Reading and writing audio files in Signal Processing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read an audio file using the soundfile library.

Signal Processing

import soundfile as sf

data, samplerate = sf.read([1])

Drag options to blanks, or click blank then click option'

Aread('audio.wav')

Baudio.wav

C'audio.wav'

Dsf.read('audio.wav')

Attempts:

3 left

💡 Hint

Common Mistakes

Not putting quotes around the filename.

Passing the function call inside the argument.

The sf.read() function requires the filename as a string, so it must be enclosed in quotes.

2fill in blank

medium

Complete the code to write audio data to a file with a given sample rate.

Signal Processing

import soundfile as sf

sf.write('output.wav', [1], 44100)

Drag options to blanks, or click blank then click option'

A44100

Bdata

C'data'

Dsamplerate

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the sample rate instead of the audio data.

Passing the variable name as a string.

The sf.write() function needs the audio data array as the second argument.

3fill in blank

hard

Fix the error in the code to read a stereo audio file and print its shape.

Signal Processing

import soundfile as sf

data, samplerate = sf.read('stereo.wav')
print(data[1])

Drag options to blanks, or click blank then click option'

A.shape

B(shape)

C[shape]

D.shape()

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses after shape like a function.

Using brackets instead of dot notation.

The shape attribute of a NumPy array is accessed without parentheses.

4fill in blank

hard

Fill in the blank to create a dictionary with filenames as keys and their sample rates as values.

Signal Processing

files = ['a.wav', 'b.wav', 'c.wav']
sample_rates = {file: [1] for file in files}

Drag options to blanks, or click blank then click option'

Asf.read(file)[1]

Bsf.read(file)[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing data and sample rate positions.

Trying to assign the whole tuple as value.

The sf.read(file) returns a tuple (data, samplerate). We want to store the sample rate as the value, which is the second item at index 1.

5fill in blank

hard

Fill all three blanks to filter audio files longer than 5 seconds and create a dictionary with filename and duration.

Signal Processing

files = ['x.wav', 'y.wav', 'z.wav']
durations = { [1] : [2] / [3] for [1] in files if (data, samplerate) := sf.read([1]) and len(data) / samplerate > 5 }

Drag options to blanks, or click blank then click option'

Afile

Blen(data)

Csamplerate

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for keys or values.

Not calculating duration correctly.

The dictionary comprehension uses the filename as key, and duration is calculated by dividing the length of data by the sample rate.

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