Hi George,
noutput_items is a number given to you by GNURadio. I would not
overwrite this variable, otherwise you no longer know how many items you
can write into the output buffer. Create a new variable nitems_written
or similar to track how many items you have written.
Also have a look at what each variable is tracking exactly. cnt seems to
be the number of input bytes you have checked for the preamble. But you
also use it to compute the position in the out array, which does not
make sense to me. cnt would increase with more spurious bytes in front
of the preamble, but the output position should not depend on the number
of spurious bytes. From the code you showed me cnt also seems to persist
between multiple calls to general_work, but the out array is always
clean when general_work is called again (meaning that on every
general_work call you should always start writing to element 0).
You loop over your entire input array and if the preamble was found you
copy the entire preamble and then you copy every following byte one by
one. That means you copy the entire preamble again (without the first
byte) and if your input buffer is longer than your message size you also
copy more bytes to the output than your message is long which could
result in missing the next preamble. Also you are potentially writing
more items into the out array than GNURadio has space for, which is why
I recommend you not to overwrite noutput_items and while you are still
learning check that your computed indices for out and in are within the
allowed bounds before every single write and read (i.e. by using
assertions).
What messages are you using in your unittest and what kind of spurious
bytes? Is is all zeros, or all ones? I would recommend you use different
bytes, e.g. an increasing counter for your message and a decreasing
counter starting from 255 for your spurious bytes so you can quickly
spot if your messages are cropped, or off in some other way. Because the
way you describe your results it seems like one message is copied twice.
I could not find any obvious bug in your code that would output a
message exactly twice, so I suppose it copies some data that look like a
message on the first glance.
Yours
Martin
On 11.09.20 03:06, George Edwards wrote:
> Hi Martin,
>
> Thanks for your detailed answer. I really appreciate the great effort
> you put into explaining how things work. I am still on the learning curve.
>
> I used your suggestions to the best of my understanding and it worked
> beautifully for the one sync pattern test vector in the QA test.
>
> Then, I took your suggestion for repeated sync patterns using an init
> flag which I reset to 0 to restart the process. For the QA test, I
> repeated the original input data twice (now I have 2 sync patterns), so
> the expected QA output should be the original output repeated twice. I
> modified the C code by adding an if statement at the end to check if
> noutput_items == Buf_size+message_size (buf_size is the length of the
> pattern, which I call preamble) and if it is, I reset the init flag to
> zero as well as other params used in the initialization section of the
> code. The QA test failed by producing an output with 3 repeated copies
> of the original output rather than the expected 2 copies. I do not
> expect you to send too much time looking at my code below, however, I
> would appreciate it very much, if you would glance at it to see if you
> can spot what I am doing wrong. The test to un-initialize (setting init
> to 0) was done towards the end of the code block after the consume method.
>
> int kk = 0;____
>
> for (int i = 0; i < ninput_items[0]; i++)____
>
> {____
>
> if(!init){__
>
> cnt += 1; // cnt number of passing bytes____
>
> kk = initialize(in[i]);____
>
> if (kk == 0){____
>
> noutput_items = cnt;____
>
> }else{____
>
> memcpy((void*)out, (const void*)preamble, buf_size);____
>
> noutput_items = cnt;____
>
> }
>
> } else {
>
> out[i-cnt+buf_size] = in[i];____
>
> noutput_items = buf_size + message_size;____
>
> }____
>
> } ____
>
> consume_each (noutput_items);____
>
> if (noutput_items == buf_size + message_size){____
>
> init = 0; // re-initialize all____
>
> cnt = 0;____
>
> kk = 0;____
>
> }____
>
> return noutput_items;____
>
> }
>
>
> Thanks very much for the help.
>
> Regards,
> George
>
> On Thu, Sep 10, 2020 at 12:06 AM Martin Luelf <mail@mluelf.de
> <mailto:mail@mluelf.de>> wrote:
>
> Dear George,
>
> this also caused me a lot of headache when I started with GNURadio, so
> here is what I learned about it.
>
> Let's start with the forecast method. This tells GNURadio how many
> input
> samples you need on each input stream to generate the requested number
> of output items. Usually GNURadio will run your forecast function a
> couple of times with different output numbers to find a good data chunk
> size to give to the general work function. Keep in mind that the number
> of required input items you give here is a minimum number and GNURadio
> might decide to give you more input data than you requested. It is also
> important to know that the number of samples you request here is
> just an
> estimate. You are not forced to actually process that much data.
