Re: [Discuss-gnuradio] GnuRadio: Clock Recovery MM: imu out of bounds

[Marcus Müller]

Hi Michael,

so, this having sat on my "what the hell, I need to look at this" list for the weekend:
I can't reproduce the behaviour, but that might simply have the cause of me using white noise instead of your recording. Could you share that?

I have two hypotheses and one observation so far:
The observation is that in many places where we want to get the closest int to a float, we use stuff like int imu= rint(mu*NSTEPS), which is wrong in respect to the return value (double), the parameter type (double) and in most cases also the side effects (rint is the version of nearbyint that sets error flags, but we don't check them). We should use something else; that's an overhaul of existing code that'll have to be done another day.

So first hypothesis:
It's your signal that will suffice for us to reproduce the error. That's the "good" case, because then one can actually inspect what goes wrong here.

Second hypothesis:
Based on above observation and VirtualBox' history of delicate floating point bugs, this might get a bit more involved, especially since floating point exception registers are involved, which historically were hard to properly multithread/make ISR save; maybe the overhaul mentioned above would solve the issue, though.

Best regards,
Marcus

On 21.08.2015 08:00, Michael B wrote:

Hi Tom,

Thanks for your help. I've adapted the sample rate of the result coming out of the low-pass filter now. However, if I decimate much further, my signal becomes a bit too weak I think.
New chart:
http://imgur.com/mbt288s,etHbYLT#0

New result:
http://imgur.com/mbt288s,etHbYLT#1

Is this a better approach?
The initial error still persists though.

Regards

 

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From: address@hidden
To: address@hidden
Date: Thu, 20 Aug 2015 09:09:42 +0200
CC: address@hidden
Subject: Re: [Discuss-gnuradio] GnuRadio: Clock Recovery MM: imu out of bounds

Hi Tom,

Thanks for your answer. That's indeed something that I need to think about more. However, as I understand it, the samples per symbol rate is equal to (samp_rate / data_rate). My sample rate is 500k, and my data rate (i think) is only around 100 or 200. See this figure of my (cleaned) signal:

http://imgur.com/8pNqCop

You see that it takes approximately 10 ms to transmit one symbol. This means 100 symbols per second. As I have 500k samples per second, this means 5k samples per symbol, right?

However, I have tried setting the samples per symbol to 4, just to try, and the 'out of bounds' error still persists.

Regards,
Michael

 

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From: address@hidden
Date: Wed, 19 Aug 2015 09:41:19 -0400
Subject: Re: [Discuss-gnuradio] GnuRadio: Clock Recovery MM: imu out of bounds
To: address@hidden
CC: address@hidden

On Wed, Aug 19, 2015 at 2:51 AM, Michael B <address@hidden> wrote:

I have created a flowgraph, based on an example of Michael Ossmann, which takes in a signal, and should output bits.

I need to use the clock recovery MM block, which I do not fully understand yet. However, after reading some blogposts, I am quite sure that I can leave most of the settings default, except for the Omega one. Here's my flowgraph:

http://imgur.com/pHRXnZu

When running this flowgraph, it gives me the following error:

thread[thread-per-block[5]:<block clock_recovery_mm_ff (9)>]: mmse_fir_interpolator_ff: imu out of bounds.



While searching, I stumbled upon this piece of code in the source of GnuRadio:


int imu = (int)rint(mu * NSTEPS);  
  if((imu < 0) || (imu > NSTEPS)) {
    throw std::runtime_error("mmse_fir_interpolator_ff: imu out of bounds.\n");
  }

So, I suspect it is not due to my Omega setting (which might be wrong, I have to play with that setting), but that it is due to my Mu setting, which is just the default (0.5). However, I understand that Mu needs to be between 0 and 1, so I do not really understand what the problem is. Anyone who does?

Environment details:

• GNU Radio Companion 3.7.7.1
• Running a GNU Radio live DVD in a virtual machine (VirtualBox 4.2.12) on Windows 7.
• Using Volk machine: ssse3_64

Michael,

I don't have an answer, but I can say where you're doing something wrong. You're samples/symbol (samp_per_sym) is definitely /not/ 2.5k. That's a massively oversampled signal and can't be right. You need to think what's the sampling rate of the system? What's the symbol rate of my signal? That will tell you the samples/symbol you need. It should small, like 2 or 4.

Tom

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