Audio signal processing in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the audioread function in MATLAB?

The audioread function reads audio files and loads the audio data into MATLAB as an array, along with the sample rate.

beginner

How do you play an audio signal stored in a variable y with sample rate Fs in MATLAB?

Use the sound(y, Fs) function to play the audio signal y at the sample rate Fs.

intermediate

What does the fft function do in audio signal processing?

The fft function computes the Fast Fourier Transform, converting a time-domain audio signal into its frequency components.

intermediate

Why is it important to normalize an audio signal before processing?

Normalization scales the audio signal to a standard range (usually -1 to 1) to prevent clipping and ensure consistent volume levels during processing.

beginner

What MATLAB function can you use to write processed audio data back to a file?

Use audiowrite(filename, y, Fs) to save the audio data y with sample rate Fs to a file named filename.

Which MATLAB function reads an audio file into the workspace?

A fft

B audiowrite

C sound

D audioread

The audioread function loads audio data from a file into MATLAB.

What does the fft function return when applied to an audio signal?

A Time-domain signal

B Audio playback

C Frequency-domain representation

D Normalized audio

The fft converts the signal from time domain to frequency domain.

How do you play an audio signal y at sample rate Fs in MATLAB?

A sound(y, Fs)

B audioread(y, Fs)

C audiowrite(y, Fs)

D fft(y, Fs)

The sound function plays audio data at the specified sample rate.

Why normalize an audio signal before processing?

A To increase file size

B To prevent clipping and keep volume consistent

C To convert to frequency domain

D To play audio

Normalization prevents distortion and keeps audio levels consistent.

Which function saves audio data to a file in MATLAB?

A audiowrite

B sound

C audioread

D fft

The audiowrite function writes audio data to a file.

Explain the basic steps to read, process, and play an audio signal in MATLAB.

Think about input, processing, and output steps.

Describe how the Fast Fourier Transform (FFT) helps in audio signal processing.

Consider what frequency information tells us about sound.

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