Merge branch 'divyekapoor-sampling_rate_fix'

The rtl-sdr driver code disallows most of the sampling rates below
900 kS/s. However, it allows a small window of sampling rates
between [250001, 300000]. The update to the conditional is along
the lines of the conditions imposed by the RTL SDR driver here:

http://cgit.osmocom.org/rtl-sdr/tree/src/librtlsdr.c

The default sampling rate of 1 MS/s causes severe buffer underruns on
a Raspberry Pi 2. However choosing a sampling rate of 250 kS/s and
lower values for PCM sampling causes successful FM demodulation of
strong FM stations on the device.

CMakeLists.txt

@@ -3,6 +3,7 @@
 cmake_minimum_required(VERSION 2.4)
 project(SoftFM)
 
+find_package(Threads)
 find_package(PkgConfig)
 find_package(ALSA REQUIRED)
 
@@ -49,6 +50,7 @@ include_directories(
     ${EXTRA_INCLUDES} )
 
 target_link_libraries(softfm
+    ${CMAKE_THREAD_LIBS_INIT}
     ${RTLSDR_LIBRARIES}
     ${ALSA_LIBRARIES}
     ${EXTRA_LIBS} )

FmDecode.cc

@@ -104,7 +104,7 @@ PilotPhaseLock::PilotPhaseLock(double freq, double bandwidth, double minsignal)
 
     // Set valid signal threshold.
     m_minsignal  = minsignal;
-    m_lock_delay = int(10.0 / bandwidth);
+    m_lock_delay = int(20.0 / bandwidth);
     m_lock_cnt   = 0;
     m_pilot_level = 0;
 

NOTES.txt

@@ -200,7 +200,31 @@ Radio4 reception improves from very bad to almost good.
 
 However, strong stations (radio3) sound slightly worse with the DIY antenna
 than with the basic DVB antenna.
-May be caused by clipping due to too strong signal from antenna.
+Theory: Distortion caused by clipping I/Q samples due to strong antenna signal.
+No, that's not it. Reducing LNA gain or IF gain does not help much;
+small DVB antenna still sounds better than DIY antenna.
+Difference only clear in stereo mode.
+Don't know what's going on here, maybe the DIY antenna is just not good.
+
+
+IF processing
+-------------
+
+Idea: Filter I/Q samples with 3rd order Butterworth filter
+instead of 10th order FIR filter.
+Implemented in branch "iirfilter".
+Speed is unchanged.
+Sound quality is not much changed for strong stations.
+Sound quality is a bit worse for weak stations (at 1 MS/s).
+Conclusion: not worthwhile.
+
+Idea: Downsample I/Q samples to ~ 250 kS/s BEFORE quadrature detection
+instead of immediately after detection. Would reduce amount of work in
+I/Q filtering for same or higher FIR filter order.
+Implemented in branch "filterif".
+CPU time reduced by ~ 25%.
+Sound quality very slightly worse.
+Conclusion: not worthwhile.
 
 
 Local radio stations

README

@@ -38,6 +38,7 @@ For the latest version, see https://github.com/jorisvr/SoftFM
 
 The Osmocom RTL-SDR library must be installed before you can build SoftFM.
 See http://sdr.osmocom.org/trac/wiki/rtl-sdr for more information.
+SoftFM has been tested successfully with RTL-SDR 0.5.3.
 
 To install SoftFM, download and unpack the source code and go to the
 top level directory. Then do like this:

