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

[Vincenzo Pellegrini]

This is Great... :)

Yup, the playback cannot be smooth because of the wrong throughput, definitely.
Did you use the USRP1 with interpolation factor = 16 ?

I can prepare a modulated signal with the correct throughput for you.. this is not a problem... :)

what hard disc are you playing your signal back from?

regards

vincenzo

2008/11/3 Martin DvH <address@hidden>

Hi,

> > 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.

I resampled and scaled your ofdm_40.dump file so it now will use 8 Mhz bandwidth with a 8 Msps samplerate.
The reception never can be perfect this way but it seems good enough for
tests.

My USB DVB-T receiver receives the transport stream without problems.
Mplayer playes the stream without problem for two loops and then crashes
with a broken frame.
My standalone settopbox DVB-T receiver now also receives the stream.
(8 MHz channel on UHF)
It has big problems displaying it. Sound is only a chop of sound now and
then and video stops, then runs for a second, then stops again.

I think this is because the timestamps and framerate (playout speed)
don't match the data throughput of the MPEG stream anymore.
(It is getting the stream too fast)

I put my resampled RF file at:
http://www.olifantasia.com/projects/gnuradio/mdvh/OTA/DVB-T/ofdm_40_bw8Mhz_samplerate_8Msps_cshort.raw

format is complex signed short integers (I 16 bit, Q 16 bit) at 8
Msamples/sec.


Greetings,
Martin

> > 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

-- 
Vincenzo Pellegrini