# Switching flow between two paths

**URL:** <https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733>\
**Category:** GNU Radio\
**Created:** [March 25, 2015, 7:56am UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733 "2015-03-25T07:56:04Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [March 25, 2015, 7:56am UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/1 "2015-03-25T07:56:04Z")

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* * *

Discuss-gnuradio mailing list  
removed\_email\_address@domain.invalid  
[https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 25, 2015, 10:16am UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/2 "2015-03-25T10:16:53Z")

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Hi Jeon,  
in principle, your flow graph does make a lot of sense.  
However, there’s one problem: GNU Radio is a series of tubes, so to  
speak;  
as you switch the “upstream” selector, there’s still items in the in-  
and output buffers of the previously selected encoder.  
Thus, you’d need to switch the downstream selector just the right amount  
of items later; the problem here is that you get absolutely no relation  
between “wall clock time” and the amount of items that have been  
processed.

Exactly for problems like that GNU Radio has stream tags, which really  
are just pieces of information logically “stuck” to a single item, as it  
passes through the block chain[1][2]. Now, if the selector was able to  
deal with such stream tags, you could just tell each selector to switch  
its ports right after a specific item has passed. Sadly, the selector  
can’t do that. You could implement that in C++ or python; however, that  
might be a bit challenging, because this requires usage of the general  
block type, which makes it harder to implement (you’ll need to build  
some kind of state machine to be able to tell GNU Radio how many input  
items you need on which input to produce the wanted amount of output).  
For a quick implementation, I’d go a different route:

source -±-\> custom\_block0 —\> encoder0 —\> add →   
±-\> custom\_block1 —\> encoder1 ------^

you’d need to implement custom\_block in python or C++, that would either  
pass through items, just like the block does that comes out of  
gr\_modtool add -l python -t sync  
or set the output items to 0. You’d toggle that behavior exactly at the  
sample that you get a tag.

Of course, this has the downside of a unnecessary copy, encoding and  
adding per sample, compared to a proper stream en/disabler, but it’d be  
easier to implement, I’d say.

Greetings,  
Marcus  
[1] You’ll find a bit of instructional information in  
[https://gnuradio.org/redmine/projects/gnuradio/wiki/Guided\_Tutorial\_Programming\_Topics#52-Stream-Tags](https://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_Programming_Topics#52-Stream-Tags)  
[2] Reference [http://gnuradio.org/doc/doxygen/page\_stream\_tags.html](http://gnuradio.org/doc/doxygen/page_stream_tags.html)

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 25, 2015, 4:18pm UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/3 "2015-03-25T16:18:24Z")

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Maybe the matrix multiplier can be of use here. Try the corresponding  
example in gr-blocks/examples.

M

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 26, 2015, 3:19pm UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/4 "2015-03-26T15:19:36Z")

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On Mar 25, 2015, at 2:15, Marcus M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> source -±-\> custom\_block0 —\> encoder0 —\> add →   
> ±-\> custom\_block1 —\> encoder1 ------^
> 
> you’d need to implement custom\_block in python or C++, that would either  
> pass through items, just like the block does that comes out of  
> gr\_modtool add -l python -t sync  
> or set the output items to 0. You’d toggle that behavior exactly at the  
> sample that you get a tag.

Note that if you don’t actually need the switch to happen at a specific  
point in the stream, but you still want the switch to be atomic  
(guaranteed not to drop or repeat any items), the existing  
multiply\_matrix block can do the switching in this flowgraph: it would  
be configured with one input and two outputs, and setting the matrix to  
either [[0, 1]] or [[1, 0]].

(Of course, the disadvantage of either of these strategies is that  
you’re paying the CPU cost of both encoders all of the time.)

