Audio Fundamentals - Basics

Slides for a workshop on audio fundamentals.

Transcript

1. Audio Fundamentals
   Chinmay Pendharkar, Sonoport, 2013
   

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3. Sound Waves
   

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4. Analog vs Digital Audio
   

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5. Analog vs Digital Audio
   

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6. Digitization
   

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7. Digitization - Why?
   ● Easier to manipulate digital audio
   ● Easier to store digital audio
   ● Easier to transport digital audio
   ● Get back perfect original analog audio
   

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8. Digital Audio
   Analog
   Audio
   Analog - Digital
   Conversion
   Digital - Analog
   Conversion
   Digital
   Audio
   

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9. Sampling Frequency
   

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10. Sampling Frequency
    

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11. Sampling Theorem
    

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12. Sampling Theorem
    

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13. Sampling Theorem
    2 x (Max Frequency of Content) ≤ Sampling Frequency
    for perfect reconstruction of analog signal from digital
    

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14. Sampling Frequency
    2 x (Max Frequency of Content) ≤ Sampling Frequency.
    2 x ( 20000 Hz ) ≤ 44100 Hz .
    Upper Limit of Human Hearing
    

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15. Sampling
    ● Sampling Frequency > 44100 : Oversampling
    ● Sampling Frequency < 44100 : Undersampling
    ● 44100Hz =>
    ○ 44100 samples per second
    ○ 44100 data points per second
    ● Less computation vs Audio Fidelity
    

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16. Quantization / Bit Depth
    uint_16
    

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17. Quantization
    0x0000 (0)
    0xFFFF (65535)
    96dBFS
    ~ 120dB Human
    Hearing Range
    65535
    65534
    65533
    65532
    0
    

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18. Digital Audio
    Discretization of Time - Sampling
    Discretization of Amplitude -
    Quantization
    

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19. Digital Audio
    Common Formats:
    Sampling Rate : 44100 Hz
    Bit Depth : 16 bits
    Sampling Rate : 192000 Hz
    Bit Depth : 24 bits
    "CD Audio"
    "Mastering
    Quality"
    

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20. Digital Audio Formats
    ● Uncompressed
    ○ AIFF
    ○ WAV
    ● Compressed (Lossless)
    ○ FLAC
    ○ ALAC
    ● Compressed (Lossy)
    ○ MP3
    ○ AAC
    

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21. Real Time Audio on PC/Mac
    

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22. Audio Stacks
    Audio Hardware
    Drivers
    Operating System Audio API
    User Program/Application
    

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23. Audio
    Stack -
    Windows
    

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24. Audio Stack - OSX/iOS
    

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25. Audio Stack - Linux
    

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26. Audio
    Stack -
    Android
    

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27. Audio Stacks
    Audio Hardware
    Drivers
    Operating System Audio API
    User Program/Application
    

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28. Audio Stack
    Audio Hardware DAC
    Digital Analog Converter
    

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29. Audio Stacks
    44.1kHz / 16bits ~= 172 kB per second
    DAC
    Digital Data Buffer
    "Magic"
    OS
    

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30. DAC Schemes
    DAC
    Digital Data Buffer #1
    OS
    Digital Data Buffer #2
    "Magic"
    1s
    

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31. Buffer Size - Latency
    ● Time taken = Max time to fill next buffer
    ● Time taken = Min time to new sound output
    Stereo = 2 channels
    

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32. Buffer Size - Latency
    Ideally we want....
    ● Fill up next buffer well before current is fully
    output. (No clicks/silence)
    ● Have no delay between audio being sent to
    the OS and output. (No buffering)
    

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33. Buffer Size - Latency
    Typical Buffer Sizes
    ● 256 * 2 * 2 = 1024 bytes = 5.8ms latency
    ● 512 *2 * 2 = 2048 byes = 11.6ms latency
    ● 1024 * 2 * 2 = 4096 byes = 23.2ms latency
    Assuming 2 channel stereo and 16bit depth
    

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34. Callback vs R/W IO
    Two common schemes for audio drivers.
    ● Read-Write
    ● Callback
    

