← IGCSE Computer Science Paper 1

Converting the length of a sound to seconds

Sound length must be in seconds before using it in a file size calculation, so multiply minutes by 60 (add any extra seconds). Length is then the third factor in the formula.

Part of: IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 25

Understanding Converting the length of a sound to seconds in IGCSE Computer Science Paper 1

What it does: The length of 4 minutes is converted to 240 seconds (4 * 60) and then multiplied with the sample rate and sample resolution. 1 minute is 60 seconds, plus 20 extra seconds gives a length of 80 seconds to use in the formula. Convert minutes to seconds by multiplying by 60: 3 x 60 = 180. Using 3 would give a file size that is far too small. There are 60 seconds in a minute. 1024 is used for storage units, not for time. 2 x 60 = 120, then add the extra 30 seconds to get 150. You cannot just write the digits side by side.

Common mistakes: The length must be in seconds before it is used in the file size formula. The line should multiply by seconds, not minutes. There are 60 seconds in a minute, not 100. It should be seconds ← minutes * 60.

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Learning
Duo
A sound file is 3 minutes long. How many seconds should you use in a file size calculation?
3 seconds180 seconds
Matches
size = 8000 * 8 * 5
↓
The sound is 5 minutes long but 5 was used as the length, so the minutes were never converted to seconds. It should be 5 * 60 = 300.
Applying
Meanings
Term
seconds = minutes x 60
Meaning
Sound length must be in seconds before using it in a file size calculation, so multiply minutes by 60 (add any extra seconds). Length is then the third factor in the formula.
Practice
minutes ← 2
Testing
Gaps
seconds = minutes *
Check Answer
Spots
3seconds ← minutes * 60 + extra
4size ← rate * res * minutes

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Quick Questions

1. A sound file is 3 minutes long. How many seconds should you use in a file size calculation?
A. 3 seconds
B. 180 seconds ✓
Convert minutes to seconds by multiplying by 60: 3 x 60 = 180. Using 3 would give a file size that is far too small.
2. Which formula correctly converts minutes to seconds?
A. seconds = minutes x 60 ✓
B. seconds = minutes x 1024
There are 60 seconds in a minute. 1024 is used for storage units, not for time.
3. A recording lasts 2 minutes 30 seconds. What length do you use in the calculation?
A. 230 seconds
B. 150 seconds ✓
2 x 60 = 120, then add the extra 30 seconds to get 150. You cannot just write the digits side by side.
4. Sound file size = sample rate x sample resolution x length. What must the length be measured in?
A. Seconds ✓
B. Minutes
The sample rate is in samples per second (hertz), so the length must also be in seconds for the units to match.
5. A song is 4 minutes long. Which working is correct for the length factor?
A. 4 x 60 = 240 ✓
B. 4 + 60 = 64
Converting minutes to seconds is a multiplication by 60, not an addition.

Match the Pairs

seconds = 3 * 60 → Converts a sound of 3 minutes into 180 seconds before using it in the file size formula
seconds = 2 * 60 + 30 → Converts a sound of 2 minutes 30 seconds into 150 seconds by multiplying the minutes by 60 and adding the extra seconds
Length is the third factor in the formula, so it must be in seconds: file size = sample rate x sample resolution x length in seconds → size = 44100 * 16 * (4 * 60)
A sound lasting 1 minute 20 seconds must be changed to seconds before it is used in a calculation → length = 1 * 60 + 20
size = 8000 * 8 * 5 → The sound is 5 minutes long but 5 was used as the length, so the minutes were never converted to seconds. It should be 5 * 60 = 300.
seconds = 2 * 60 * 30 → The extra 30 seconds were multiplied instead of added. For 2 minutes 30 seconds it should be 2 * 60 + 30 = 150.

Key Term

seconds = minutes x 60
Sound length must be in seconds before using it in a file size calculation, so multiply minutes by 60 (add any extra seconds). Length is then the third factor in the formula.

Fill in the Blanks

1. seconds = minutes * 60
Hint: Type the number of seconds in one minute.
2. seconds = minutes * 60
Hint: Type the variable that holds the sound length before it is converted.
3. # 3 minutes 20 seconds seconds = 3 * 60 + 20
Hint: Type the number of extra seconds left over after the whole minutes.
4. size = rate * resolution * seconds
Hint: Type the variable for the third factor, the sound length in the correct unit.
5. seconds = minutes * 60
Hint: Type the operator that converts minutes into seconds.

Spot the Error

Question 1
A minutes ← 3
B extra ← 20
C seconds ← minutes * 60 + extra
D size ← rate * res * minutes
Line D — The length must be in seconds before it is used in the file size formula. The line should multiply by seconds, not minutes.
Question 2
A minutes ← 2
B seconds ← minutes * 100
C rate ← 44100
D bits ← rate * 16 * seconds
Line B — There are 60 seconds in a minute, not 100. It should be seconds ← minutes * 60.
Question 3
A minutes ← 5
B extra ← 30
C seconds ← minutes * 60 - extra
D OUTPUT seconds
Line C — Extra seconds must be added to the converted minutes, not subtracted. It should be minutes * 60 + extra.
Question 4
A minutes ← 4
B seconds ← minutes * 60
C bits ← 8000 * 8
D bits ← bits + seconds
Line D — The length in seconds is the third factor and is multiplied with the sample rate and sample resolution. It should be bits * seconds, not bits + seconds.
Question 5
A minutes ← 1
B seconds ← minutes / 60
C size ← 8000 * 16 * seconds
D OUTPUT size
Line B — Converting minutes to seconds needs multiplication, not division. It should be seconds ← minutes * 60.

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