TODO.txt

@@ -1,8 +1,2 @@
-* (quality) solve issues with bad sound due to strong antenna signal
-            (experiment with adaptive LNA gain or adaptive IF gain)
 * (speedup) maybe replace high-order FIR downsampling filter with 2nd order butterworth followed by lower order FIR filter
 * (feature) implement RDS decoding
-* (quality) consider FM demodulation with PLL instead of phase discriminator
-* (quality) consider pulse-counting discriminator
-* (speedup) consider downsampling of IF signal before FM detection
-            (work in progress in branch "filterif")

main.cc

@@ -144,7 +144,7 @@ void adjust_gain(SampleVector& samples, double gain)
  * This code runs in a separate thread.
  * The RTL-SDR library is not capable of buffering large amounts of data.
  * Running this in a background thread ensures that the time between calls
- * to RtlSdrSource::get_samples() is very short. 
+ * to RtlSdrSource::get_samples() is very short.
  */
 void read_source_data(RtlSdrSource *rtlsdr, DataBuffer<IQSample> *buf)
 {
@@ -219,7 +219,8 @@ void usage()
             "  -d devidx     RTL-SDR device index, 'list' to show device list (default 0)\n"
             "  -g gain       Set LNA gain in dB, or 'auto' (default auto)\n"
             "  -a            Enable RTL AGC mode (default disabled)\n"
-            "  -s ifrate     IF sample rate in Hz (default 1000000, min 900001)\n"
+            "  -s ifrate     IF sample rate in Hz (default 1000000)\n"
+            "                (valid ranges: [225001, 300000], [900001, 3200000]))\n"
             "  -r pcmrate    Audio sample rate in Hz (default 48000 Hz)\n"
             "  -M            Disable stereo decoding\n"
             "  -R filename   Write audio data as raw S16_LE samples\n"
@@ -355,8 +356,11 @@ int main(int argc, char **argv)
                 }
                 break;
             case 's':
-                // NOTE: RTL does not suppor sample rate 900 kS/s or lower
+                // NOTE: RTL does not support some sample rates below 900 kS/s
-                if (!parse_dbl(optarg, ifrate) || ifrate <= 900000) {
+                // Also, max sampling rate is 3.2 MS/s
+                if (!parse_dbl(optarg, ifrate) ||
+                     (ifrate < 225001) || (ifrate > 3200000) ||
+                     ((ifrate > 300000) && (ifrate < 900001))) {
                     badarg("-s");
                 }
                 break;

pyfm.py

@@ -1,8 +1,18 @@
 """
 Test lab for FM decoding algorithms.
+
+Use as follows:
+
+    >>> graw  = pyfm.lazyRawSamples('rtlsdr.dat', 1000000)
+    >>> gtune = pyfm.freqShiftIQ(graw, 0.25)
+    >>> bfir  = scipy.signal.firwin(20, 0.2, window='nuttall')
+    >>> gfilt = pyfm.firFilter(gtune, bfir)
+    >>> gbase = pyfm.quadratureDetector(gfilt, fs=1.0e6)
+    >>> fs,qs = pyfm.spectrum(gbase, fs=1.0e6)
 """
 
 import sys
+import datetime
 import types
 import numpy
 import numpy.fft
@@ -11,6 +21,20 @@ import numpy.random
 import scipy.signal
 
 
+def sincw(n):
+    """Return Sinc or Lanczos window of length n."""
+
+    w = numpy.zeros(n)
+    for i in xrange(n):
+        if 2 * i == n + 1:
+            w[i] = 1.0
+        else:
+            t = 2 * i / float(n+1) - 1
+            w[i] = numpy.sin(numpy.pi * t) / (numpy.pi * t)
+
+    return w
+
+
 def readRawSamples(fname):
     """Read raw sample file from rtl_sdr."""
 
@@ -364,3 +388,435 @@ def modulateAndReconstruct(sigfreq, sigampl, nsampl, fs, noisebw=None, ifbw=None
 
