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

[Vincenzo Pellegrini]

Hi Martin, 
sorry for the delayed replies but now I've passed my first cluster of PhD tests (went well) and I've got to carry out some work + preparing the second group of tests.

Well, really glad to know that you managed to receive my signals.

Yup dvb-t sticks can actually receive 7 MHz channels everywhere,
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. 

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.

more details will follow as soon as I find some time...

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