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Re: [Discuss-gnuradio] Questions on US digital cable ...

From: David I. Emery
Subject: Re: [Discuss-gnuradio] Questions on US digital cable ...
Date: Mon, 13 Aug 2007 20:06:31 -0400
User-agent: Mutt/1.4.1i

On Mon, Aug 13, 2007 at 10:52:54AM -0400, Vijay Ramasami wrote:
> Hi,
> I have a couple of questions:
> 1. Does the US digital cable system follow the DVB-C standard (or one
> of its annexes) ? Is there any information (website) on the typical
> symbol rates, bandwidths (I am guessing approx 6 MHz), used ?

        There is a Cablelabs spec that defines what is used.   The ETSI
standards (ITU-T J.83B) also define the particular parameters as well.

        I can look up detailed references if needed...

        The are actually only two standard modulations in wide use 64QAM
with a particular set of parameters that yields a 30 Mb/s signal with a
5.056941 Msym/symbol rate and a 256QAM signal which yields a 38.9 Mb/s
signal with a symbol rate of 5.360537 Msym/s

        Both signals are 6 MHz wide as US CATV is universally 6 MHz
channel based.

> 2. Has anyone successfully captured (preferably unencrypted) digital
> QAM transmissions using the USRP ? If so, can you please send me a
> link to the data ? Given that the symbol rates are in the range of 5-6
> Ms/s, it must be possible to use 16 MHz sampling frequency to
> demodulate the signals.

        I have used a number of purpose built demods, but not yet tried
a USRP solution.   Some of the cable transport streams have open
channels, but you will find most are encrypted except the local OTA HD
signals and a few freebie promos.

        It is also possible to MODULATE QAM cable standard signals,
something that gets more useful every month as more QAM/ATSC tuners are
shipped for cable ready setups with CableCards rather than set top
boxes.   This of course allows direct input of MPEG transport streams
into the digital domain of LCD/plasma panels with no analog step...

  Dave Emery N1PRE/AE, address@hidden  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."

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