bi-directional capacity. One should not confuse
this fact with what is popularly known as consumer
DSL
service that is asymmetric and provides a high speed download but a
low speed upload. The "T1" line is
terminated by AT&T with a CSU, commonly
from Adtran or a similar T1 access vendor. The
CSU ("Channel Service Unit") receives power from
the line and presents a "DSX" interface to connection to a router. The router
uses PPP to establish a connection to the
AT&T network, using configuration information provided
by AT&T. The router is configured such that a range of IP addresses are available, and the encoder is configured to use one of these addresses. The router is connected
to an Ethernet switch, and the encoder is given the target IP address
and also connected to the switch. Thus, AT&T can pull the stream from the
encoder's server. (Unfortunately, AT&T does not support the push model, which makes setup vastly
easier). One should also realize that the U-verse
people and the "T1" people are different. The
AT&T people that provision and
install the "T1" may think they are giving you public
Internet access, and indeed this is what
you are getting. The U-verse people then use this new public Internet
connection to pull the video into U-verse.
Note: do NOT be tempted to use the connection for general Internet access: virtually all of its capacity
will be used for video delivery and any other use will
likely adversely affect your video
performance.
The encoder parameters are unusual, but are optimal for the
consumer Set Top Box that U-verse uses. Here are the settings:
- Audio/Video Rate: Approx 1.25 Mbps (AT&T Recommendation)
- Video Rate: 1.1 Mbps
- Audio Rate: 96 Kbps Stereo, 44.1 KHz sampling rate (48K possible
too)
- Video Resolution: 480 x 480
- Aspect Ratio: 4x3
There is often confusion around the video rate. Sometimes people say
"video rate" when they mean the "payload rate". The "payload
rate" is video rate + audio rate + overhead. As already
discussed, the maximum rate a "T1" can support is 1.536 Mbps. This
number is not the
payload rate into
the router, however.
There is packet overhead (each IP packet has headers that does not
carry your data), video overhead (information to keep the audio and
video in sync), and depending on the content the video rate can vary
a little bit (bursts). You must set the total payload rate
from the encoder to under 1.33 Mbps, which leaves approximately 134 Kbps for all
of the transmission and video overhead. AT&T recommends 1.25
Mbps rate ("A/V rate"), but this is not the full payload
rate which must be under 1.33 Mbps. The encoder video rate could be
anything from 20 Kbps to 4 Mbps, and the audio rate could be 8 Kbps to 384
Kbps -- so you must consider both (plus overhead) when you
think about the payload rate. Since our intent is to deliver the
maximum quality possible within the 1.536 Mbps bandwidth available, we know
the optimum settings for the encoder (see above).
Some say it is not possible to deliver a television broadcast at
only 1.1 Mbps. The advances in video compression has indeed made
this possible, however it is certainly true that a higher quality
video image would be produced at a higher bit rate. The quality of
the video on the U-verse system is surprisingly good considering the
bandwidth used to deliver it, and it often rivals the highly compressed video delivered on
satellite and other systems.
The video is encoded and delivered as progressive, square pixel
video. This is not to imply the video input is not
interlaced...it is conventional NTSC video. The U-verse Set Top Box is optimized to
receive this resolution, and it, of course, delivers conventional
NTSC interlaced video to the subscriber's TV monitor. No
broadcaster should panic or be concerned about the encoder
resolution...this is a behind-the-scenes configuration and the
result is surprising good (assuming you are using a high quality
device such as a VBrick for the encoding).
AT&T uses port 5000 for streaming, and uses no resource identifier.
Audio/Video Inputs
The encoder accepts conventional analog composite or S-Video input,
and line-level audio input. The audio/video settings in the encoder
can be adjusted to improve brightness, contrast, color saturation,
hue, etc. and the audio levels can be adjusted. Commonly, it
is sufficient to simply "T" the audio/video that is presented to RF
modulators, or to use a separate audio/video output from a
production switching system.
Simultaneous Internet Streaming
Discover Video's unique proposition is that our solution delivers
simultaneous live streaming of your PEG station on the public
Internet. Thanks to built-in advanced routing capability, the
Internet streaming uses a conventional public Internet access
connection that you already have or can get (consumer DSL service)
to push the live video stream at a different rate and resolution to
the public Internet. You can see an example
here.
Complete Kit
Discover Video provides a complete U-verse kit that includes the T1 Router, Ethernet switch, encoder, key
wiring and adapters, and install assistance where we work directly with AT&T to make it easy
to get "on the air" with U-verse.
When you obtain your system from Discover Video, you can have confidence that you are getting a complete solution
that has been tested and deployed in the real world, and that you won't suffer from the long-duration
troubles found in PC-based systems. |
