Hi Martin,
I was out of town for the weekend, so I am just getting at my email.
Thanks for your detailed reply as usual. I really appreciate your help
and I know it is a lot of effort and time on your part so I really
appreciate it.
:
The param cnt works well and is used to synch up the indices for the
in and out arrays. For example, suppose the input data was
{1,2,3,0,1,3,2,5,6} and the preamble pattern is {3,0,1,3,2}.
Then after initialization, cnt = 7 and i = 7 and of course the number of
sync bytes = buf_size = 5. Before leaving the initialization loop I
memcpy the 5 preamble bytes to the out array, which means the next index in
"out" to write on the next input data sample is out[5], however, this
sample comes from in[i] = in[7]. Immediately after initialization, hence
the relationship:
out[i-cnt+buf_size] = in[i] is correct because out[5] = in[7] and as i
increases, we get out[6] = in[8], etc.
However, I see one big problem that will cause my logic to fail and it
is based on your explanation that each time the general_work method is
called it brings in a new set of data and i starts over in the "for loop".
I will need to change my logic, but please allow me to ask another
question. Let's say, I have a source which generates 100 bytes and it
Repeats. In Gnuradio, does this mean that each time the general_work
method is called it receives a block of100 bytes of data? Thus, in the C
logic test "for (int i = 0; i < ninput_items[0]; i++)", can I assume the
Gnuradio parameter ninput_items[0] is equal to 100 and each function call?
Thanks for the help.
Best Regards,
George
On Sun, Sep 13, 2020 at 2:51 AM Martin Luelf <mail@mluelf.de
<mailto:mail@mluelf.de>> wrote:
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>
> <mailto: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
>