Stream tags

GNU Radio
1

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?

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?

BR,
Patrik E.

2

On Thu, Mar 24, 2011 at 6:29 AM, Patrik E.
<[email protected]

wrote:

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.

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.

BR,

Patrik E.

Thanks!
Tom

3

On Thu, Mar 24, 2011 at 6:29 AM, Patrik E.
<[email protected]mailto:[email protected]> 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 E.

Thanks!
Tom

4

On Tue, Mar 29, 2011 at 3:05 AM, Patrik E.
<[email protected]

wrote:

relative rate, and if it is not 1.0, it will adjust the value by multiplying
instead of being the float value, just as you said.

Great! Glad you got it.

I found that it is possible to change the policy of how the tags
propagate

occur when you have multiply inputs and multiply outputs. For example, a
input(0) --> output(0)
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.

We discussed that, but the solution that we came up with is to use a
third
policy (the TPP stands for tag propagation policy), which is TPP_DONT.
If
you create a block that uses this policy, it does not pass on any tags.
Instead, if you want to pass on tags, you basically have to do it
yourself.
It’s really the opposite of what you are saying: instead of
cherry-picking
to remove tags, you cherry-pick which tags to continue to propagate.

I’m willing to entertain the idea of creating a method to remove tags
from
the list, but I’ll have to think about it some. Largely, there isn’t
much of
a body of work built up that uses these tags, yet, so we implemented it
one
way under some assumptions about their uses. Mainly, we said that there
won’t be that many tags overall, so it’s easy enough to pick which ones
to
propagate if you are a TPP_DONT block.

That said, as you and others use the tags in other ways and build up
some
working knowledge about what you want to do with them, we can review
these
kinds of things.

Thanks,
Tom