     return ampl1, phase1, noise1
 
+
+def rdsDemodulate(d, fs):
+    """Demodulate RDS bit stream.
+
+    d       :: block of baseband samples or lazy baseband sample stream
+    fs      :: sample frequency in Hz
+
+    Return (bits, levels)
+    where bits is a list of RDS data bits
+          levels is a list of squared RDS carrier amplitudes
+    """
+
+    # RDS carrier in Hz
+    carrier = 57000.0
+
+    # RDS bit rate in bit/s
+    bitrate = 1187.5
+
+    # Approximate nr of samples per bit.
+    bitsteps = round(fs / bitrate)
+
+    # Prepare FIR coefficients for matched filter.
+    #
+    # The filter is a root-raised-cosine with hard cutoff at f = 2/bitrate.
+    #   H(f) = cos(pi * f / (4*bitrate))   if f <  2*bitrate
+    #   H(f) = 0                           if f >= 2*bitrate
+    #
+    # Impulse response:
+    #   h(t) = ampl * cos(pi*4*bitrate*t) / (1 - 4 * (4*bitrate*t)**2)
+    #
+    wlen = int(1.5 * fs / bitrate)
+    w    = numpy.zeros(wlen)
+    for i in xrange(wlen):
+        t = (i - 0.5 * (wlen - 1)) * 4.0 * bitrate / fs
+        if abs(abs(t) - 0.5) < 1.0e-4:
+            # lim {t->0.5} {cos(pi*t) / (1 - 4*t**2)} = 0.25 * pi
+            w[i] = 0.25 * numpy.pi - 0.25 * numpy.pi * (abs(t) - 0.5)
+        else:
+            w[i] = numpy.cos(numpy.pi * t) / (1 - 4.0 * t * t)
+
+    # Use Sinc window to reduce leakage.
+    w *= sincw(wlen)
+
+    # Scale filter such that peak output of filter equals original amplitude.
+    w /= numpy.sum(w**2)
+
+    demod_phase = 0.0
+    prev_a1 = 0.0
+
+    prevb = numpy.array([])
+    pos   = 0
+
+    bits = [ ]
+    levels = [ ]
+
+    if not isinstance(d, types.GeneratorType):
+        d = [ d ]
+
+    for b in d:
+
+        n = len(b)
+
+        # I/Q demodulate with fixed 57 kHz phasor
+        ps  = numpy.arange(n) * (carrier / float(fs)) + demod_phase
+        dem = b * numpy.exp(-2j * numpy.pi * ps)
+        demod_phase = (demod_phase + n * carrier / float(fs)) % 1.0
+
+        # Merge with remaining data from previous block
+        prevb = numpy.concatenate((prevb[pos:], dem))
+        pos   = 0
+
+        # Detect bits.
+        while pos + bitsteps + wlen < len(prevb):
+
+            # Measure average phase of first impulse of symbol.
+            a1 = numpy.sum(prevb[pos:pos+wlen] * w)
+
+            # Measure average phase of second impulse of symbol.
+            a2 = numpy.sum(prevb[pos+bitsteps//2:pos+wlen+bitsteps//2] * w)
+
+            # Measure average phase in middle of symbol.
+            a3 = numpy.sum(prevb[pos+bitsteps//4:pos+wlen+bitsteps//4] * w)
+
+            # Calculate inner product of first impulse and previous symbol.
+            sym = a1.real * prev_a1.real + a1.imag * prev_a1.imag
+            prev_a1 = a1
+
+            if sym < 0:
+                # Consecutive symbols have opposite phase; this is a 1 bit.
+                bits.append(1)
+            else:
+                # Consecutive symbols are in phase; this is a 0 bit.
+                bits.append(0)
+
+            # Calculate inner product of first and second impulse.
+            a1a2 = a1.real * a2.real + a1.imag * a2.imag
+
+            # Calculate inner product of first impulse and middle phasor.
+            a1a3 = a1.real * a3.real + a1.imag * a3.imag
+
+            levels.append(-a1a2)
+
+            if a1a2 >= 0:
+                # First and second impulse are in phase;
+                # we must be woefully misaligned.
+                pos += 5 * bitsteps // 8
+            elif a1a3 > -0.02 * a1a2:
