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MATLAB 程式設計入門篇音訊讀寫、錄製與播放 (Audio Reading, Writing, Recording, and Playback) 張智星 (Roger Jang) 台大資訊系多媒體檢索實驗室 CSIE/NTU, MIR Lab

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音訊的基本介紹聲音訊號（Audio Signals）簡稱音訊，泛指由人耳聽到的各種聲音的訊號。音訊的基本聲學特徵音量（Volume）：聲音的大小稱為音量，又稱為力度、強度（Intensity）或是能量（Energy）。音量越大，代表音訊波形的震幅越大。音高（Pitch）：聲音的基本頻率（Fundamental Frequency）越高，代表音高越高（例如女高音的歌聲）；反之，聲音的基本頻率越低，代表音高越低（例如男低音的歌聲）。音色（Timbre）：音訊波形在每個週期內的變化，就形成了此音訊的音色。不同的音色即代表不同的音訊內容，例如不同的字有不同的發音，或是不同的歌手有不同的特色，這些都是由於音色不同而產生。 Demo via CoolEdit

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Basics about Audio Signals Audio signals: signals that are audible to human Basic perceptible acoustic features of speech Volume (音量): the amplitude of audio signals Also known as intensity, or energy. Pitch (音高): Fundamental frequency (the number of fundamental periods in a second) in audio signals. Usually males have a lower pitch while females have a higher one Timbre (音色): Waveform inside a fundamental period. Different vowels have different timbres Different singers also have different timbres. Demo via CoolEdit

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Time-domain Features Time-domain audio features presented in a frame (analysis window of 20-40 ms)

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Frequency-domain Features Frequency-domain audio features in a frame Energy: Sum of power spectrum Pitch: Distance between harmonics Timbre: Smoothed spectrum

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音檔的讀取、寫檔與播放使用audioread讀取wav檔案，畫出音訊的波形並播放此音訊： audioRead01.m

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Read, Write and Playback Use “audioread” to read a .wav file, plot its waveform and play the sound. audioRead01.m

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Read Metadata from .wav Files Reading metadata info=audioInfo('file'); Different types of audio files may return different fields of info. Two types of reading data from audio files Read audio signals y= audioread('file') Read metadata info=audioinfo('file')

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Metadata of Other Audio Files *.aif audioInfo02.m

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Scaling of Audio Signals by “audioread” Internal data types of audio signals in a file (音訊檔案內部儲存方式) 8 bits  uint8, [0, 28-1] 16 bits  int16, [-215, 215-1] MATLAB’s method to scale raw audio signals to the range [-1, 1] 8 bits  (y-128)/128 16 bits  y/32768

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讀取雙聲道檔案 audioread 可以讀取雙聲道或立體聲（Stereo）的音檔，此時傳回的變數具有兩直行，每一直行代表一個聲道的音訊。

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Read Stereo Audio Files “audioread” can also read stereo audio files. The returned variable has two columns representing two channels of audio signals.

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讀取部分音檔如果音檔很大，無法一次讀入記憶體，我們也可以使用 audioread 來讀出音檔的一部份，例如： audioRead05.m

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Read a Portion Only If the audio file is too big, we can read a portion from the whole file: audioRead05.m

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聲音訊號的播放一旦我們可以讀入音訊檔案，就可以對聲音訊號進行各種處理，例如增大或減小音量、提高或降低音高、消除雜訊等。要確認處理後的聲音訊號是否符合所需，就要能夠把音訊直接透過接到電腦的喇叭播放出來，本節就是要介紹如何使用 MATLAB 來進行音訊的播放。

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Playback of Audio Signals Once we have read audio signals, we can perform all kinds of processing, such as volume modification, pitch scaling, noise reduction, etc. To verify the result, you need to play the audio via speakers, as shown in the following examples.

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播放聲音 (1/2) 一旦 MATLAB 讀入音訊資料，並將之設定成工作空間中的變數後，我們就可以使用 sound 指令來直接播放此變數。

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Playback (1/2) We can use “sound” to play audio signals that has been read and stored as a variable in MATLAB’s workspace.

