REM. Real-time audio spectrum analyser in BBC BASIC for SDL 2.0
      REM. Richard Russell, 26th April 2023
      
      HIMEM = PAGE + 12 * 1024 * 1024
      MODE 7
      SYS "SDL_SetWindowTitle", @hwnd%, "Audio Spectrum Analyser", @memhdc%
      
      REM. Find number of audio capture devices:
      SYS "SDL_GetNumAudioDevices", 1, @memhdc% TO numDevices%
      IF numDevices% = 0 ERROR 100, "No audio capture devices are available"
      
      REM. List the capture device names and put them into an array:
      DIM deviceName$(numDevices%)
      PRINT "Available audio capture devices:"'
      FOR index% = 1 TO numDevices%
        SYS "SDL_GetAudioDeviceName", index%-1, 1, @memhdc% TO pname%%
        IF @platform% AND &40 ELSE pname%% = !^pname%%
        deviceName$(index%) = $$pname%%
        PRINT ;index% ": " deviceName$(index%)
      NEXT
      PRINT
      
      REM. Get selected device from user:
      REPEAT
        INPUT "Enter device number: " index%
      UNTIL index% >= 1 AND index% <= numDevices%
      
      MODE 8
      OFF
      
      ON ERROR PROCcleanup : MODE 7 : PRINT REPORT$ : END
      ON CLOSE PROCcleanup : QUIT
      
      Window% = 1024
      DIM In(Window%-1), Out(Window%-1), DFT(Window%-1, Window%-1), Hanning(Window%-1)
      DIM want{freq%, format{l&,h&}, channels&, silence&, samples%, size%, callback%%, userdata%%}
      DIM have{} = want{}
      want.freq% = 22050
      want.format.h& = &80 : REM AUDIO_S16LSB
      want.format.l& = &10 : REM AUDIO_S16LSB
      want.channels& = 2
      want.samples% = Window%
      
      SYS "SDL_OpenAudioDevice", deviceName$(index%), 1, want{}, have{}, 9, @memhdc% TO Device%
      IF Device% = 0 ERROR 100, "Couldn't open audio device " + deviceName$(index%)
      
      SamplingRate% = have.freq%
      
      REM. Draw axes and labels:
      ORIGIN 128,64
      LINE -2,-2,-2,903  : REM Y-axis
      LINE -2,-2,1022,-2 : REM X-axis
      VDU 5
      FOR F = 0 TO 11
        X% = F/5.5125*512
        LINE X%-2,-2,X%-2,-12      : REM X ticks
        MOVE X%-10,-20 : PRINT ;F*SamplingRate%/11025/2; : REM X labels
      NEXT
      PRINT " kHz";
      FOR D = 0 TO -90 STEP -10
        Y% = 903 + D*10
        LINE -2,Y%-2,-12,Y%-2      : REM Y ticks
        MOVE -80,Y%+12 : PRINT ;D; : REM Y labels
        IF D = 0 PRINT " dB";
      NEXT
      COLOR 1,128,255,128
      GCOL 1
      VDU 28,8,29,71,0     : REM Set text viewport
      VDU 24,0;0;1022;958; : REM Set graphics viewport
      
      REM. Prepare DFT coefficients:
      FOR I% = 0 TO Window%-2 STEP 2
        r = -PI*I%/Window%
        FOR J% = 0 TO Window%-1
          DFT(I%+0,J%) = COS(r*J%)
          DFT(I%+1,J%) = SIN(r*J%)
        NEXT
      NEXT I%
      
      REM. Prepare Hanning window function:
      FOR I% = 0 TO Window%-1
        Hanning(I%) = COS(PI*2*(I%/Window%-0.5))+1
      NEXT
      
      *REFRESH OFF
      REM. Wait for and process audio data:
      SYS "SDL_PauseAudioDevice", Device%, 0, @memhdc%
      REPEAT
        PROCprocess(Device%, Window%)
      UNTIL FALSE
      END
      
      DEF PROCcleanup
      *REFRESH ON
      Device% += 0
      IF Device% THEN
        SYS "SDL_PauseAudioDevice", Device%, 1, @memhdc%
        SYS "SDL_CloseAudioDevice", Device%, @memhdc%
        Device% = 0
      ENDIF
      ENDPROC
      
      REM. Acquire audio and perform Fourier Transform:
      DEF PROCprocess(D%, N%)
      LOCAL I%, L%, P%, R%, U%, V%, b%(), v
      DIM b%(N%-1) : R% = N% * 4
      REPEAT
        SYS "SDL_DequeueAudio", D%, ^b%(P%), R%, @memhdc% TO I%
        P% += I% DIV 4 : R% -= I%
      UNTIL R% <= 0
      FOR I% = 0 TO N%-1
        U% = (b%(I%) << 16) >> 16 : V% = b%(I%) >> 16
        In(I%) = (U% + V%)/N%
      NEXT
      REPEAT
        SYS "SDL_DequeueAudio", D%, ^b%(0), N% * 4, @memhdc% TO I%
      UNTIL I% = 0
      In() *= Hanning()    : REM Window Function
      Out() = DFT() . In() : REM Fourier Transform
      CLS
      P% = 4
      FOR I% = 0 TO N%-2 STEP 2
        v = Out(I%)^2 + Out(I%+1)^2
        IF v=0 L%=0 ELSE L%=100*LOGv
        PLOT P%,I%,L% : P%=5
      NEXT
      *REFRESH
      ENDPROC