+                # Middle phasor is in phase with first impulse;
+                # we are sampling slightly too early.
+                pos += (102 * bitsteps) // 100
+            elif a1a3 > -0.01 * a1a2:
+                pos += (101 * bitsteps) // 100
+            elif a1a3 < 0.02 * a1a2:
+                # Middle phasor is opposite to first impulse;
+                # we are sampling slightly too late.
+                pos += (98 * bitsteps) // 100
+            elif a1a3 < 0.01 * a1a2:
+                pos += (99 * bitsteps) // 100
+            else:
+                # Middle phasor is zero; we are sampling just right.
+                pos += bitsteps
+
+    return (bits, levels)
+
+
+def rdsDecodeBlock(bits, typ):
+    """Decode one RDS data block.
+
+    bits    :: list of 26 bits
+    typ     :: expected block type, "A" or "B" or "C" or "C'" or "D" or "E"
+
+    Return (block, ber)
+    where block is a 16-bit unsigned integer if the block is correctly decoded,
+          block is None if decoding failed,
+          ber is 0 if the block is error-free,
+          ber is 1 if a single-bit error has been corrected,
+          ber is 2 if decoding failed.
+    """
+
+# TODO : there are still problems with bit alignment on weak stations
+# TODO : try to pin down the problem
+
+    # Offset word for each type of block.
+    rdsOffsetTable = { "A": 0x0fc, "B": 0x198,
+                       "C": 0x168, "C'": 0x350,
+                       "D": 0x1B4, "E": 0 }
+
+    # RDS checkword generator polynomial.
+    gpoly = 0x5B9
+
+    # Convert bits to word.
+    assert len(bits) == 26
+    w = 0
+    for b in bits:
+        w = 2 * w + b
+
+    # Remove block offset.
+    w ^= rdsOffsetTable[typ]
+
+    # Calculate error syndrome.
+    syn = w
+    for i in xrange(16):
+        if syn & (1 << (25 - i)):
+            syn ^= gpoly << (15 - i)
+
+    # Check block.
+    if syn == 0:
+        return (w >> 10, 0)
+
+    # Error detected; try all single-bit errors.
+    p = 1
+    for k in xrange(26):
+        if p == syn:
+            # Detected single-bit error in bit k.
+            w ^= (1 << k)
+            return (w >> 10, 1)
+        p <<= 1
+        if p & 0x400:
+            p ^= gpoly
+
+    # No single-bit error can explain this syndrome.
+    return (None, 2)
+
+
+class RdsData(object):
+    """Stucture to hold common RDS data fields."""
+
+    pi  = None
+    pty = None
+    tp  = None
+    ta  = None
+    ms  = None
+    af  = None
+    di  = None
+    pin = None
+    pserv   = None
+    ptyn    = None
+    ptynab  = None
+    rtext   = None
+    rtextab = None
+    time    = None
+
+    tmp_afs = None
+    tmp_aflen = 0
+    tmp_afmode = 0
+
+    ptyTable = [
+        'None',             'News',             
+        'Current Affairs',  'Information',
+        'Sport',            'Education',        
+        'Drama',            'Cultures',
+        'Science',          'Varied Speech',    
+        'Pop Music',        'Rock Music',
+        'Easy Listening',   'Light Classics M', 
+        'Serious Classics', 'Other Music',
+        'Weather & Metr',   'Finance',          
+        "Children's Progs", 'Social Affairs',
+        'Religion',         'Phone In',         
+        'Travel & Touring', 'Leisure & Hobby',
+        'Jazz Music',       'Country Music',    
+        'National Music',   'Oldies Music',
+        'Folk Music',       'Documentary',      
+        'Alarm Test',       'Alarm - Alarm !' ]
+
+    def __str__(self):
+
+        if self.pi is None:
