[opendtv] Re: From Broadcast Engineering - WRAL tests mobile DTV
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 8 Aug 2008 16:20:18 -0400
Dan Grimes wrote:
> What is the reason for only getting one 600 Kb/s and one
> 300Kb/s channel out of 4.5 Mb/s? Are the rest of the bits
> required for overhead or did they just not fill all the M/H
> channels possible in the 4.5 Mb/s subchannel bandwidth?
Without knowing the specific details of the MPH standard, the general
answer is that the 4.5 Mb/s figure represents a capacity that was
available when the data bits coded using 2/3 convolutional FEC. However
in the M/H stream, to attain the greater robustness, MPH (and E-VSB and
A-VSB) layer an additional convolutional code ON TOP OF the 2/3 trellis
code. Which makes these streams more robust and compatible with the rest
of the 8T-VSB multiplex.
Or put another way, had the entire 4.5 Mb/s been available for the M/H
stream, then where would the extra robustness have come from? Even if
you only added some extra training or synchronization overhead, which
typically requires a lot less overhead that convolutional FEC, you'd
still expect SOME loss of data carrying capacity in the more robust
subchannel.
You can make a direct comparison of the effectiveness of these schemes
vs COFDM HM. Again, you need to consider both the capacity and the C/N
threshold of the different channels.
So, in COFDM HM, the robust stream along with the wider 64-QAM results
in a loss of capacity of the 64-QAM subchannel *IF* you want that 64-QAM
subchannel to remain as robust as before you had switched on the HM. (As
happens with the remaining 8T-VSB capacity when you use any of the M/H
schemes.)
Using numbers from 8 MHz channel tables, a typical 64-QAM multiplex in
Europe uses 2/3 convolutional FEC, and has 24 Mb/s capacity (which means
a small GI of 1/32). The resulting robustness is 16.5 dB of C/N
threshold, in a gaussian channel.
Now turn on HM. To retain 16.5 dB of C/N, that 64-QAM capacity drops to
12.06 Mb/s, still with the same 1/32 GI.
Where did the 12 Mb/s of lost capacity go? It went into creating the
more robust HM channel, which in turn has its own robustness and bit
rate tradeoffs.
Bert
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