[opendtv] Re: Food for thought
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 23 Feb 2007 16:00:05 -0500
Bob Miller wrote:
> But they actually "pack" 9 program channels in on 64
> QAM channels.. They may only be doing 8 today but they
> consider then that they have a spare unused for sale
> slot available. With 16 QAM they "pack" 6 programs
> channels in and again if they are not using one or two
> those slots are for sale, up for grabs.
So that would adjust the numbers downward a little bit for how much Mb/s
are used in each SD channel, and also downward a little bit for the
lowest possible Mb/s they deem adequate for HDTV.
However, I'm not convinced until you show me your source. If you go to
this site:
http://www.dgtvi.it/stat/Europa/Gran_Bretagna/Page1.html
and scroll down to the bottom of the page, you will see a picture of the
Freeview offerings. And a paragraph (in Italian) that describes this,
including the existence of some regional multiplexes.
Freeview channels seem to be divided as follows:
Multiplex 1: 5 streams
Multiplex 2: 5 streams
Multiplex 3: 5 streams
Multiplex 4: 4 streams
Multiplex 5: 4 streams
Multiplex 6: 4 streams
Multiplex 7: 5 streams
Total: 32 program streams.
Now go to the mabb site, to see if this bears any resemblance to what
the Germans determine adequate SDTV quality to be. German DTT is also
16-QAM, so the comparison should be valid.
http://www.garv.de/
shows a similar graphic of the German DTT offerings. Note that these are
a consistent 4 program streams per multiplex, which again suggests that
good 576i quality is achieved at bit rates of around 3 to 4 Mb/s per SD
stream. And the half of the Channel 59 multiplex is dedicated to radio
streams.
Detail: The German scheme used in Berlin needed to increase the GI to
1/8, in order to support their SFN. Therefore, the bit rate available
dropped to a MAXIMUM of 14.75 Mb/s, with a few multiplexes less than
that, which gives each SD stream an average 3.7 Mb/s maximum capacity.
The UK system I'm not positive about, so I assumed the GI to be 1/16,
giving them some extra spectral efficiency.
However you do thes numbers, 3 to 4 Mb/s are the consistent bit rate
deemed adequate for SDTV in 576i. So it always comes out to 9 to 16 Mb/s
required for H.264 HDTV quality, by BBC's own reckoning (not mine).
Bert
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