>
> Now to the general_work function. noutput_items tells you how many
> samples GNURadio would like you to produce (this is a maximum number
> you
> may produce less). It also tells you how much memory is allocated in
> every array in the output_items vector. If you have only one output and
> you used the default <out type> *out = (<out type> *) output_items[0];
> definition this tells you how many items you can place (at most) into
> the out array.
>
> The ninput_items[] array tells you how many input items are
> available in
> the input arrays. Again if you just have one input and you use const
> <in
> type> *in = (const <in type> *) input_items[0]; ninput_items[0] is the
> number of inputs available in the in array. You may not read more items
> than that from the array.
>
> Within the given input symbols you can start looking for your sync
> pattern. If you generate output you have to write (in your case
> copy) it
> to the out array. At the end of general_work you call consume(0, K)
> with
> the number of input items K that you have consumed on input 0. That is
> how many of the input items you have used and do not need to see again.
> If you consume 0 symbols the next call to general_work will show you
> the
> exact same input samples again. If you consume ninput_items[0] you will
> see completely new input samples on the first input the next time
> general_work is called. And then you return the number of samples you
> generated (i.e. how much samples you put into the out array). This
> number must be smaller or equal noutput_items, because otherwise you
> would have written out of the allocated memory which might result in a
> segfault/core dump. Note that you don't have to call consume at the
> very
> end of work and there is also another way of telling GNURadio how many
> samples you have produced, but let's leave that for another day.
>
> So a very easy (but not the most efficient) setup for your problem
> could be:
> Add a boolean flag to your _impl class that both forecast and
> general_work can read/write to. This flag will indicate whether or not
> you have found the sync pattern or not. You initialize this flag
> with false.
> Assume you have a sync pattern of length L and a message with M data
> symbols afterwards.
>
> In forecast if the flag is set (meaning you have found the sync
> pattern)
> you need L+M symbols of input. If the flag is not set you need L input
> samples, regardless of how many output samples GNURadio wants you to
> generate.
>
> In general work if the flag is false you search the input for the sync
> pattern. If you found it at position i (counting from 0) you set the
> flag to true, consume i samples (i.e. everything before the sync
> marker). If the sync marker is not found you keep the flag to false and
> consume the inputs that you have searched so far. In both cases you
> return 0 since you have not generated any output yet.
>
> If the flag is true you copy the first L+M samples from the input to
> the
> output, you set the flag to false (because after the data you have to
> start searching for the sync marker again) you consume L+M samples and
> return L+M samples.
>
> Note: This is a very easy to understand scheme, but unfortunately not
> very efficient. You only process a single block of either unwanted
> spurious symbols, or one sync marker and data at a time. So once you
> have a good understanding of how this works you should tweak that block
> to be able to process multiple blocks of spurious symbols and sync
> patterns/data within once call to general_work. It uses the same
> kind of
> logic, but requires more housekeeping of counters and indices.
>
> If your input is symbols rather than bits/bytes you should also look at
> the paper from J. Massey "Optimum Frame Synchronization" from 1972 on
> how to perform optimum sync marker detection, which performs better
> than
> the intuitive correlation search.
>
> Hope that gets you started.
>
> Yours
> Martin
>
>
> On 10.09.20 04:34, George Edwards wrote:
> > Hello,
> >
> > I am writing an OOT block in C++ that receives a sequence of
> numbers and
> > searches through for a sync pattern and passes to its output the
> sync
> > pattern and the bytes of data which follows it. The QA test
> shows the
> > block recognizes the pattern and will pass the pattern along with
> the
> > data that follow it, but there is a problem. The block does not
> know a
> > priori the number of spurious bytes preceding the sync pattern of
> bytes,
> > so I cannot set up the true relationship between the ninput_items
> and
> > noutput_items. However, the block can count the number of bytes that
> > came in before the pattern. This is my problem:
> >
> > 1) In the OOT general_work method: If I set noutput_items =
> > ninput_items[0], then in addition to passing the correct data to the
> > output, it passes trailing zeros equal to the number of spurious
> bytes
> > that entered before the pattern.
> >
> > 2) If I set the return noutput_items = ninput_items - cnt (where
> cnt is
> > the number of spurious bytes before the pattern) depending on
> where I
> > put noutput_items in the code, it either throws a core dump or
> cuts off
> > the number of true data.
> >
> > Also, within the forecast method, I simply use a _for_ loop and set
> > ninput_items_required[i]=noutput_items;
> >
> > I will appreciate any help to point me in the right direction in
> dealing
> > with this non-regular output/ inputs relationship.
> >
> > Thank you!
> >
> > George
>
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