From 881d4a9f72912f6446929ecbf78f52d9f0590cb6 Mon Sep 17 00:00:00 2001 From: Andreas Eversberg Date: Sat, 25 Aug 2018 08:13:16 +0200 Subject: [PATCH] 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 --- src/libsdr/sdr.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/src/libsdr/sdr.c b/src/libsdr/sdr.c index a316d95e..4d6d99a8 100644 --- a/src/libsdr/sdr.c +++ b/src/libsdr/sdr.c @@ -51,6 +51,9 @@ enum paging_signal; /* usable bandwidth of IQ rate, because no filter is perfect */ #define USABLE_BANDWIDTH 0.75 +/* limit the IQ level to prevent IIR filter from exceeding range of -1 .. 1 */ +#define LIMIT_IQ_LEVEL 0.95 + int sdr_rx_overflow = 0; typedef struct sdr_thread { @@ -446,8 +449,8 @@ static void *sdr_write_child(void *arg) out = sdr->thread_write.out; for (s = 0, ss = 0; s < num; s++) { for (o = 0; o < sdr->oversample; o++) { - sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out]; - sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1]; + sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out] * LIMIT_IQ_LEVEL; + sdr->thread_write.buffer2[ss++] = sdr->thread_write.buffer[out + 1] * LIMIT_IQ_LEVEL; } out = (out + 2) % sdr->thread_write.buffer_size; }