Audio signal processing in MATLAB - Mini Project: Build & Apply

Audio Signal Processing Basics

📖 Scenario: You are working on a simple audio processing task. You have a short audio signal represented as a vector of sample values. You want to analyze and modify this signal step-by-step.

🎯 Goal: Build a MATLAB script that loads a short audio signal, sets a threshold, filters the signal based on this threshold, and then outputs the filtered signal.

📋 What You'll Learn

Create a vector variable with exact audio sample values

Create a threshold variable with an exact value

Use a logical indexing operation to filter the audio samples above the threshold

Display the filtered audio samples using disp

💡 Why This Matters

🌍 Real World

Audio signal processing is used in music editing, noise reduction, and voice recognition to analyze and improve sound quality.

💼 Career

Understanding basic audio processing helps in roles like audio engineering, software development for media apps, and research in signal processing.

1

Create the audio signal vector

Create a vector called audioSignal with these exact sample values: [0.1, 0.5, 0.3, 0.7, 0.2, 0.9, 0.4]

MATLAB

% Create the audioSignal vector with the given values
% Your code here

Hint

Use square brackets to create a vector with the exact numbers separated by commas.

2

Set the threshold value

Create a variable called threshold and set it to the value 0.5

MATLAB

audioSignal = [0.1, 0.5, 0.3, 0.7, 0.2, 0.9, 0.4];
% Create the threshold variable and set it to 0.5
% Your code here

Hint

Use the assignment operator = to set the variable threshold.

3

Filter the audio signal above the threshold

Create a new vector called filteredSignal that contains only the values from audioSignal that are greater than threshold using logical indexing

MATLAB

audioSignal = [0.1, 0.5, 0.3, 0.7, 0.2, 0.9, 0.4];
threshold = 0.5;
% Filter audioSignal to keep values greater than threshold
% Your code here

Hint

Use audioSignal(audioSignal > threshold) to select values above the threshold.

4

Display the filtered audio signal

Use disp to display the contents of filteredSignal

MATLAB

audioSignal = [0.1, 0.5, 0.3, 0.7, 0.2, 0.9, 0.4];
threshold = 0.5;
filteredSignal = audioSignal(audioSignal > threshold);
% Display filteredSignal
% Your code here

Hint

Use disp(filteredSignal) to show the filtered values in the command window.

Practice

1. What does the MATLAB function audioread do in audio signal processing?

easy

A. It saves audio data to a file.

B. It plays the audio sound through speakers.

C. It reads an audio file and loads the sound data into MATLAB.

D. It filters the audio signal to remove noise.

Solution

1. Step 1: Understand the purpose of audioread

   The function audioread is used to load audio data from a file into MATLAB as a vector or matrix.

2. Step 2: Differentiate from other audio functions

   Functions like sound play audio, audiowrite saves audio, and filter processes signals. audioread only reads data.

3. Final Answer:

   It reads an audio file and loads the sound data into MATLAB. -> Option C

4. Quick Check:

   audioread = read audio data [OK]

Hint: Remember: 'read' means load data, not play or save [OK]

Common Mistakes:

• Confusing audioread with sound
• Thinking it saves audio instead of loading
• Assuming it filters or changes the audio

2. Which of the following is the correct syntax to apply a simple low-pass filter b, a to an audio signal x in MATLAB?

easy

A. y = filter(b, a, x);

B. y = filter(x, b, a);

C. y = filter(a, b, x);

D. y = filter(x, a, b);

Solution

1. Step 1: Recall the filter function syntax

   The MATLAB filter function syntax is y = filter(b, a, x); where b and a are filter coefficients and x is the input signal.

2. Step 2: Identify correct argument order

   Options A, C, and D have the arguments in wrong order, which will cause errors or wrong output.

3. Final Answer:

   y = filter(b, a, x); -> Option A

4. Quick Check:

   filter(b, a, x) = correct syntax [OK]

Hint: Filter syntax is always filter(b, a, x) [OK]

Common Mistakes:

• Swapping filter coefficients order
• Putting signal before coefficients
• Using wrong variable names

3. What will be the output size of y after running this MATLAB code?

x = randn(44100,1); % 1 second noise at 44.1kHz
b = [1 0 0.5]; a = 1;
y = filter(b, a, x);

medium

A. An error due to filter coefficients

B. A vector of size 44102x1

C. A scalar value

D. A vector of size 44100x1

Solution

1. Step 1: Understand input and filter output length

   The input x is a 44100x1 vector. The filter function outputs a vector of the same length as input.

