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

[Vijay Ramasami]

On 8/17/07, Jan Schiefer <address@hidden> wrote:
> Vijay Ramasami wrote:
> > Thanks for the information David. I will look up ITU-J.83B ...
> >
> > Do you happen to have any captured QAM cable data (or any website that
> > lists the data) ? I wanted to see if I can put together a software
> > demod for digital cable ...
> >
> > On 8/13/07, David I. Emery <address@hidden> wrote:
> >
> >> 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."
> >>
> >>
> >>
> If you can't find any, let me know and I might be able to help. I have
> access to 2 QAM-64 channels and 15 or so QAM256 channels, some HD, all
> unencrypted. I probably have some encrypted ones too, but I don't know
> how many, as they are encrypted...  If you can write a little capture
> program, I can try to hook up the USRP to the cable jack and see whether
> I can capture something. Problem is, I don't have a TVRX (yet), but this
> that may be a good excuse to order one. :-). Provided the phase response
> of the TVRX is sufficiently flat for this kind of signal. Anybody have a
> guess?
>
> Cheers,
> Jan
>

Hi Jan,

I would highly appreciate it if you can capture a few QAM-64
snapshots. I was able to successfully demodulate signals captured from
a QAM modulator, but I don't have access to a real-world cable source.
I guess the python script "usrp_rx_cfile.py" (in the examples
directory) can be used to capture samples. We need at least 16 MHz
sampling frequency for symbol timing recovery to work properly.

Thanks,
Vijay.