# A two blocks synchronizing problem

**URL:** <https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513>\
**Category:** GNU Radio\
**Created:** [June 29, 2015, 5:20pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513 "2015-06-29T17:20:55Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [June 29, 2015, 5:20pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/1 "2015-06-29T17:20:55Z")

</div>

Hello

I created 2 new blocks, one is an encoder and the other one a decoder. I  
needed the encoder block to had 8 input items (from a vector source) and  
11  
output items, so I set: consume each(8) and return(11). I did not have  
any  
problem with it.

Now, for the decoder block, I needed to had 11 input items and 8 output  
items, so I set: consume each(11) and return (8).

I printed out the outputs with “cout \<\< out[i];” .

The problem: The encoder´s outputs are fine, they are what I expected  
them  
to be.  
But, the decoder´s outputs are giving me trouble: At first, it behaves  
well,  
the decoder´s input corresponds with encoder´s output and the decoder´s  
output corresponds with the vector source´output. But then, after a  
while,  
the decoder´s input do not begin in the first value as the encoder´s  
output  
and thus, the decoder´s output is not equal to the vector source´s  
output.

For example, if the encoder´s output is: 20.57, 11.45, 14.82, 17.81,  
16.95,  
16.63, 20.57, 15.14, 16.95, 11.45, 20.57. At first, the decoder´s input  
is  
exactly the same, but after a while, are: 17.81, 16.95, 16.63, 20.57,  
15.14,  
16.95, 11.45, 20.57, 20.57, 11.45, 14.82. It begins in the fourth value  
instead the first.

Is there a way to synchronize these two bloks? So the decoder´s input is  
always the same as the encoder´s output?

am I missing something? What should I do?

Thank you

David

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [June 29, 2015, 6:14pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/2 "2015-06-29T18:14:04Z")

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Hi David,

GNU Radio really makes sure you are only greeted with samples you did  
not already consume, and you don’t miss a single one, so in principle,  
this should work.

Do you check that you a) don’t produce more samples than you are allowed  
to (noutput\_items) in your encoder and b) don’t consume more input items  
then there are (ninput\_items) in your decoder?

There’s a handy trick to ensure that GNU Radio always calls you with a  
multiple of 8 (encoder) or 11 (decoder) items:  
set\_output\_multiple as well as set\_relative\_rate allow you to define how  
GNU Radio handles the amount of items.

Best regards,  
Marcus

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [July 1, 2015, 8:19pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/3 "2015-07-01T20:19:23Z")

</div>

Hello Marcus.

Thank you for your response.

\<There’s a handy trick to ensure that GNU Radio always calls you with a  
multiple of 8 (encoder) or 11 (decoder) items:  
set\_output\_multiple as well as set\_relative\_rate allow you to define how  
GNU Radio handles the amount of items.\>

I I had already done that when I made the question, I’m sorry I didn’t  
specified it.  
This is how I set it:  
\<void set\_output\_multiple (int tamano\_entrada + nsimbolo);  
set\_relative\_rate (double relative\_rate);\>

In another topic, you wrote me that was syntactically incorrect and you  
advice me to set it like this:  
set\_output\_multiple(tamano\_entrada+nsimbolo);

But this way I get the errors \<‘tamano\_entrada’ is not a type\> and  
\<expected  
‘,’ or ‘…’ before ‘+’ token\>

Again, when I set this parameters the first time I asked (void  
set\_output\_multiple (int tamano\_entrada + nsimbolo);  
void set\_relative\_rate (double relative\_rate)😉 I did not get any  
errors,  
but I had the problems I wrote in the original message.  
\<The problem: The encoder´s outputs are fine, they are what I expected  
them

> 16.95,  
> 16.63, 20.57, 15.14, 16.95, 11.45, 20.57. At first, the decoder´s input  
> is  
> exactly the same, but after a while, are: 17.81, 16.95, 16.63, 20.57,  
> 15.14,  
> 16.95, 11.45, 20.57, 20.57, 11.45, 14.82. It begins in the fourth value  
> instead the first. \>

Is there something else I am missing?

