[Discuss-gnuradio] Serious bug in tag propagation with non-integer relative rate

[Andy Walls]

Hi Eugene

> From:         Eugene Grayver
> Date:         Thu, 9 Nov 2017 19:52:35 +0000
> 
> There is a major problem with the way tags are propagated in blocks
> with non-integer relative rate. If the ratio is not a power of two,
> the numerical accuracy of the floating point will cause the output
> tags to diverge from the input tags.  Consider the fractional
> resampler. It accumulates the timing offset in the range of 0 to 1.
> However tag propagation multiplies the ratio by the sample number. As
> the sample number grows the LSB accuracy of the ratio gets scaled by
> the ever larger value. For a ratio of 1.001 we saw divergence of
> 1000s of samples over a few minutes at 10msps.

Could you please test the following branch to see if it fixes the
problem?  Maybe test something simple first, like an FIR filter
decimating by 5 or 3?

https://github.com/awalls-cx18/gnuradio.git branch: tag_fix3

Or if you have a GRC or python script I can use myself for testing,
that would be great.

>  I rewrote tag propagation for the resampler but did not rework the
> generic logic. I think the key point is to use the delta between read
> and written items to take out the large integer difference and then
> apply the scaling to a local delta within the current window.
> 

The fix that I have made stores the relative_rate as an integer
numerator and an integer denominator, and it uses integer arithmetic to
propagate tags. (Except if enable_update_rate() is True, in which case,
precision tag placement was abandonded by the block author anyway.)

Thanks,
Andy