On Fri, Mar 16, 2012 at 8:41 AM, Zhonghua <zhong...@sics.se> wrote: > Hi all, > > After a period of using gnu radio, I have a problem of how to understand > the stream. I got an information from a literature says: 'From the high > level point-of-view, infinite streams of data flow through the ports. At > the C++ level,streams are dealt with in convenient sized pieces, > represented as contiguous arrays of the underlying type.' In almost each > signal processing block we can see the definition of 'general_work' has > this format: > general_work(int noutput_items, > gr_vector_int &ninput_items, > gr_vector_const_void_star &input_items, > gr_vector_void_star &output_items) > Some literatures say the 'noutput_items' variable represents the items > number of one piece of the stream. So the first question is what on earth > the size of this variable? where dose it be specified? For example if A > block connects to B block, how can ensure the 'noutput_items' of A block > equal to the 'ninput_items' (presume B block only has one input stream). > In an concrete instance, in gr_squelch_base_cc.cc, we can see the last > program as: > if (d_state != ST_MUTED) > out[j++] = in[i]*gr_complex(d_envelope, 0.0); > else > if (!d_gate) > out[j++] = 0.0; > If we set d_gate as False(Actually in case of high sample rate it has to > be set as False), when the input power is lower than the threshold(means > the d_state should be as ST_MUTED), the out[j] shall get nothing. That > means at this point there is no output data. So how the sequential block > recognise this point? For instance the sequential block is > ieee802_15_4_demod, how could it know where is this lost point? If only one > point lost from the squelch block, is that means the whole piece of stream > and even the whole package could not be demodulated correctly? > We can set the instance in more precise condition. If there are two pieces > of stream: stream A and stream B. Assume each noutput_items is 10, that is > to say we have A[0], A[1] ....to A[9] and B[0] to B[9]. If A[0] has lost, > then the stream entering into the sequential demodulation block is > A[0](lost),A[1]....A[9]? Or is A[1],A[2]....B[0]? > If A[0] is valid, A[1] has lost, dose that mean the first piece of stream > would not be demodulated correctly? If one package need both these two > pieces of stream, dose it mean this package would not be demodulated > correctly? > > Any answer is greatly appreciated. > > Sincerely, > > Zhonghua >
Zhonghua, This is a really complicated question to answer without giving you a full essay. All of this happens in the scheduler, so if you want to know more, study the code for the thread-per-block scheduler in gnuradio-core/src/lib/runtime. It's the gr_block_executor that looks at the read/write pointers to each blocks buffer and determines how many samples are available to be read and how many samples a block can write. Tom
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