Re: [Discuss-gnuradio] Getting the Count of FFTs

[Hasini Abeywickrama]

Hi Nathan,

Thank you very much for the suggestions. I will try them.

Regards,

Hasini

On Tue, Sep 20, 2016 at 11:44 PM, West, Nathan <address@hidden> wrote:

Do you really need the count, or do you just need some code to process FFTs as the block computes them? I suspect a little bit more understanding of GNU Radio will help you achieve what you want.

Assuming that I have some DSP stuff I want to operate on an FFT output, I would go about this in one of the following ways:

Option 1) Write my DSP stuff in GNU Radio blocks in my own out of tree module. This is the GNU Radio way to do things. The GNU Radio scheduler will call my code to process FFT chunks as they are available.

Option 2) Connect the output of the GNU Radio FFT block to something like a gr-zeromq block or TCP block and write a receiver for this on my application. This is better than a file (but can actually *look* like a file if I use a something like zeromq's ipc:// protocol or a FD block because an actual file flushes buffers (actually commits the write to disk) at unknown times, so it may not actually be particularly close to real-time. Then instead of reopening a file and seeking to the end every time I can just read from the socket/icp FD to get all of the latest stuff.

There's no way to get a count of FFTs that are written to the file because the FFT block and the file block are not deterministicly scheduled. You could use the performance counters to see how many items the file block has read, but that's a horrible abuse of that counter and still doesn't guarantee the number actually written to disk because you don't know when the file buffer is actually being flushed. Rethinking the problem is likely the best way forward.

I highly recommend option 1 and option 2 as a last resort. Option 2 is better for when you're no longer doing DSP and passing something like bits to another process that's great at handling bits.

On Mon, Sep 19, 2016 at 9:07 PM, Hasini Abeywickrama <address@hidden> wrote:

Hi Nathan,

That's not quite what I want. I do not want the file to have a certain number of FFTs. I want to have a counter which is dynamically updated as the FFTs are written to a file. Ideally I want to know when a new FFT is written to the file so I can trigger another function to process it. I need this to be done real time and that's why I'm looking for the most efficient way.

Regards,

Hasini

On Tue, Sep 20, 2016 at 10:34 AM, West, Nathan <address@hidden> wrote:

On Mon, Sep 19, 2016 at 6:57 AM, Hasini Abeywickrama <address@hidden> wrote:

Hi Marcus,

That's what I'm currently doing. It's not very efficient as I have to constantly check the file size. If I can have a counter that is increments each time an FFT is written to the file by the flow graph I would be able to trigger an event based on the counter value. That was my concern. 

Regards,

Hasini

On Sep 19, 2016, at 3:29 PM, Marcus Müller <address@hidden> wrote:

If you need to do that externally, I'd just watch the file size and divide it by the item size (4B per floating point number) and by the FFT length.

Best regards,

Marcus

On 09/18/2016 11:21 PM, Hasini Abeywickrama wrote:

Hi Marcus,

It's like I need to start reading and processing the file after a certain number of FFTs are being written to the file. For that I would need to maintain a counter and I'm trying to figure out how to do it.

Regards,

Hasini

You can do this with the head block. It let's N number of items through and then stops the flowgraph. Set N to your certain number and then the file will have N FFTs.

 

On Mon, Sep 19, 2016 at 2:58 PM, Marcus Müller <address@hidden> wrote:

What would you need that counter for? There's the 1/(FFT length) fixed relation between samples processed and numbers of FFTs done, so this is pretty much a redundant piece of info, but maybe I'm just missing the use case.

Best regards,

Marcus

On 09/18/2016 10:23 PM, Hasini Abeywickrama wrote:

Hi all,

I have a GNU Radio flowgraph which uses a USRP to receive a signal, converts it to log power FFT and then writes the result to a file, using File Sink. I want to have a counter which increments with each FFT written to the file.

Is there a way of maintaining and increasing count of the FFTs written to the file, while the flowgraph is still running?

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