On Wed, Sep 17, 2008 at 1:20 PM, BOUWSMA Barry <freebeer.bouwsma@xxxxxxxxx> wrote: > On Wed, 17 Sep 2008, alireza ghahremanian wrote: > >> Is it possible to access the sampling subsystem of a dvb card >> as like skystar 2 or any other ? > > I'm not sure I understand your question. > > The way a SkyStar 2 card works can be described, in the case > of radio -- assuming the same radio which I tune with it is > that which you want to demodulate -- is sort of like this... > > Baseband analogue audio is sampled at 48kHz, 16bit, and then > compressed into a stream, usually MPEG 1/2 Layer II, but > sometimes Dolby AC3 or similar. From this, an ES (Elementary > Stream) is created. > > This Elementary Stream is then multiplexed into an MPEG > Transport Stream with other services. > > The resulting datastream is then modulated onto a RF carrier, > using something like QPSK or whatever is appropriate to the > delivery system, sent to a satellite (SkyStar) and effectively > bounced back. Error correction and whatnot is added at this > point. > > Your hardware tunes into the RF signal, and itself demodulates > the QPSK modulation and recovers the Transport Stream, or at > least part of it. > > The Linux-DVB API gives you access to this demodulated signal. > > Then, an application can work with this TS, extract the payload > from it (the Layer II or equivalent audio), and decode that into > a PCM stream. > > OF course, I could be wrong... > > >> I want to make a software radio and i want to do demodulation >> and decoding in software? > > The demodulation is performed by the hardware in your SkyStar > card. > > The demultiplexing and decoding are already handled either by > all-in-one programs, or you could chain together building-block > tools that already exist, in order to listen to radio... > > I use separate, existing utilities to control the frontend (tune > to a transponder); I'm delivered some part of a transport stream > which I can either play, with, for example, `mplayer', or I can > hand it to `ts_es_audio_demux' (if that's not part of the DVB > libdvb package, then it's a hack based on the existing routines > to extract ESen from a TS in that package); that's either an mp2 > or ac3 stream which I can pass directly to, for example, `mpg123' > or in a slightly different fashion, to `ac3dec' > > > The demodulation is performed in the hardware. You can, of > course, re-invent the wheel if you like and perform the > demultiplexing and decoding to PCM in software, if that is > what you are asking. > > > The sampling is done at the broadcaster/uplink end, and the > corresponding samples are the PCM output, if I understand > what you are asking. > > > Sorry if I do not understand your question correctly. > Barry, have you had a look at: http://www.gnu.org/software/gnuradio/ Markus _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
