Reading and writing audio files in Signal Processing

Introduction

We read audio files to listen, analyze, or change sounds. We write audio files to save our changes or create new sounds.

You want to analyze a song to find its beats or notes.

You need to save a recorded voice as a file.

You want to change the speed or pitch of a sound and save it.

You are building a program that plays sounds from files.

You want to convert audio from one format to another.

Syntax

Signal Processing

import soundfile as sf

# Read audio file
data, samplerate = sf.read('filename.wav')

# Write audio file
sf.write('newfile.wav', data, samplerate)

The sf.read function returns the sound data and the sample rate.

The sf.write function saves the sound data with the given sample rate.

Examples

This reads the audio data and sample rate from 'speech.wav'.

Signal Processing

data, samplerate = sf.read('speech.wav')

This writes the audio data to a new file called 'copy.wav'.

Signal Processing

sf.write('copy.wav', data, samplerate)

You can read different audio formats like MP3 if supported.

Signal Processing

data, samplerate = sf.read('music.mp3')

Sample Program

This program reads an audio file, reduces its volume by half, and saves it as a new file.

Signal Processing

import soundfile as sf
import numpy as np

# Read an audio file
input_file = 'input.wav'
data, samplerate = sf.read(input_file)

# Print basic info
print(f'Original sample rate: {samplerate}')
print(f'Audio shape: {data.shape}')

# Make audio half volume
data_quiet = data * 0.5

# Save the quieter audio
output_file = 'output_quiet.wav'
sf.write(output_file, data_quiet, samplerate)

print(f'Wrote quieter audio to {output_file}')

Important Notes

Audio data is usually a NumPy array with values between -1 and 1.

Sample rate is how many samples per second the audio has, like 44100 Hz.

Make sure the output file extension matches the format you want to save.

Summary

Reading audio files lets you get sound data and sample rate for analysis or changes.

Writing audio files saves your sound data with a sample rate to a file.

Use libraries like soundfile to easily read and write many audio formats.

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