On 03/16/2012 09:35 PM, Tom Rondeau wrote:
On Fri, Mar 16, 2012 at 3:28 PM, Zhonghua <zhong...@sics.se
<mailto:zhong...@sics.se>> wrote:
On 03/16/2012 06:37 PM, Tom Rondeau wrote:
On Fri, Mar 16, 2012 at 8:41 AM, Zhonghua <zhong...@sics.se
<mailto: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
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 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
OUT[0](IN[1]),OUT[1](IN[3]),OUT[2](IN[4]),OUT[3](IN[5]). 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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