Re: [Discuss-gnuradio] Continuously Write FFT Samples to a File

On Fri, Jan 20, 2017 at 2:33 PM, Marcus Müller <address@hidden> wrote:

Hello Mallesham,

I'm afraid not, since I'm afraid that to my current understanding, what you want is mathematically impossible. Either you want much data – and that seems to be the case, since you want to record 24h of raw IQ data – or you can store it in what comparably little RAM modern computers have.

Maybe, however, we haven't fully understood the problem. Can you, mathematically, define what you want to observe and record?

Best regards,

Marcus
On 01/20/2017 08:28 PM, Mallesham Dasari wrote:
Hello everyone,

Can anyone give some solution for this? Even writing to the ramdisk is not enough for running the flow graph for so long. I am facing the same issue.

Thank you!
On Thu, Jan 12, 2017 at 5:41 PM, Hasini Abeywickrama <address@hidden> wrote:
Hi all,

Thank you very much for the informative responses.

My requirement is to run the flowgraph for a long time (ideally 24 hours) and store the FFT data in the memory (ramdisk) to they can be processed later or in chunks, not everything at the same time.

So far, I have increased the size of the ramdisk and it works fine for a few hours. But it still is not the solution I'm looking for.

Regards,

Hasini
On Thu, Jan 12, 2017 at 8:30 PM, Marcus Müller <address@hidden> wrote:
But if you do a single 1024-FFT, you'd only operate on 1024 of the input samples!

And: the FFT doesn't just give you power values, but complex values; mathematically, the FFT is a DFT, and the DFT is an invertible linear operator $\mathcal F$ :

$\mathcal F: \mathbb C^{N} \mapsto \mathbb C^{N}$

which maps complex vectors to complex vectors of size $N,\quad 0<N\in\mathbb N$ . It is, in fact, representable as square matrix with column (and row) vectors being samples of the orthogonal complex sinusoids $e^{j\frac{2\pi}N nk},\, k=0,\ldots,N-1$; that is, it can also be understood as a base change matrix, that just represents the "input vector" according to a different base, orthogonal base.
In the physical sense: the input vector base was represented by the standard basis $\mathbf e_N$, meaning that each base vector represents a single point in time – the sample time of the respective entry; the "output" of the transform is represented on a base of orthogonal frequencies. This is an invertible operation – really just another way to look at the same signal. I think this is really important to keep in mind:

The Fourier transforms are not magical by any means. What they do is represent the same signal from a different point of view. It can be interpreted as transform between time and frequency domain (or space and impulse, or...). The DFT is still just a boring, old, square, orthogonal, invertible matrix that produces output of the same dimensionality as it takes input.

As you can see, the DFT/FFT itself never reduces the amount of data.

What you might be referring to is some kind PSD estimate done by first |·|² a lot of DFTed vectors and then averaging them. The data reduction here lies in the magnitude square operation and the average, not in the DFT.
The point here is that you're throwing away a whole lot of information, and I'm not convinced that's what Hasini needs!

Best regards,

Marcus

On 12.01.2017 05:54, Mallesham Dasari wrote:
Hi Marcus,

Raw IQ samples take lots of memory because each sample will be around 8Bytes. Suppose, if we 1Msps sample rate, just for 10 minutes of data, we get 10*60*1M*8B = 4.8GB data. On the other hand, if you store just FFT with 1024 bin, we get 4.8GB/1024 power values right (which has very less size)?

Please correct me if I am wrong.

Thanks
On Wed, Jan 11, 2017 at 7:32 AM, Marcus Müller <address@hidden> wrote:
Hi Mallesham,

I don't understand – the raw IQ samples and their FFT have the same size, and data type.

Maybe you've understood something that I (and Martin) didn't – could you elaborate?

Best regards,
Marcus
On 01/11/2017 12:56 AM, Mallesham Dasari wrote:
Hi Hasini,

If you are trying to print just the FFT, it should not be an issue. If you print raw iq samples, then you will run out of memory. By long, you mean how long? Days?

On Tue, Jan 10, 2017 at 3:16 PM, Martin Braun <address@hidden> wrote:

Hasini,

can you please re-state what you're trying to do? That might help you
getting some answers. It is not quite clear from this email.

Cheers,
Martin

On 01/02/2017 09:16 PM, Hasini Abeywickrama wrote:
> Hi all,
>
> I have a flowgraph that reads a signal and writes its FFT samples to a
> file. I need to run this continuously (for a long time), without running
> out of memory.
>
> I tired deleting the earlier FFT samples from the file but that messes
> up with reading the data. I also tried starting writing to a different
> file after some time so the initial file can be completely deleted. But
> it did not work as well.
>
> What would be the best approach for this? Any thought would be very much
> appreciated.
>
> Regards,
> Hasini
>
>

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Best Regards,
Mallesham Dasari

Department of Computer Science

Stony Brook University

USA - 11794
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Department of Computer Science
Stony Brook University
USA - 11794

Best Regards,

Mallesham Dasari

Department of Computer Science

Stony Brook University

USA - 11794

From:	Marcus Müller
Subject:	Re: [Discuss-gnuradio] Continuously Write FFT Samples to a File
Date:	Fri, 20 Jan 2017 20:46:18 +0100
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