[opendtv] Re: Mediaflo Technology
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 10 Nov 2006 12:22:26 -0400
At 11:02 AM -0500 11/9/06, Mark Aitken wrote:
In-line response.
Craig Birkmaier wrote:
14 live streaming national interest
5 live streaming local interest
50 national and 15 local services that will deliver ~20 minutes of
content per day to receiver cache.
What does local mean?
I presume that this is content that is localized to the specific
transmitter - for now it looks like Mediaflo will have one
transmitter per market served; later they claim they will build out
SFNs around markets. There is more detail about the service
opportunities in the resources on the website, however, it is
important to note that MediaFlo is NOT the operator of the system,
per se. They are offering the service to existing system operators in
each market as an adjunct to a cellular service. The operator in each
market will have a strong say in what is carried, and the ability to
partner with local content providers within that market to fill the
"local" channels.
NTSC is 'effectively' 320 x 240? Really?
You could argue that there is more horizontal resolution, but not
vertical resolution The cable guys started out with 1/2 D1 MPEG-2
encoding, which was 352 samples per line. I looked at several sites
about the horizontal resolution of NTSC and the consensus sees to be
about 330 dots per screen width when transmission losses are taken
into consideration.
There are two factors at work here:
1. Transmission bandwidth - Ron did the math to show what is
"possible " if you can receive an NTSC signal perfectly. About 440
pixels per line. The 240 pixels vertically does not take into account
any positive benefit from "the interlace factor," which could add a
bit more vertical detail, however, with Mediaflo, the signal must be
de-interlaced for encoding as 320 x 240 progressive frames, thus it
is unlikely that any additional detail will make it through the
conversion process.
2. NTSC displays - the typical NTSC receiver has no more than 330
dots/stripes per screen width. With higher quality tubes (usually
with S-video inputs) the number of dots/stripes may increase to
approximately 450-500.
Bottom line, you are wasting bandwidth if you are inputting more than
480 samples per line into an NTSC encoder. We can use the full 704
samples per line (ATSC) if we are encoding for DTV, but even here
this may be overkill if we don't provide enough channel bandwidth
(bit rate) to assure that the detail makes it through the encoder.
MediaFlo says that the input source for their network will be
primarily from satellite, and they expect MPEG-2 encoding at 704 x
480 or 720 x 480. If this stuff is not overly compressed, there
should be enough detail left to produce a much higher quality 320 x
240 progressive stream than would be possible if one started with
NTSC source (analog or encoded using MPEG-2).
This all brings back memories from the early '90s when we noted that
a properly encoded 480P signal (i.e. 854 x 480) would provide a very
high quality viewing experience comparable to HDTV on many consumer
displays.
How quick we forget how poor NTSC quality actually is...
Regards
Craig
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