RE: [Discuss-gnuradio] Stream tags

[Patrik Eliardsson]

 

On Thu, Mar 24, 2011 at 6:29 AM, Patrik Eliardsson <address@hidden> wrote:

Hi,

I've tried the new stream tags and like them, but when I've a block in my flowgraph that change the stream rate I'm not sure how the absolute sample number is calculated.

Consider the flowgraph below where B0, B1 and B4 are gr_sync_block and B2 is a gr_block that increases the stream rate in the flowgraph.
B0 -> B1 -> B2 -> B4

If I set tags on sample 0 and 200 in block B0 and check the tags in block B1 everything is correct, I find them at sample 0 and 200. But if I check the tags in block B4 I find them at 0 and 400 while I expected to find them on 0 at 527. The B2 block consumes 200 samples and produces 527 samples, this is a rate of 2.635 and the tag absolute sample number is increased by a ratio of 2. How is the absolute sample number calculated?

Patrik, glad you've gotten into using the stream tags. To answer your question, the sample numbers are changed based on the relative rate you set for your block. When you use a gr_sync_decimator with a decimation rate of D, the relative_rate of that block is set in the constructor to be 1.0/(float)D. Likewise, when using a gr_sync_interpolator, the relative rate is set to (float)I. 

When a stream tag is passed between blocks, the schedule looks at the relative rate, and if it is not 1.0, it will adjust the value by multiplying by relative_rate.

It strikes me that possible block B2 is having its relative_rate set incorrectly. It could be a casting problem that has truncated the math to 2 instead of 2.635. You can ask a block it's rate with relative_rate() to see what it thinks its rate is.  

 

Thanks Tom! I found it in the source code how the sample number was calculated. The problem I had was that my relative rate was casted to 2 instead of being the float value, just as you said.

I found that it is possible to change the policy of how the tags propagate between blocks. My default setting was TPP_ALL_TO_ALL. When I changed it to TPP_ONE_TO_ONE I got the following error "gr_block_executor: propagation_policy 'ONE_TO_ONE' requires ninputs == noutputs", I tested it on the flowgraph above. With the TPP_ONE_TO_ONE policy, shouldn't the tags only propagate from B0 to B1?

Yes, you are correct in how it should work, so I can't say why it is failing. Basically, if you have a graph with a single path, then TPP_ALL_TO_ALL should function the same as TPP_ONE_TO_ONE. The differences occur when you have multiply inputs and multiply outputs. For example, a block with 1 in and 2 outs will not work with TPP_ONE_TO_ONE, but with TPP_ALL_TO_ALL, all tags appearing on input(0) will be sent to output(0) and output(1).

The TPP_ONE_TO_ONE says that if you have 2 inputs and 2 outputs, then tags flow:

input(0) --> output(0)

input(1) --> output(1)

While the TPP_ALL_TO_ALL would send this:

input(0) --> output(0)

input(0) --> output(1)

input(1) --> output(0)

input(1) --> output(1)

 

We have code that checks both cases out, but either something is wrong or not obvious in your case. If you can provide more details about what you are doing, we can work to track this down.

Okey, propagation policy was not as I first thought! I was thinking about how to limit the number of blocks that the stream tags would propagate to. As it is now the stream tags is visible in all blocks in the flowgraph. Is it possible to introduce some function like consume_stream_tag(stream_tag) that ensures that a certain stream tag would not propagate further. Or some policy for tags that the stream tag should only propagate X blocks down stream? It was just a thought..

 

The reason why I got the error with TPP_ONE_ONE is that I have a block down streams that doesn't use stream tags but have 1 inputs and 2 outputs.

BR,
Patrik Eliardsson

Thanks!

Tom