SDR: Fix level range of IQ date; must not exceed range of -1 .. 1
LimeSDR mini (and maybe others) do not want IQ data to become a value below -1 or above 1. This happens slightly when applying the IIR filter after upsampling. To prevent this, we scale down the IQ level a little bit. To test the problem: - set the scale level to 1.0, so it does not take effect - use LimeSDR mini - use upsampling (sample rate differs SDR sample rate) - run loopback mode 2 (-l 2) - IIR filter must be initialized with 2 or more iterations to take effect - check linear IQ graph; it will then plot dots inside the circle
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@ -51,6 +51,9 @@ enum paging_signal;
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/* usable bandwidth of IQ rate, because no filter is perfect */
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#define USABLE_BANDWIDTH 0.75
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/* limit the IQ level to prevent IIR filter from exceeding range of -1 .. 1 */
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#define LIMIT_IQ_LEVEL 0.95
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int sdr_rx_overflow = 0;
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typedef struct sdr_thread {
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@ -446,8 +449,8 @@ static void *sdr_write_child(void *arg)
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out = sdr->thread_write.out;
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for (s = 0, ss = 0; s < num; s++) {
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for (o = 0; o < sdr->oversample; o++) {
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sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out];
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sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1];
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sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out] * LIMIT_IQ_LEVEL;
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sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1] * LIMIT_IQ_LEVEL;
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}
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out = (out + 2) % sdr->thread_write.buffer_size;
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}
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