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35. Read-Write
    ● Direct R/W access to "digital data buffer".
    ● Blocking
    ● PaError Pa_WriteStream ( PaStream *
    stream, void * buffer, long frames )
    ● PaError Pa_ReadStream ( PaStream *
    stream, void * buffer, long frames )
    

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36. Callback
    ● Callback when more data is needed.
    ● Non-Blocking
    ● PaError Pa_OpenStream (PaStream **
    stream, ... , double sampleRate, long
    framesPerBuffer, ... ,PaStreamCallback *
    streamCallback, void * userData)
    ● typedef int PaStreamCallback(const void
    *input, void *output, unsigned long
    frameCount, ... , void *userData)
    

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37. Read-Write Example - C
    // From http://portaudio.com/docs/v19-doxydocs/blocking_read_write.html
    err = Pa_Initialize();
    if( err != paNoError ) goto error;
    /* --SKIPPED-- -- Initalize Variables -- --SKIPPED-- */
    /* -- setup stream -- */
    err = Pa_OpenStream(
    &stream, &inputParameters,&outputParameters,
    SAMPLE_RATE,FRAMES_PER_BUFFER, paClipOff,
    NULL, /* no callback, use blocking API */
    NULL ); /* no callback, so no callback userData */
    /* -- start stream -- */
    err = Pa_StartStream( stream );
    printf("Wire on. Will run one minute.\n"); fflush(stdout);
    

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38. Read-Write Example - C
    /* -- Here's the loop where we pass data from input to output -- */
    for( i=0; i<(60*SAMPLE_RATE)/FRAMES_PER_BUFFER; ++i )
    {
    err = Pa_WriteStream( stream, sampleBlock, FRAMES_PER_BUFFER );
    err = Pa_ReadStream( stream, sampleBlock, FRAMES_PER_BUFFER );
    }
    /* -- Now we stop the stream -- */
    err = Pa_StopStream( stream );
    /* -- don't forget to cleanup! -- */
    err = Pa_CloseStream( stream );
    Pa_Terminate();
    return 0;
    }
    

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39. Callback Example - C
    // http://portaudio.com/docs/v19-doxydocs/paex__sine_8c_source.html
    err = Pa_Initialize();
    if( err != paNoError ) goto error;
    err = Pa_OpenStream( &stream,NULL,&outputParameters,
    SAMPLE_RATE, FRAMES_PER_BUFFER, paClipOff,
    patestCallback, &data );
    if( err != paNoError ) goto error;
    printf("Setup Done\n");
    err = Pa_SetStreamFinishedCallback( stream, &StreamFinished );
    err = Pa_StartStream( stream );
    printf("Play for %d seconds.\n", NUM_SECONDS );
    Pa_Sleep( NUM_SECONDS * 1000 );
    err = Pa_StopStream( stream );
    err = Pa_CloseStream( stream );
    Pa_Terminate();
    return err;
    

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40. Callback Example - C
    static int patestCallback( const void *inputBuffer, void *outputBuffer,
    unsigned long framesPerBuffer,
    const PaStreamCallbackTimeInfo* timeInfo,
    PaStreamCallbackFlags statusFlags,
    void *userData )
    {
    for( i=0; i*out++ = data->sine[data->left_phase]; /* left */
    *out++ = data->sine[data->right_phase]; /* right */
    data->left_phase += 1;
    if( data->left_phase >= TABLE_SIZE ) data->left_phase -=
    TABLE_SIZE;
    data->right_phase += 3;
    if( data->right_phase >= TABLE_SIZE ) data->right_phase -=
    TABLE_SIZE;
    }
    return paContinue;
    }
    

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41. Callback Example - AS3
    var mySound:Sound = new Sound();
    function sineWaveGenerator(event:SampleDataEvent):void {
    for ( var c:int=0; c<8192; c++ ) {
    event.data.writeFloat(Math.sin((Number(c+event.
    position)/Math.PI/2))*0.25);
    event.data.writeFloat(Math.sin((Number(c+event.
    position)/Math.PI/2))*0.25);
    }
    }
    mySound.addEventListener(SampleDataEvent.SAMPLE_DATA,
    sineWaveGenerator);
    mySound.play();
    

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42. Hands On
    

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