Re: [Discuss-gnuradio] RDS / TMC / DAB / DVB-S/T ?

[Robert McGwier]

David I. Emery wrote:
> On Sat, Oct 14, 2006 at 06:49:03AM -0700, Eric Blossom wrote:
>   
> > The USRP could receive the raw signals for DVB-T in the 6 and 7 MHz wide
> > channel format.  The 8 MHz wide version could be somewhat degraded
> > because of filter rolloff at the edges of the passband of the digital
> > downconverter in the FPGA.  Shifting to 8-bit I&Q would allow the full
> > 8 MHz wide signal to fit across the USB at the expense of dynamic range.
> >
> > I'm not sure of the bandwidth required for DVB-S and thus can't comment.
> >     
>
>         DVB-S as used for video is usually upwards of 3 megasymbols/sec
> as this kind of data rate is required to support sufficient transport
> stream bandwidth for adequate distribution grade video.  Many of the
> actual signals on satellite, however,  are multiplexes of multiple video
> and audio streams sent at symbol rates as high as 26 to 29
> megasymbols/sec.
>
>         Currently almost all signals are QPSK (NOT DQPSK, absolute phase
> counts) although the standards permit 8PSK and even 16QAM.  8PSK IS used
> for some HDTV feeds (notably the US broadcast networks ABC and NBC and
> the Canadian network CBC) and some HDTV backhauls.
>
>         Roughly speaking, the actual rf bandwidth required to demodulate
> QPSK is perhaps 1.1 to 1.2  times the symbol rate with a higher sampling
> rate than that probably necessary to get good results.
>         
>         This implies that the USRP with a 6 to 8 mhz bandwidth might be
> able to successfully demodulate the SCPC DVB-S QPSK video transmissions
> at symbol rates like 3.9876 megasymbols/sec (5.5 mbs transport stream at
> FEC 3/4) commonly used by satellite trucks for news feeds.   Most of
> these signals carrying single channel NTSC video (720 by 480 MPEG 2) run
> at either 3.9 megasymbols/sec or 4.2 megasymbols/sec.
>
>         

--- snip ----

As is typical of many signals of this type (geostationary satellite 
signals),  it needs very little dynamic range, mostly enough to 
accomodate weather and multipath fades but not what would expect to have 
to accommodate on a different type of FDM channel where you could see 
serious near-far problems.  I suspect the 8 bit DDS might work nicely 
indeed here.

Bob


--
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"You see, wire telegraph is a kind of a very, very long cat.
You pull his tail in New York and his head is meowing in Los
Angeles. Do you understand this? And radio operates exactly
the same way: you send signals here, they receive them there.
The only difference is that there is no cat." - Einstein