Questions on US digital cable

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 ?

  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.

Regards,
Vijay.

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.

  1. 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, [email protected] 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."

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 Tue, Aug 14, 2007 at 10:55:40AM -0400, 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 …

Not conveniantly available…


Dave Emery N1PRE/AE, [email protected] 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?

The tuner in the TVRX was designed for cable modems, so I would assume
it is flat enough.

Matt

On 8/17/07, Jan S. [email protected] wrote:

signals and a few freebie promos.
"An empty zombie mind with a forlorn barely readable weatherbeaten
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.

Vijay Ramasami wrote:

Hi,
I can look up detailed references if needed…
2. Has anyone successfully captured (preferably unencrypted) digital

‘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

Vijay,

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.

My TVRX is on a UPS truck somewhere. As soon as it arrives, I’ll give it
a shot.

Cheers,
Jan

Vijay Ramasami wrote:

On 8/13/07, David I. Emery [email protected] wrote:

of its annexes) ? Is there any information (website) on the typical
5.056941 Msym/symbol rate and a 256QAM signal which yields a 38.9 Mb/s
Ms/s, it must be possible to use 16 MHz sampling frequency to
something that gets more useful every month as more QAM/ATSC tuners are

of the TVRX is sufficiently flat for this kind of signal. Anybody have a
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.

Hi Vijay,

sorry this is taking so long, but I think I have what you need now. With
a 16MHz sampling frequency you get only 8 bits of resolution, so I was a
little skeptical as to whether this would be sufficient. I played around
with a 10 second QAM64 snapshot (640MB, stored as 16-bit signed ints,
which gzips down to around 270MB). I put a chunk of this data into the
Agilent 89600 Vector Signal Analyzer software, and with equalization
turned on, the constellation actually looks pretty reasonable. So what I
captured must not be complete garbage :-).

Let me know if you still need this and I find a shady spot on the web to
put it. Or if you’d rather have something shorter, or stored as floats,
let me know.

Cheers,
Jan

Jan,

Sorry. I have been out of touch with GNURadio for a while. I am very
happy to hear that you have some data available to test. I would
really appreciate it if you can find time to upload your data
(whatever size you are comfortable with - I have a fast connection and
hence 270MB should download fairly quickly). I am ready to jump back
in.

Thanks again,
Vijay.

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