Audio signal processing in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read an audio file into MATLAB.

MATLAB

[y, Fs] = audioread([1]);

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

A'sound.wav'

Baudiofile

Csound

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the filename in quotes

Using a variable name without quotes

The audioread function requires the filename as a string, including quotes.

2fill in blank

medium

Complete the code to play the audio stored in variable y with sampling rate Fs.

MATLAB

sound([1], Fs);

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

Aaudio

BFs

Csignal

Dy

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of arguments

Passing the sampling rate as the first argument

The sound function plays the audio data stored in y at the sampling rate Fs.

3fill in blank

hard

Fix the error in the code to compute the duration of the audio signal y sampled at Fs.

MATLAB

duration = length([1]) / Fs;

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

Ay

Bsignal

Cduration

DFs

Attempts:

3 left

💡 Hint

Common Mistakes

Using the sampling rate variable inside length()

Using an undefined variable

The length function should be applied to the audio data y to get the number of samples.

4fill in blank

hard

Fill both blanks to create a vector of time values for the audio signal y sampled at Fs.

MATLAB

t = (0:[1]) / [2];

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Alength(y)-1

Blength(y)

CFs

D1/Fs

Attempts:

3 left

💡 Hint

Common Mistakes

Using length(y) instead of length(y)-1 for the last index

Using 1/Fs instead of Fs as the divisor

The time vector is generated as sample indices from 0 to length(y)-1 divided by the sampling rate Fs.

5fill in blank

hard

Fill all three blanks to create a new audio signal y_filtered by applying a simple moving average filter of length N.

MATLAB

N = 5;
filter = ones(1, [1]) / [2];
y_filtered = conv([3], filter, 'same');

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

AN

B5

Cy

Dlength(y)

Attempts:

3 left

💡 Hint

Common Mistakes

Using length(y) instead of N for filter length

Dividing by length(y) instead of filter length

Applying convolution to the wrong variable

The filter length is N, the divisor is N to normalize the moving average, and the convolution is applied to the audio data y.

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