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Re: [Discuss-gnuradio] is there possible to have item drops?

From: Mostafa Alizadeh
Subject: Re: [Discuss-gnuradio] is there possible to have item drops?
Date: Thu, 16 Oct 2014 15:17:02 +0330

Hi Marcus, 

I tested again my problem with connecting a data file (file source) to the fir filter and read the output with the one resulted from Matlab. The error which appears due to the data precision is in acceptable order. 

However, when I directly connect my tx to the fir filter, it did something strange after sometime which causes a great error with respect to the Matlab one. 

I think the problem is with my tx! I really confused!! I just wanna know how could I identify the problem and how could I fix it? 

My scheme:
Inline image 1


On Wed, Oct 15, 2014 at 9:03 PM, Marcus Müller <address@hidden> wrote:
Hello Mostafa,

filtering is a inherently lossy operation: it's an operation that involves multiplication and addition of floating point numbers.
Matlab internally uses double (float64) values, whereas GNU Radio, usually, uses single precision (float32) numbers for performance reasons.
Now, without knowing what kind of signal you are filtering with what kind of taps you are using with what kind of GNU Radio filters and what Matlab functions, there's nothing to guess on how these differences occur.
What I *can* say though is that I've never seen an item drop in GNU Radio's runtime, and we have strong tests against that. Now, it's entirely possible that any of our filter implementations might have bugs, but I kind of doubt that. It's most probably simply numeric accuracy that kills your samples.


On 15.10.2014 18:07, Mostafa Alizadeh wrote:
Hi all,

It seems this is a ridiculous question, however, I didn't find any solution
for my problem.

I have already a bunch of custom blocks connected to each other which play
role as a transmitter (tx). I want to connect tx to a fir filter by using
fir_filter_ccc block. I set the filter taps with specific ones. I also have
this filter in Matlab. The output of the filter is connected to a sink file.

When I see the output (tx) and filtering it by Matlab and comparing it with
the output of the gnuradio filter, I see both of the following:

1) Both matlab and gnuradio filtered output, have little difference in a
range of 0.01. (I'm using gr_complex data).

2) After a while, Matlab output and GNURadio's output will differ as high
as 1000 in magnitude!!

I guess, there is item drops in this filtering. I know, the probability of
this event is so weak, anyway, item drop even can happen in GNURadio?

Where is my problem?


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