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播放聲音 (2/2) 若要控制聲音的播放模式，則必須採用功能較為強大的指令： audioplayer play playblocking

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Playback (2/2) If you want to control the playback mode, you need to invoke other commands: audioplayer play playblocking

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改變音訊的震幅我們在第一節提到過，聲音的音量是由聲波的震幅來決定，因此我們可藉由震幅的大小來改變音量，例如： playVolume01.m

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Change of Audio Amplitude Volume of audio signals is determine by their amplitude. Here are modify amplitude to change the volume: playVolume01.m

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改變音訊播放的取樣率 (1/2) 如果在播放時，改變取樣頻率，就會改變整個音訊的時間長度，進而影響到音高。在下例中，我們漸漸提高播放時的取樣頻率，聽到的聲音就會越來越快、越來越高，最後出現像唐老鴨的聲音。為什麼？ playFs01.m

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Change of Sample Rates (1/2) Change of sample rate during playback  Change of duration  Change of the perceived pitch Increase the sample rate during playback, and you’ll hear Donald Duck (唐老鴨). Why? playFs01.m

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改變音訊播放的取樣率 (2/2) 反之，如果漸漸降低播放的頻率，聽到的聲音就會越來越慢、越來越低，最後出現像牛叫的聲音。 playFs02.m

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Change of Sample Rates (2/2) On the other hand, decrease the sample rate during playback, and you’ll hear cow moo… playFs02.m

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Observations Observations Larger sample rate for playback leads to… Shorter duration and higher pitch Smaller sample rate for playback leads to... Longer duration and lower pitch How to… Generate higher pitch without duration change?  Pitch modification Create longer duration without pitch change?  Duration modification Demo

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改變符號及改變時序如果我們將聲波訊號上下顛倒，聽到的聲音基本上是一樣的，但是如果前後顛倒，聽到的聲音就如同錄音帶「倒放」的聲音，聽起來很像是某種外國語音，請試試下列範例：範例20-11： playReverse01.m

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Change of Sign & Time Sequence Change of sign  No perceptional difference Reverse sequence  Sounds like another spoken language? 範例20-11： playReverse01.m

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同步及非同步播放通常在播放音訊時，MATLAB 停止進行其他動作，直到音訊播放完畢後，才會再進行其他指令的運算，此種運作方式稱為「同步式」（Synchronous）。若需要一邊播放、一邊進行其他運算，就必須使用「非同步式」（Asynchronous）的播放方式。範例20-12： playSync01.m

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非同步播放在此例中，我們會聽到類似男女兩部合唱，一快一慢，這是因為 sound 指令的預設播放方式就是「非同步」。範例20-13： playSync02.m

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Playback Modes There are two playback modes Synchronous mode: Block everything till the end of playback. Asychronous mode: Nonblocking playSync01.m playSync02.m

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音量自動調整另一個類似的指令是 soundsc，可針對音訊變數的數值先進行正規化（介於 –1 和 1 中間）後，再送到喇叭播放，以達到最好的播放效果。 soundsc01.m

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Automatic Volume Adjustment “soundsc” adjusts the volume (by normalizing the signals to have max of 1 or min of -1) before playback soundsc01.m

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聲音訊號的錄製我們在第一節已經說明了如何讀取音訊檔案，並在第二節說明如何播放。MATLAB 也支援直接由麥克風讀取訊號，因此可以直接進行聲音的錄製，所使用的指令是 audiorecorder recordblocking

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Recording of Audio Signals We can use the following MATLAB commands for recording from the microphone directly: audiorecorder recordblocking

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音訊的錄製範例 (1/2) 使用預設參數，由麥克風進行3秒錄音： audioRecord01.m 預設錄音參數取樣頻率為 8000 Hz 取樣點解析度為 8 bits 單聲道錄音

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Recording (1/2) Use default setting for 3-sec recording: audioRecord01.m Default settings for recording Sample rate: 8000 Hz Bit resolution: 8 bits Mono

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音訊的錄製範例 (2/2) 設定各項錄音參數來進行3秒錄音，並畫出波形： audioRecord02.m

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Recording (2/2) Set recording parameters, record, plot the waveform: audioRecord02.m

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聲音訊號的寫檔 (1/2) 我們也可以經由 MATLAB 將音訊資料直接儲存為音訊檔案，以便直接在電腦播放。寫入音訊檔案的指令是 audiowrite，其格式為: audiowrite(audioFile, y, fs) audioFile 則是欲寫入資料的檔案名稱，y 是音訊變數，fs 是取樣頻率。範例：audioWrite01.m

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Storing Audio Files (1/2) We can use “audiowrite” to save audio files, with the following I/O format: audiowrite(audioFile, y, fs) audioFile: file to write to, y: audio signals, fs: sample rate 範例：audioWrite01.m

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聲音訊號的寫檔 (2/2) 錄音、播放、存檔的範例：範例：audioWrite02.m

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Storing Audio Files (2/2) Example of recording, playback, and saving: 範例：audioWrite02.m

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Cross-version Issues File mapping for different versions of MATLAB wavread  audioread wavwrite  audiowrite wavplay  audioplayer, sound wavrecord  audiorecorder

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Supplementary Material Other resources ASPR: Audio Signal Processing and Recognition Texts for this set of slides can be found at Chapter 4. Pitch tracking by visual inspection can be found at Section 4 of Chapter 5. (Slides)

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