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Re: [Discuss-gnuradio] Question about stream to complex and fft

From: Kyeong Su Shin
Subject: Re: [Discuss-gnuradio] Question about stream to complex and fft
Date: Fri, 14 Jun 2019 14:18:14 +0000

Hello S. L.:

Short Version:

Yes, you are correct!

Some picky details:

Due to technical limitations, the output timing of the Throttle block is not perfect (but should be good enough, for most practical use cases). Also, due to this, the Throttle block may output more than one samples at a time.

GNU Radio blocks use buffers to communicate. A block outputs data by writing the generated data into its output buffer. The next block in the chain considers this buffer as an input buffer, and reads data from it. It then tells the previous block that the data has been read, so as the previous block can generate more data. The Throttle block intentionally slows down this process.

But in overall, your idea is correct.


Kyeong Su Shin

보낸 사람: S. L. <address@hidden>
보낸 날짜: 2019년 6월 14일 금요일 오후 9:26:45
받는 사람: Kyeong Su Shin; address@hidden; rear1019
제목: RE: Question about stream to complex and fft
Thank you very much for your responses

So, if for example, I want to simulate 1 second of a cosine wave of 1K, with sample rate of 32K as the example and I have a very powerful computer the signal source block (cosine) could generate the 32k samples in 0.5 seconds , but the 32K wave samples of a 1K would correspond to real (good) values as if it had been sampling for 1 second ... Is that correct?

And if I put a Throttle block at the output of the signal source block at 32K of sample rate, the throttle block will receive the 32k samples as input at a lower time, store them in a buffer and direct them to the output at the precise moment, it is say every 1 / 32K ms ... Is this correct?

Thank you very much for your time and for your kindness

De: Kyeong Su Shin <address@hidden>
Enviado: viernes, 14 de junio de 2019 13:31
Para: S. L.; address@hidden
Asunto: RE: Question about stream to complex and fft

Hello S.L:

That sounds correct to me.

One note, however: the sampling rate in GNU Radio is just a conceptual thing, and does not determine the 'actual time that GNU Radio takes to generate or consume the samples', if you do not use hardware sinks/sources (like SDR sources) and/or throttle blocks. If your flowgraph does not have such blocks, then the flowgraph will execute 'as fast as it can'. The data generated by your program will be still correct; it is just that the 'time' is just a conceptual thing (so as you don't have to deal with normalized sampling rates) and may not agree with the actual execution speed of the program.

Kyeong Su Shin

보낸 사람: S. L. <address@hidden> 대신 Discuss-gnuradio <discuss-gnuradio-bounces+ksshin=address@hidden>
보낸 날짜: 2019년 6월 14일 금요일 오후 6:08:55
받는 사람: address@hidden
제목: [Discuss-gnuradio] Question about stream to complex and fft
Hello, I have a question about the real-time operation of gnuradio and the "stream-to-vector" and FFT modules "

In the example image I have:

variable samp_rate to 32K

and the following flowchart:

Signal source (waveform: cosine, sample_rate: 32K, frecuency: 1K) -> stream-to-vector (Num items: 1024) -> FFT (size: 1024) -> Complex-to-Mag (Vector lenght: 1024) - > QT GUI Vector Sink (Vector size: 1024)

Then cosine has 1K of frequency and sample rate is 32K and FFT size is 1024

I'm not really sure how the gnuradio simulation works so I'd like to check it with you and see if I'm right or I'm wrong

A whole cosine wave would take 1ms
since the sample rate is 32K, a cosine wave would be 32 samples
Then 1024 samples would be 1024/32 = 32 cosine waves, that is 32 * 1 ms = 32 ms

Since I have vector-to-stream and fft to 1024 samples, I think or I guess it stores the first 1024 samples (from 0 ms to 32 ms) and the vector-to-stream module passes those samples to the FFT module that performs the FFT of those 1024 samples
Then another 1024 samples (form 32 ms to 64 ms) would arrive and I would pass those samples to the FFT

That is, the FFT block performs its function every 32 ms with 1024 sample vectors

Is this operating logic on the stream-to-vector and fft block correct? or I'm wrong

Thank you very much everyone for your time
a greeting

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