Method readStream

src/sdr/SoapySDRThread.cpp:195–402

Called in an infinite loop, read SaopySDR device to build a 'this.numElems' sized batch of samples (SDRThreadIQData) and push it into iqDataOutQueue. this batch of samples is built to represent 1 frame / TARGET_DISPLAY_FPS.

// a 'this.numElems' sized batch of samples (SDRThreadIQData) and push it into  iqDataOutQueue.
//this batch of samples is built to represent 1 frame / TARGET_DISPLAY_FPS.
int SDRThread::readStream(const SDRThreadIQDataQueuePtr& iqDataOutQueue) {
    
    int flags(0);
    
    long long timeNs(0);

    // Supply a huge timeout value to neutralize the readStream 'timeout' effect
    // we are not interested in, but some modules may effectively use. 
    //TODO: use something roughly (1 / TARGET_DISPLAY_FPS) seconds * (factor) instead.?
    long timeoutUs = (1 << 30);

    int n_read = 0;
    int nElems = numElems.load();
    int mtElems = mtuElems.load();

    // Warning: if MTU > numElems, i.e if device MTU is too big w.r.t the sample rate, the TARGET_DISPLAY_FPS cannot
    //be reached and the CubicSDR displays "slows down". 
    //To get back a TARGET_DISPLAY_FPS, the user need to adapt 
    //the SoapySDR Device to use smaller buffer sizes, because  
    // readStream() is suited to device MTU and cannot be really adapted dynamically.
    //TODO: Add in doc the need to reduce SoapySDR device buffer length (if available) to restore higher fps.

    //0. Retrieve a new batch 
    SDRThreadIQDataPtr dataOut = buffers.getBuffer();

    //resize to the target size immedialetly, to minimize later reallocs:
    assureBufferMinSize(dataOut.get(), nElems);

    //1.If overflow occurred on the previous readStream(), transfer it in dataOut directly. 
    if (numOverflow > 0) {
        int n_overflow = std::min(numOverflow, nElems);
        
        //safety
        assureBufferMinSize(dataOut.get(), n_overflow);

        ::memcpy(&dataOut->data[0], &overflowBuffer.data[0], n_overflow * sizeof(liquid_float_complex));
        n_read = n_overflow;

        //is still > 0 if MTU > nElements (low sample rate w.r.t the MTU !)
        numOverflow -= n_overflow;

        // std::cout << "SDRThread::readStream() 1.1 overflowBuffer not empty, collect the remaining " << n_overflow << " samples in it..." << std::endl;
        
        if (numOverflow > 0) { // still some left, shift the remaining samples to the beginning.
            ::memmove(&overflowBuffer.data[0], &overflowBuffer.data[n_overflow], numOverflow * sizeof(liquid_float_complex));

        //    std::cout << "SDRThread::readStream() 1.2 overflowBuffer still not empty, compact the remaining " << numOverflow << " samples in it..." << std::endl;
        }
    } //end if numOverflow > 0
    
    //default means blocking.
    int readStreamCode = 0;

    //2. attempt readStream() at most nElems, by mtElems-sized chunks, append in dataOut->data directly.
    while (n_read < nElems && !stopping) {
        
        //Whatever the number of remaining samples needed to reach nElems,  we always try to read a mtElems-size chunk,
        //from which SoapySDR effectively returns n_stream_read.