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 bouwsma _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