Thank you very much!

David

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [July 1, 2015, 8:46pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/4 "2015-07-01T20:46:16Z")

</div>

Hello again

I’m sorry, in the last message, tamano\_entrada is 8 and nsimbolo is 3  
(this  
is for the encoder).

Also, I had probed doing this scheme in two parts:  
First I took a vector source as a binary entry for the encoder. The  
encoder’s output was what I have expected it to be. I had the same 11  
float  
values in the same order all the time.  
Then, I took a vector source with the float values that would be in the  
encoder’s output, and I connected it to the decoder’s input. The values  
at  
the decoder’s output were corrects too (were the same bits that I put  
into  
the vector source in the first part).

So, I think the problem is when I connect both encoder and decoder  
blocks  
but I don’t know how to solve it.

Than you.  
David.

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [July 1, 2015, 10:49pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/5 "2015-07-01T22:49:31Z")

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Hi!

> In another topic, you wrote me that was syntactically incorrect and you  
> advice me to set it like this:  
> set\_output\_multiple(tamano\_entrada+nsimbolo);
> 
> But this way I get the errors \<‘tamano\_entrada’ is not a type\> and \<expected  
> ‘,’ or ‘…’ before ‘+’ token\>  
> Different things are correct to do different things: When you call a  
> function, you must not say which types the parameters have.  
> If you do something that your compiler thinks is a function declaration,  
> you must.

> Again, when I set this parameters the first time I asked (void  
> set\_output\_multiple (int tamano\_entrada + nsimbolo);  
> void set\_relative\_rate (double relative\_rate)😉 I did not get any errors,  
> I’d like to address these two lines, and then remove myself from this  
> discussion. You make C++ beginner’s mistakes, which is totally ok, and  
> can be overcome with a little training, but this is really not the place  
> to discuss this, I’m afraid. I must apologize if I came across harsh at  
> times, but you must understand that there’s a gap between the level of  
> questions you’re asking and your ability to autonomously write correct  
> C++ code, and I can’t help you bridge that.

void set\_output\_multiple (int tamano\_entrada + nsimbolo);

This doesn’t really make sense. Either you’re in a situation where you  
need to supply the type (you _declare_ or _define_ the function/method  
set\_output\_multiple), then your arguments must have types but need to be  
identifiers and can be expressions that give you a value (such as  
tamano\_entrada + nsimbolo).

OR you’re in a place to _use_ that already existing function or method  
(that’s the case here), and the typename is wrong.  
I can’t find any way that GCC won’t give an error for this line, sorry;  
I think you might be missing some compiler errors.

void set\_relative\_rate (double relative\_rate);

That is **not** a call to gr::block::set\_relative\_rate. That is a  
function declaration, possibly hiding the original method. C++ in some  
compilers/versions supports local-scope function declarations like  
these. A call would simply look like  
set\_relative\_rate(12.12);  
or  
set\_relative\_rate(relative\_rate);  
but can _never_ declare the type of either the arguments or the return  
value of that function. That’s logical, because the function must  
already be declared if you want to use it. The fact that this seems to  
work better than what I told you to do means that you have more mistakes  
that get hidden by this mistake.

Best regards,  
Marcus

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [July 1, 2015, 10:54pm UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/6 "2015-07-01T22:54:47Z")

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Sorry, typo:

…

> then your arguments must have types but need to be identifiers and can be  
> expressions that give you a value  
> …  
> is wrong,  
> needs to read  
> can _not_ be expressions that give you a value  
> Full text, corrected:

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**Author:** ![dcardona](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a698b9/32.png) [@dcardona](https://www.ruby-forum.com/u/dcardona)\
**Post date:** [July 2, 2015, 8:12am UTC](https://www.ruby-forum.com/t/a-two-blocks-synchronizing-problem/242513/7 "2015-07-02T08:12:39Z")

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Thank you Marcus.

I saw my mistakes, now both blocks are finally working fine! Thank you  
for  
all your help. And thank to all that helped me.

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