Reading and writing audio files in Signal Processing - Mini Project: Build & Apply

Reading and writing audio files

📖 Scenario: You are working on a simple audio processing project. You need to read an audio file, process it, and save the result as a new audio file.

🎯 Goal: Learn how to read an audio file into a data array, set up a configuration variable for processing, apply a simple operation on the audio data, and write the processed audio back to a new file.

📋 What You'll Learn

Use the scipy.io.wavfile module to read and write WAV audio files

Create a variable to hold the sample rate and audio data

Create a configuration variable for volume scaling

Apply volume scaling to the audio data

Write the processed audio data to a new WAV file

Print the sample rate and shape of the audio data

💡 Why This Matters

🌍 Real World

Reading and writing audio files is essential in music production, podcast editing, and any audio data analysis.

💼 Career

Audio engineers, data scientists working with sound, and software developers use these skills to manipulate and analyze audio data.

1

Read the audio file

Use scipy.io.wavfile.read to read the audio file named 'input.wav'. Create two variables called sample_rate and audio_data to store the sample rate and audio data respectively.

Signal Processing

# Your code here to read 'input.wav' into sample_rate and audio_data

Hint

Use wavfile.read from scipy.io to load the WAV file.

2

Set volume scaling factor

Create a variable called volume_scale and set it to 0.5 to reduce the audio volume by half.

Signal Processing

from scipy.io import wavfile
sample_rate, audio_data = wavfile.read('input.wav')
# Your code here to set volume_scale to 0.5

Hint

Just assign 0.5 to volume_scale.

3

Apply volume scaling to audio data

Create a new variable called scaled_audio by multiplying audio_data with volume_scale. Convert the result to the same data type as audio_data using .astype().

Signal Processing

from scipy.io import wavfile
sample_rate, audio_data = wavfile.read('input.wav')
volume_scale = 0.5
# Your code here to create scaled_audio by scaling audio_data

Hint

Multiply audio_data by volume_scale and convert back to original data type.

4

Write the scaled audio and print info

Use wavfile.write to save scaled_audio to a file named 'output.wav' with the sample rate sample_rate. Then print the sample rate and the shape of audio_data.

Signal Processing

from scipy.io import wavfile
sample_rate, audio_data = wavfile.read('input.wav')
volume_scale = 0.5
scaled_audio = (audio_data * volume_scale).astype(audio_data.dtype)
# Your code here to write scaled_audio and print sample_rate and audio_data shape

Hint

Use wavfile.write('output.wav', sample_rate, scaled_audio) to save the file. Then print sample_rate and audio_data.shape.

Practice

1. What does reading an audio file using a library like soundfile typically provide you?

easy

A. The audio data array and the sample rate

B. Only the file name

C. The audio file size in bytes

D. The duration of the audio in minutes

Solution

1. Step 1: Understand the purpose of reading audio files

   Reading audio files extracts the sound data and the sample rate for processing.

2. Step 2: Identify what the library returns

   Libraries like soundfile return the audio data as an array and the sample rate as an integer.

3. Final Answer:

   The audio data array and the sample rate -> Option A

4. Quick Check:

   Reading audio files = data + sample rate [OK]

Hint: Remember: reading audio gives data and sample rate [OK]

Common Mistakes:

• Thinking reading returns only file metadata
• Confusing sample rate with file size
• Assuming reading returns audio duration only

2. Which of the following is the correct way to read an audio file using the soundfile library in Python?

easy

A. data, samplerate = soundfile.read('audio.wav')

B. data = soundfile.load('audio.wav')

C. samplerate, data = soundfile.read('audio.wav')

D. soundfile.open('audio.wav')

Solution

1. Step 1: Recall the soundfile.read function signature

   The function returns two values: data array and sample rate, in that order.

2. Step 2: Match the correct syntax

   data, samplerate = soundfile.read('audio.wav') correctly assigns data and sample rate from soundfile.read.

3. Final Answer:

   data, samplerate = soundfile.read('audio.wav') -> Option A

4. Quick Check:

   Correct syntax = data, samplerate = read(file) [OK]

Hint: Remember order: data first, then sample rate [OK]

Common Mistakes:

• Swapping data and sample rate order
• Using non-existent load method
• Not unpacking returned tuple

3. What will be the output type of the variable data after running this code?

import soundfile as sf
data, samplerate = sf.read('example.wav')

medium

A. An integer representing duration

B. A list of integers

C. A string with audio info

D. A NumPy array of floats

Solution

1. Step 1: Understand sf.read output

   The soundfile.read function returns audio data as a NumPy array, usually with float values normalized between -1 and 1.

2. Step 2: Identify the data type

   The variable data will be a NumPy array of floats representing audio samples.

3. Final Answer:

   A NumPy array of floats -> Option D

4. Quick Check:

   Audio data = NumPy float array [OK]

Hint: Audio data is a NumPy float array, not list or string [OK]

Common Mistakes:

• Assuming data is a list instead of array
• Thinking data is a string or integer
• Confusing sample rate with data type

4. Identify the error in this code snippet that tries to write audio data to a file:

import soundfile as sf
sf.write('output.wav', samplerate, data)

medium

A. Missing import statement for NumPy

B. Arguments order is incorrect; data should come before samplerate

C. File extension must be .mp3, not .wav

D. No error; code is correct

Solution

1. Step 1: Check sf.write function signature

   The correct order is sf.write(file, data, samplerate), data comes before samplerate.

2. Step 2: Compare with given code

   The code passes samplerate before data, which is incorrect and will cause an error.

3. Final Answer:

   Arguments order is incorrect; data should come before samplerate -> Option B

4. Quick Check:

   Write args order = file, data, samplerate [OK]

Hint: Remember: write(file, data, samplerate) order [OK]

Common Mistakes:

• Swapping data and samplerate arguments
• Assuming file extension must be .mp3
• Thinking missing NumPy import causes write error

5. You want to read an audio file, double its volume, and save it back. Which code snippet correctly does this using soundfile?

hard

A. sr, data = sf.read('input.wav')
data = data * 2
sf.write('output.wav', sr, data)

B. data = sf.read('input.wav')
data = data * 2
sf.write('output.wav', data)

C. data, sr = sf.read('input.wav')
data = data * 2
sf.write('output.wav', data, sr)

D. data, sr = sf.read('input.wav')
sf.write('output.wav', sr, data * 2)

Solution

1. Step 1: Read audio correctly

   Use data, sr = sf.read('input.wav') to get data and sample rate.

2. Step 2: Double the volume

   Multiply the data array by 2 to increase volume.

3. Step 3: Write audio correctly

   Use sf.write('output.wav', data, sr) with data first, then sample rate.

4. Final Answer:

   data, sr = sf.read('input.wav') data = data * 2 sf.write('output.wav', data, sr) -> Option C

5. Quick Check:

   Read data, multiply, write data with sample rate [OK]

Hint: Read data, multiply, write with correct argument order [OK]

Common Mistakes:

• Swapping data and sample rate in write
• Not unpacking read return values
• Forgetting to multiply data before writing