2. Step 2: Check filter coefficients validity

   Coefficients b and a are valid. No error occurs. Output y matches input size.

3. Final Answer:

   A vector of size 44100x1 -> Option D

4. Quick Check:

   filter output length = input length [OK]

Hint: Filter output length equals input length [OK]

Common Mistakes:

• Assuming output length changes with filter order
• Expecting error from coefficients
• Confusing vector with scalar output

4. Identify the error in this MATLAB code that tries to add an echo effect to an audio signal x:

delay = 5000;
echoGain = 0.6;
y = x;
y(delay:end) = y(delay:end) + echoGain * x(1:end-delay);

medium

A. The length of y(delay:end) and x(1:end-delay) do not match.

B. The variable echoGain should be greater than 1.

C. The indexing delay:end is invalid for vector y.

D. The echo effect requires a filter function, not direct addition.

Solution

1. Step 1: Check vector lengths in addition

   The segment y(delay:end) has length length(y) - delay + 1. The segment x(1:end-delay) has length length(x) - delay. They differ by 1, causing a dimension mismatch error.

2. Step 2: Eliminate other options

   A: Indexing is valid. B: echoGain=0.6 <1 is appropriate for decaying echo. D: Direct indexing addition works for simple echo.

3. Final Answer:

   The length of y(delay:end) and x(1:end-delay) do not match. -> Option A

4. Quick Check:

   len(y(delay:end))=N-d+1 vs len(x(1:end-d))=N-d [OK]

Hint: Check lengths of vectors before adding [OK]

Common Mistakes:

• Assuming echoGain must be >1
• Thinking filter is required for echo
• Ignoring vector length mismatch

5. You want to create a MATLAB function that adds an echo effect to an audio signal x sampled at Fs. The echo delay is 0.3 seconds and echo gain is 0.5. Which code correctly implements this?

hard

A. function y = addEcho(x, Fs)
delaySamples = round(0.3 * Fs);
y = x;
y(delaySamples+1:end) = y(delaySamples+1:end) + 0.5*x(1:end-delaySamples);
end

B. function y = addEcho(x, Fs)
delaySamples = round(0.3 * Fs);
y = filter([1 zeros(1,delaySamples-1) 0.5], 1, x);
end

C. function y = addEcho(x, Fs)
delaySamples = round(0.3 * Fs);
y = x + 0.5 * [zeros(delaySamples,1); x];
end

D. function y = addEcho(x, Fs)
delaySamples = round(0.3 * Fs);
y = zeros(length(x)+delaySamples,1);
y(1:length(x)) = x;
y(delaySamples+1:end) = y(delaySamples+1:end) + 0.5*x;
end

Solution

1. Step 1: Understand echo effect implementation

   Echo is created by adding a delayed and scaled copy of the signal to itself. Using filter with coefficients [1 zeros(1,delaySamples-1) 0.5] adds original plus delayed echo.

2. Step 2: Evaluate each option

   function y = addEcho(x, Fs) delaySamples = round(0.3 * Fs); y = zeros(length(x)+delaySamples,1); y(1:length(x)) = x; y(delaySamples+1:end) = y(delaySamples+1:end) + 0.5*x; end extends length but adds full x without delay in echo part. function y = addEcho(x, Fs) delaySamples = round(0.3 * Fs); y = x; y(delaySamples+1:end) = y(delaySamples+1:end) + 0.5*x(1:end-delaySamples); end correctly indexes x to match lengths. function y = addEcho(x, Fs) delaySamples = round(0.3 * Fs); y = x + 0.5 * [zeros(delaySamples,1); x]; end adds vectors of different lengths causing error. function y = addEcho(x, Fs) delaySamples = round(0.3 * Fs); y = filter([1 zeros(1,delaySamples-1) 0.5], 1, x); end correctly uses filter with proper coefficients.

3. Final Answer:

   function y = addEcho(x, Fs) delaySamples = round(0.3 * Fs); y = filter([1 zeros(1,delaySamples-1) 0.5], 1, x); end -> Option B

4. Quick Check:

   Use filter with delayed coefficients for echo [OK]

Hint: Use filter with delayed coefficients for echo [OK]

Common Mistakes:

• Adding vectors of different lengths directly
• Forgetting to delay the echo signal
• Incorrect indexing causing errors