Try to understand the stream in Gnu Radio

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 A0,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

On Fri, Mar 16, 2012 at 8:41 AM, Zhonghua [email protected] wrote:

                gr_vector_int &ninput_items,
  out[j++] = in[i]*gr_complex(d_envelope, 0.0);

and even the whole package could 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

On 03/16/2012 06:37 PM, Tom R. wrote:

underlying type.' In almost each signal processing block we can
(presume B block only has one input stream).
shall get nothing. That means at this point there is no output
is A[1],A[2]....B[0]?

Tom

Hi Tom,

Thank you for your information. I think you have answered the first
question that where and how dose the size be determined. To the detailed
question, do you think if one item lost, all this package will lose? And
in my last instance, do you think which assemblage will be sent to the
sequential block? Thank you again!

Zhonghua

On Fri, Mar 16, 2012 at 3:28 PM, Zhonghua [email protected] wrote:

the C++ level,streams are dealt with in convenient sized pieces,
block connects to B block, how can ensure the ‘noutput_items’ of A block
the d_state should be as ST_MUTED), the out[j] shall get nothing. That
If A[0] is valid, A[1] has lost, dose that mean the first piece of stream

Hi Tom,

Thank you for your information. I think you have answered the first
question that where and how dose the size be determined. To the detailed
question, do you think if one item lost, all this package will lose? And in
my last instance, do you think which assemblage will be sent to the
sequential block? Thank you again!

Zhonghua

I’m honestly not sure what you are getting at. Where/how would you lose
an
item?

Tom

Hi Zhonghua,

If you find any answer to you question?
If you have found please let me know. As I am working in CDMA I need
continuous block of data to despread the required information.

Thank You.

Regards
M.G.Praveen

On 03/16/2012 09:35 PM, Tom R. wrote:

    understand the stream. I got an information from a literature
    Some literatures say the 'noutput_items' variable represents
          if (!d_gate)
    could not be demodulated correctly?

you a full essay. All of this happens in the scheduler, so if you
Thank you for your information. I think you have answered the

Tom

Hi Tom,

The background is that we lose some packets when use USRP E100 to
implement IEEE802.15.4 protocol(4M sample rate). The computation
capacities of the E100 platform is not very good. We think the high
sample rate gives the demodulation block too high input load so packets
will lose. To decrease the input load of demodulation block, we manage
to raise the threshold of the squelch block. But if the threshold is too
high, maybe some valid input items of squelch block will be incorrectly
filtered. So we need to make sure if only one item lost will make the
whole packet lose.
From the original code of gr_squelch_base_cc.cc I think the first
output item is always at the first position of the output piece no
matter what the condition of the input piece. All output items in one
piece should be consecutive. For example if input piece has
IN[0],IN[1]…IN[5] in which IN[0] and IN[2] are lower than the
threshold, the output piece should be
OUT0,OUT1,OUT2,OUT3. There are no
OUT[4] and OUT[5]. I don’t know how does the subsequent demodulation
sample this piece since it has only 4 items although one piece should
has 6 items. One thing I think maybe sure is that if IN[2] is necessary
but lost just because it is lower than the threshold, the input of
demodulation block has changed so the result would not be correct.
I don’t know is my understanding right or not.

Zhonghua

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