SoapyRTLSDR Number of samples to read

[Mas313]

I have a problem reading samples from the dongle since in rx_fm there is a large number of samples (131,072) to read passed as an argument to the function as shown here in rtl_fm.c https://github.com/rxseger/rx_tools/blob/master/src/rtl_fm.c

define DEFAULT_BUF_LENGTH (1 * 16384)

#define MAXIMUM_OVERSAMPLE      16
#define MAXIMUM_BUF_LENGTH      (MAXIMUM_OVERSAMPLE * DEFAULT_BUF_LENGTH)
samples_per_buffer = MAXIMUM_BUF_LENGTH/2
r = SoapySDRDevice_readStream(s->dev, s->stream, buffs, samples_per_buffer, &flags, &timeNs, timeoutNs);
        //fprintf(stderr, "ret=%d\n", r);

        if (r >= 0) {
            // r is number of elements read, elements=complex pairs, so buffer length in bytes is twice
            rtlsdr_callback(buf, r * 2, s);
        }

However, my program with same lines of code hangs until and unless i pass a very small value of samples to read from the dongle such as 512. Plz note that 131,0722 (is passed to the function rtlsdr_callback(buf, r 2, s); ) Can anyone tell what could be the issue? Also i don't see any callback function in soapyrtlsdr that i can use to get samples from rtl sdr dongle. Plz guide me if there is any. Thanks

[zuckschwerdt]

The 128k is actually the default transfer size for RTL-SDR, so not that large. The readStream API is correct, if you want callback behavior then run the blocking call in a thread an then notify the main thread with the data frame (but know how to use threads). Otherwise, guessing that rtlsdr_callback() is converted librtlsdr code, this is already the wanted control flow. But maybe you'd want to loop on readStream until a full buffer is completed. https://github.com/pothosware/SoapySDR/blob/9c4fa3241b220341479a1a76057025a8264e50d9/include/SoapySDR/Device.h#L377-L403

You omitted quite some interesting parts here. Is dev and stream setup and active? How is buffs defined? What's in timeout? Are you running that code in a loop?

[Mas313]

Thanks for the prompt reply, i am running code from c example ( https://github.com/pothosware/SoapySDR/wiki/C_API_Example ) which is shown below and even with 1024 samples to read I get error which is actually related to violation when reading samples, its pointer related. I am running this code in windows 10 compiled using GCC

//create a re-usable buffer for rx samples complex float buff[1024];

    //receive some samples
    for (size_t i = 0; i < 10; i++)
    {
        void *buffs[] = {buff}; //array of buffers
        int flags; //flags set by receive operation
        long long timeNs; //timestamp for receive buffer
        int ret = SoapySDRDevice_readStream(sdr, rxStream, buffs, 1024, &flags, &timeNs, 100000);
        printf("ret=%d, flags=%d, timeNs=%lld\n", ret, flags, timeNs);
    }

[zuckschwerdt]

with 1024 samples to read I get error which is actually related to violation when reading samples, its pointer related.

Yes, that's what I suspected but was not visible in the source snippet. If you are reading CF32 (default is CS16 and native is CU8) then the buffer is way too small. 1024 Elements of I/Q is 2048 Samples.

Edit: oh, wait complex? Well that should work then?

[Mas313]

Its strange that at time the code is run successfully but right now I am getting this error again. When I select option to debug upon receiving message that my exe has stopped working. i get this error although I have not modified or did processing of samples. Unhandled exception at 0x7732F583 (ntdll.dll) in test.exe: 0xC0000005: Access violation writing location 0x00000014.

I wonder how SDR sharp is running with this dongle correctly?

[zuckschwerdt]

It's more likely a typo in your code. The deref of a address 0x14 (=20) pointer sounds like you passed an index/length where a buffer address would be expected. Maybe push the full code to a github project, too much guesswork otherwise.

[Mas313]

What i have observed now that when i handle all SOAPY errors MACROS then I no more have these issues. Thanks for the prompt replies

[Mas313]

The issue seems fixed when I handle all soapysdr error macros. These were not handled in example C API and caused errors due to non handling. Thanks a lot for the prompt replies.

[zuckschwerdt]

Yes, the handling of ret <= 0 in https://github.com/pothosware/SoapySDR/wiki/C_API_Example is missing -- but that should not be an issue. The example works for me, what did you need to add?

[Mas313]

I am getting these two errors frequently reading 1024 samples although I get 1024 samples in between. i have not tried reading a large number of samples

OSOAPY_SDR_OVERFLOW
SOAPY_SDR_TIMEOUT

Also I would be grateful if you can refer to any simple working formula for FM demodulation using these complex samples

[zuckschwerdt]

I'd use FFTW and a Blackman-Harris window for a true FM demod, or otherwise an Instant Phase Angle computation for a speedy approximation.

[Mas313]

can you provide Instant Phase Angle computation for FM fm demodulation in writing or link to equations