+            return str(None)
+
+        s = 'RDS PI=%-5d' % self.pi
+
+        s += ' TP=%d' % self.tp
+
+        if self.ta is not None:
+            s += ' TA=%d' % self.ta
+        else:
+            s += '     '
+
+        if self.ms is not None:
+            s += ' MS=%d' % self.ms
+        else:
+            s += '     '
+
+        s += ' PTY=%-2d %-20s' % (self.pty, '(' + self.ptyTable[self.pty] + ')')
+
+        if self.ptyn is not None:
+            s += ' PTYN=%r' + str(self.ptyn).strip('\x00')
+
+        if self.di is not None or self.pserv is not None:
+            s += '\n   '
+            if self.di is not None:
+                distr = '('
+                distr += 'stereo' if self.di & 1 else 'mono'
+                if self.di & 2:
+                    distr += ',artificial'
+                if self.di & 4:
+                    distr += ',compressed'
+                if self.di & 8:
+                    distr += ',dynpty'
+                distr += ')'
+                s += ' DI=%-2d %-37s' % (self.di, distr)
+            else:
+                s += 45 * ' '
+            if self.pserv is not None:
+                s += ' SERV=%r' % str(self.pserv).strip('\x00')
+
+        if self.time is not None or self.pin is not None:
+            s += '\n   '
+            if self.time is not None:
+                (day, hour, mt, off) = self.time
+                dt = datetime.date.fromordinal(day + datetime.date(1858, 11, 17).toordinal())
+                s += ' TIME=%04d-%02d-%02d %02d:%02d UTC ' % (dt.year, dt.month, dt.day, hour, mt)
+            else:
+                s += 27 * ' '
+            if self.pin is not None:
+                (day, hour, mt) = self.pin
+                s += ' PIN=d%02d %02d:%02d' % (day, hour, mt)
+            else:
+                s += 14 * ' '
+
+        if self.af is not None:
+            s += '\n    AF='
+            for f in self.af:
+                if f > 1.0e6:
+                    s += '%.1fMHz ' % (f * 1.0e-6)
+                else:
+                    s += '%.0fkHz ' % (f * 1.0e-3)
+
+        if self.rtext is not None:
+            s += '\n    RT=%r' % str(self.rtext).strip('\x00')
+
+        return s
+
+
+def rdsDecode(bits, rdsdata=None):
+    """Decode RDS data stream.
+
+    bits    :: list of RDS data bits
+    rdsdata :: optional RdsData object to store RDS information
+
+    Return (rdsdata, ngroups, errsoft, errhard)
+    where rdsdata is the updated RdsData object
+          ngroup  is the number of correctly decoded RDS groups
+          errsoft is the number of correctable bit errors
+          errhard is the number of uncorrectable bit errors
+    """
+
+    if rdsdata is None:
+        rdsdata = RdsData()
+
+    ngroup = 0
+    errsoft = 0
+    errhard = 0
+
+    p = 0
+    n = len(bits)
+    while p + 4 * 26 <= n:
+
+        (wa, ea) = rdsDecodeBlock(bits[p:p+26], "A")
+        if wa is None:
+            errhard += 1
+            p += 1
+            continue
+
+        (wb, eb) = rdsDecodeBlock(bits[p+26:p+2*26], "B")
+        if wb is None:
+            errhard += 1
+            p += 1
+            continue
+
+        if (wb >> 11) & 1:
+            (wc, ec) = rdsDecodeBlock(bits[p+2*26:p+3*26], "C'")
+        else:
+            (wc, ec) = rdsDecodeBlock(bits[p+2*26:p+3*26], "C")
+        if wc is None:
+            errhard += 1
+            p += 1
+            continue
+
+        (wd, ed) = rdsDecodeBlock(bits[p+3*26:p+4*26], "D")
+        if wd is None:
+            errhard += 1
+            p += 1
+            continue
