Nuts and bolts

FFT-calculation

• calculated using fftw3
• 1024 points
• averaged over 6 consecutive frames
• no window function (i.e. boxcar)
• new averaged fft is calculated every 250 ms

FM-demodulation

• original signal from the receiver is decimated with first-order CIC
• down factor = orig. sample rate / 192 kHz
• angle approximation is done to the decimated signal with fast atan2
• angles are differentiated
  • y(n) = x(n) - x(n-1)
• resulting differences are clipped (to 1 and -1) to remove spurious values
• this has suprisingly large effect on the audio
• signal is then decimated twice with factor 2, using short half-band filter
• resulting audio has sample rate of approx. 48 kHz

Websockets and WebAudio

• server to ui: binary mode
• ui to server: text mode
• frequency, bandwidth and spectrum gain are sent with spectrum data
• variables are passed as strings
• one fft point = one byte (unsigned integer)
• one audio frame is ~16 kB
• one audio sample = four bytes (single-precision float)
• audio is played using scriptprocessor