lib: check for validity of sample rates

Thanks to Joris van Rantwijk for finding what seems to be
a hardware limitation/bug (bit 28 of the rsamp register being
forced to the value of bit 27).

Signed-off-by: Steve Markgraf <steve@steve-m.de>

include/rtl-sdr.h

@@ -243,7 +243,17 @@ RTLSDR_API int rtlsdr_set_tuner_if_gain(rtlsdr_dev_t *dev, int stage, int gain);
  */
 RTLSDR_API int rtlsdr_set_tuner_gain_mode(rtlsdr_dev_t *dev, int manual);
 
-/* this will select the baseband filters according to the requested sample rate */
+/*!
+ * Set the sample rate for the device, also selects the baseband filters
+ * according to the requested sample rate for tuners where this is possible.
+ *
+ * \param dev the device handle given by rtlsdr_open()
+ * \param samp_rate the sample rate to be set, possible values are:
+ * 		    225001 - 300000 Hz
+ * 		    900001 - 3200000 Hz
+ * 		    sample loss is to be expected for rates > 2400000
+ * \return 0 on success, -EINVAL on invalid rate
+ */
 RTLSDR_API int rtlsdr_set_sample_rate(rtlsdr_dev_t *dev, uint32_t rate);
 
 /*!

src/librtlsdr.c

@@ -337,8 +337,6 @@ static rtlsdr_dongle_t known_devices[] = {
 #define MIN_RTL_XTAL_FREQ	(DEF_RTL_XTAL_FREQ - 1000)
 #define MAX_RTL_XTAL_FREQ	(DEF_RTL_XTAL_FREQ + 1000)
 
-#define MAX_SAMP_RATE		3200000
-
 #define CTRL_IN		(LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_ENDPOINT_IN)
 #define CTRL_OUT	(LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_ENDPOINT_OUT)
 #define CTRL_TIMEOUT	300
@@ -1053,20 +1051,24 @@ int rtlsdr_set_sample_rate(rtlsdr_dev_t *dev, uint32_t samp_rate)
 {
 	int r = 0;
 	uint16_t tmp;
-	uint32_t rsamp_ratio;
+	uint32_t rsamp_ratio, real_rsamp_ratio;
 	double real_rate;
 
 	if (!dev)
 		return -1;
 
-	/* check for the maximum rate the resampler supports */
-	if (samp_rate > MAX_SAMP_RATE)
-		samp_rate = MAX_SAMP_RATE;
+	/* check if the rate is supported by the resampler */
+	if ((samp_rate <= 225000) || (samp_rate > 3200000) ||
+	   ((samp_rate > 300000) && (samp_rate <= 900000))) {
+		fprintf(stderr, "Invalid sample rate: %u Hz\n", samp_rate);
+		return -EINVAL;
+	}
 
 	rsamp_ratio = (dev->rtl_xtal * TWO_POW(22)) / samp_rate;
-	rsamp_ratio &= ~3;
+	rsamp_ratio &= 0x0ffffffc;
 
-	real_rate = (dev->rtl_xtal * TWO_POW(22)) / rsamp_ratio;
+	real_rsamp_ratio = rsamp_ratio | ((rsamp_ratio & 0x08000000) << 1);
+	real_rate = (dev->rtl_xtal * TWO_POW(22)) / real_rsamp_ratio;
 
 	if ( ((double)samp_rate) != real_rate )
 		fprintf(stderr, "Exact sample rate is: %f Hz\n", real_rate);