–  
Kevin R. [http://switchb.org/kpreid/](http://switchb.org/kpreid/)

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 27, 2015, 6:25am UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/5 "2015-03-27T06:25:11Z")

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After thinking about it for a couple of days, I’ve decided not to use  
two  
separated RLL blocks.  
Instead, I made a block as below:

In work() function:

get\_tags\_in\_range(tags, 0, nitems\_read(0),  
nitems\_read(0) + ninput\_items[0],  
pmt::mp(“rll\_coding”));  
RLL\_CODE rll\_code = (RLL\_CODE)pmt::to\_long(tags[0].value);

// … omitted trivial things

switch (rll\_code) {  
case RLLMANCHESTER:  
for(int i = 0; i \< ninput\_items[0]; i++) {  
out[2 \* i] = !!in[i];  
out[2 \* i + 1] = !in[i];  
}  
break;  
case RLL4B6B:  
for(int i = 0; i \< ninput\_items[0] / 4; i += 4) {  
int rll4B = 0;  
for (int j = 0; j \< 4; j++) {  
// make four bits into one bytes  
rll4B |= (in[i + j] & 1) \<\< j;  
}  
for (int j = 0; j \< 6; j++) {  
// put six bits corresponding to rll4B  
out[6 \* i + j] = !!(rll4B6B[rll4B] & (1 \<\< j));  
}  
}  
break;  
}

// mapping definition

const char rll4B6B[16] = {  
0b011100, 0b101100, 0b110010, 0b011010,  
0b101010, 0b110001, 0b011001, 0b101001,  
0b100110, 0b010110, 0b001110, 0b100011,  
0b010011, 0b100101, 0b010101, 0b001101  
};

If the code are not formatted well and hard to read, please refer:  
[https://gist.github.com/gsongsong/f3e34991a55f29dac5d2](https://gist.github.com/gsongsong/f3e34991a55f29dac5d2)  
There might be some syntax violation and variable definitions missing, I  
apologize for that.  
And please note that each in[i] is either 0b00000000 or 0b00000001.

By using a tag, RLL coding scheme are not changing during the stream.  
Currently, I haven’t built it and tested yet.

What do you think of it?

I will make some notes after build and test the entire system.

Regrads,  
Jeon.

2015-03-26 23:17 GMT+09:00 Kevin R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 27, 2015, 4:18pm UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/6 "2015-03-27T16:18:25Z")

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Dear, Marcus

Here’s my codes

include/rll.h: [https://gist.github.com/gsongsong/07e28c797bb65572982b](https://gist.github.com/gsongsong/07e28c797bb65572982b)  
lib/rll\_impl.h: [https://gist.github.com/gsongsong/615649f628262c44c8cf](https://gist.github.com/gsongsong/615649f628262c44c8cf)  
lib/rll\_impl.cc: [https://gist.github.com/gsongsong/14e7bdab27216bbe7a17](https://gist.github.com/gsongsong/14e7bdab27216bbe7a17)

Please excuses some typos and syntax violation if any.

Regards,  
Jeon.

2015-03-27 16:30 GMT+09:00 Marcus Müller [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 28, 2015, 1:51pm UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/7 "2015-03-28T13:51:21Z")

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Hi Jeon,

this looks like good code; it’s an excellent design choice to use a  
tagged stream block here, where you’d want GNU Radio to make sure you  
get the whole “packet” that you need to encode into a single work call.

I think it should work; of course, I’m not your computer 😉 . With the  
existing tagged stream block framework, it should be quite  
straightforward to define a python test case for this, so I’d encourage  
you to actually test it!

Best regards,  
Marcus

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 27, 2015, 8:31am UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/8 "2015-03-27T08:31:56Z")

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Hi Jeon,

could you paste the whole C++ file to the gist?  
Basically, what I suspect is that you’re not checking whether  
get\_tags\_in\_range() produced a tag at all, or/and might have an error in  
your state machine that stores the “current” coder.

Greetings,  
Marcus

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**Author:** ![Jeon](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/e8c25b/32.png) [@Jeon](https://www.ruby-forum.com/u/Jeon)\
**Post date:** [March 28, 2015, 3:57pm UTC](https://www.ruby-forum.com/t/switching-flow-between-two-paths/234733/9 "2015-03-28T15:57:27Z")

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Dear, Marcus

Thank you for your advice.

I will test it as soon as possible after fixing another problem in the  
entire module (not related with RLL.)  
And will report the result on the discuss-gnuradio.

Regards,  
Jeon.

2015-03-28 21:50 GMT+09:00 Marcus Müller [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):