Hi Eugene,
yup, fully agree, the whole concept is slightly broken.
So, first of all, I really think the key problem we're constantly
working around is that tags have an integer offset – which leads
to rounding errors, even in relatively benign scenarios.
I'd propose we add a fractional part: that would only extend
tag_t by another integer field, so existing blocks wouldn't break,
and would allow non-1:1-sync blocks to properly rewrite tag
positions.
As you say, floating point is a bad idea when dealing with times
(it always has been – see uhd::time_spec_t, where we're constantly
paying off the technical debt of having floating point as
"suggested" default representation of time, albeit underneath
things should (and to some degree, are) counted in ticks). Thus,
I'd imagine a tag_t like
struct tag_t {
uint64_t offset; //integer offset, as had
uint32_t fractional_offset; //interpret as fractional part, i.e. ·2^{-32}
pmt::pmt_t key;
pmt::pmt_t tag;
pmt::pmt_t srcid; // might as well drop this; maybe someone is using this, but I haven't met them
std::vector<long> marked_deleted;
}
Would the fractional offset solve the issues you're seeing, assuming
that we add proper handling to all the existing resamplers?
Best regards,
Marcus
On 09.11.2017 20:52, Eugene Grayver
wrote:
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.
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.
_______________________
Eugene Grayver, Ph.D.
Aerospace Corp., Sr. Eng. Spec.
Tel: 310.336.1274
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