[opendtv] Re: Letter to TV Technology
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 8 Oct 2007 18:37:03 -0400
Bob Miller wrote:
> Most countries don't have mandated receivers for instance.
> The US does.
All that means is that some TV customers will be surprised when their
TVs suddenly quit working. There is no nagic going on here. If analog TV
goes off, either you plan ahead by creating transition mechanisms, or
you have to deal with more last-minute problems.
> In the article Hoffman rights ...
>
> "Time marches on, technology advances, and today's ATSC
> receivers deal with such conditions quite capably. So
> capably, in fact, that we are contemplating the possibility
> of implementing single frequency network (SFN) television
> broadcasting in the United States."
>
> He seems to be saying that the latest 8-VSB receiver
> capability makes possible the contemplation of using SFN's
> with 8-VSB.
You haven't been paying attention, then, because this has been covered
on many occasions.
It's true that better receivers help in making SFNs work, but MOSTLY
this is true for passively synchronized SFNs. That's why, for example,
the CRC demoed their gap fillers working in Ottawa, with the LG 5th gen
receivers. These gap fillers depended on receivers with good pre-echo
tolerance, and the 4th and 5th gen receivers were the first ones to
provide this capability.
The innovation that made true SFNs possible with 8-VSB, i.e. more than
just gap fillers, is the one we have talked about here for a long time,
Bob. It is cheap, effective, transmitter synchronization. It is A/110.
Obviously, better receivers will also help, because propagation paths
are often quirky. However there never was anything IMPOSSIBLE about SFNs
with 8-VSB.
In short, that is what the TV Technology article should have emphasized.
> DVB-T which was chosen by Hong Kong earlier was replaced
> by DTMB after they tested the two BTW.
And DTMB uses training sequences and powerful equalizers, and actually
dials back the inherent echo tolerance of COFDM. It not only replaces
the GI with a training sequence, but it also goes "backwards" from 8K to
4K mode, or even a single carrier mode, further impacting negatively on
COFDM's inherent echo tolerance.
So it simply brings together all the bags of tricks that have evolved
since the early 1990s, to provide a system that is at the same time
robust and spectrally efficient.
With transmitter synchronization for SFNs, and better equalizers as well
as schemes such as A-VSB for the low bitrate streams, 8T-VSB can have
its life extended. Which is the only way to go when deploying new
systems like this. You don't stop and start from scratch every time
someone has a good idea.
Bert
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