+
+        errsoft += ea + eb + ec + ed
+        ngroup += 1
+
+        # Found an RDS group; decode it.
+        typ  = (wb >> 12)
+        typb = (wb >> 11) & 1
+
+        # PI, TP, PTY are present in all groups
+        rdsdata.pi  = wa
+        rdsdata.tp  = (wb >> 10) & 1
+        rdsdata.pty = (wb >> 5) & 0x1f
+
+        if typ == 0:
+            # group type 0: TA, MS, DI, program service name 
+            rdsdata.ta = (wb >> 4) & 1
+            rdsdata.ms = (wb >> 3) & 1
+            dseg = wb & 3
+            if rdsdata.di is None:
+                rdsdata.di = 0
+            rdsdata.di &= ~(1 << dseg)
+            rdsdata.di |= (((wb >> 2) & 1) << dseg)
+            if rdsdata.pserv is None:
+                rdsdata.pserv = bytearray(8)
+            rdsdata.pserv[2*dseg]   = wd >> 8
+            rdsdata.pserv[2*dseg+1] = wd & 0xff
+
+        if typ == 0 and not typb:
+            # group type 0A: alternate frequencies
+            for f in ((wc >> 8), wc & 0xff):
+                if f >= 224 and f <= 249:
+                    rdsdata.tmp_aflen = f - 224
+                    rdsdata.tmp_aflfmode = 0
+                    rdsdata.tmp_afs = [ ]
+                elif f == 250 and rdsdata.tmp_aflen > 0 and len(rdsdata.tmp_afs) < rdsdata.tmp_aflen:
+                    rdsdata.tmp_aflfmode = 1
+                elif f >= 1 and f <= 204 and rdsdata.tmp_aflen > 0 and len(rdsdata.tmp_afs) < rdsdata.tmp_aflen:
+                    if rdsdata.tmp_aflfmode:
+                        rdsdata.tmp_afs.append(144.0e3 + f * 9.0e3)
+                    else:
+                        rdsdata.tmp_afs.append(87.5e6 + f * 0.1e6)
+                    if len(rdsdata.tmp_afs) == rdsdata.tmp_aflen:
+                        rdsdata.af = rdsdata.tmp_afs
+                        rdsdata.tmp_aflen = 0
+                        rdsdata.tmp_afs = [ ]
+                    rdsdata.tmp_aflfmode = 0
+
+        if typ == 1:
+            # group type 1: program item number
+            rdsdata.pin = (wd >> 11, (wd >> 6) & 0x1f, wd & 0x3f)
+
+        if typ == 2:
+            # group type 2: radio text
+            dseg = wb & 0xf
+            if rdsdata.rtext is None or ((wb >> 4) & 1) != rdsdata.rtextab:
+                rdsdata.rtext = bytearray(64)
+            rdsdata.rtextab = (wb >> 4) & 1
+            if typb:
+                rdsdata.rtext[2*dseg]   = (wd >> 8)
+                rdsdata.rtext[2*dseg+1] = wd & 0xff
+            else:
+                rdsdata.rtext[4*dseg]   = (wc >> 8)
+                rdsdata.rtext[4*dseg+1] = wc & 0xff
+                rdsdata.rtext[4*dseg+2] = (wd >> 8)
+                rdsdata.rtext[4*dseg+3] = wd & 0xff
+
+        if typ == 4 and not typb:
+            # group type 4A: clock-time and date
+            rdsdata.time = (((wb & 3) << 15) | (wc >> 1),
+                            ((wc & 1) << 4) | (wd >> 12),
+                            (wd >> 6) & 0x3f, (wd & 0x1f) - (wd & 0x20))
+
+        if typ == 10 and not typb:
+            # group type 10A: program type name
+            dseg = wb & 1
+            if rdsdata.ptyn is None or ((wb >> 4) & 1) != rdsdata.ptynab:
+                rdsdata.ptyn = bytearray(8)
+            rdsdata.ptynab = (wb >> 4) & 1
+            rdsdata.ptyn[4*dseg]    = (wc >> 8)
+            rdsdata.ptyn[4*dsseg+1] = wc & 0xff
+            rdsdata.ptyn[4*dseg+2]  = (wd >> 8)
+            rdsdata.ptyn[4*dsseg+3] = wd & 0xff
+        
+        # Go to next group.
+        p += 4 * 26
+
+    return (rdsdata, ngroup, errsoft, errhard)
+