[Discuss-gnuradio] Re: Soft-DVB DVB-T transmitter

[Martin DvH]

Hi Vincenzo,

On Sun, 2008-11-02 at 01:28 +0100, Vincenzo Pellegrini wrote:
> Hi Martin, 
> sorry for the delayed replies but now I've passed my first cluster of
> PhD tests (went well)

Congratulations.
>  and I've got to carry out some work + preparing the second group of
> tests.
Success.
> Well, really glad to know that you managed to receive my signals.
> Yup dvb-t sticks can actually receive 7 MHz channels everywhere,
I found that some of them need special "australian 7MHz at UHF" firmware
to get this to work.
Luckily my DVB-T usb stick doesn't need this trick.
> Well, actually any DVB-T chipset can but typically manufacturers
> impose strange limitations on set-top-boxes such as "7 MHz chanels
> accepted only in VHF" I don't really know why.
> 
> The signal I provided you with is suitable for both 7 and 8 MHz
> channels without any modification needed. The only thing you have to
> do is to set your sampling frequency a bit higher. this should be
> possible with USRP2. 
The receiver might still have a problem that it is getting in the MPEG
streams at a higher rate then realtime.

> In fact: 8 complex Msps implement a 7 MHz channel while 9.142857143
> complex Msps implement an 8 MHz channel.
> Just try to go as close as possible to such sampling frequency by
> using USRP2 and let me know what happens... it could turn out that we
> need a resampler block.
So if I use a fractional rate resampler with interpolation factor
10/8=1.25 I get a 7 Mhz channel with 10 Msps samplerate.
If I use a fractional rate resampler with interpolation factor
10/9.142857143=1.09375 I get a 8 Mhz channel with 10 Msps samplerate 

If I use a fractional rate resampler with DECIMATION factor
9.142857143/8=8/7=1.142857143 I get a 8 Mhz channel with 8 Msps
samplerate with the out-of-band skirts folded back at the sides.

Would be interesting to see if this last one works with a USRP1.

I'll let you know how the experiments go.

> more details will follow as soon as I find some time...
Thanks and success with your second group of tests.

Martin
> best regards and greetings 
> to all fellow GNURadioers
> 
> vincenzo 
> 
> PS
> Rafael, just have a look back a this thread and you'll find all the
> info you need to do your test broadcast. Thanks for your interest
> 
> 
> 
> 
> 
> 
> 2008/10/31 Martin Dudok van Heel <address@hidden>
>         Hi Vincenzo.
>         How are things going with your exams.
>         
>         I hope well.
>         
>         Thanks for your help so far.
>         
>         I finally got your DVB-T dump streams working.
>         I first tried using an undersampled basicTX but never got it
>         to work.
>         (use a niquist mirror in the VHF range on channel 11 or 12
>         (219.5 Mhz or 226.5 Mhz))
>         
>         I now use a RFX900 and that works with a pinnacle PCTV-Solo
>         72e usb DVB-T receiver card plugged into my PC.
>         I use 858.0 Mhz (channel 69)
>         I used a 10 dB attenuator on the antenna output to limit
>         output power.
>         I also modified the RFX900 to enable transmitting outside of
>         the ISM band. (disable saw-filter. add 220 pF capacitor)
>         
>         Apparantly the pinnacle 72e can receive 7 Mhz channels on the
>         UHF channels.
>         My standalone settopbox DVB-T receiver can't handle it.
>         
>         I noticed you don't use the full possible range in your 16 bit
>         streams.
>         (only goes from -80 to +80 while you could use -8192 to 8192)
>         Is this on purpose?
>         I can multiply samples by 64 and get a cleaner signal. (But
>         also more output power)
>         
>         
>         I do have a request, I hope it is not too much work.
>         Could you make a stream with 10 Msamples/sec samplerate and 8
>         Mhz wide channel.
>         This way I can use standard standalone DVB-T receivers and
>         don't have the 7Mhz bandwith on UHF problem.
>         
>         For the 10 Msps stream I would have to use my USRP2 to output
>         it.
>         It has a 100 Mhz DAC (in stead of 64 Msps in the USRP1)
>         It has a gbit ethernet connection for the samples, so I can go
>         up to 25 Msps.
>         It can only do fixed interpolation rates so I have to choose
>         from the table below.
>         (8 Msamples/sec is not supported on the USRP2)
>         
>         
>         USRP2
>         dac_rate        interp  ethernet_sample_rate
>         100             4       25
>         100             5       20
>         100             6       16.67
>         100             7       14.29
>         100             8       12.5
>         100             9       11.11
>         100             10      10      <----I think 10 Msamples/sec
>         should be optimal
>         100             11      9.09
>         100             12      8.33
>         100             13      7.69
>         100             14      7.14
>         
>         
>         I think 10 Msamples/sec would be a good candidate.
>         
>         Have you also tried using 8 Msamples/sec on the USRP1?
>         I know there would be no room left for IF channel filtering,
>         but it could in theory still work.
>         If this works I would also very much appreciate a 8Mhz
>         bandwidth stream with 8 MSPS samplerate so I can demonstrate
>         with a standard USRP1.
>         
>         Thanks for your help so far.
>         I appreciate it very much.
>         
>         And good luck with your exams.
>         
>         Have a nice weekend.
>         
>         Greetings,
>         Martin
>         
> 
> 
> 
> -- 
> Vincenzo Pellegrini