← ACARA Digital Technologies Years 7–8

Bit rate of digital audio

The number of bits used to store each second of sound: sample rate x sample resolution. A higher bit rate means better accuracy but more storage per second.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 55

Understanding Bit rate of digital audio in ACARA Digital Technologies Years 7–8

What it does: The formula for bit rate multiplies the number of samples each second by the number of bits in each sample. Multiply: 100 x 8 = 800 bits per second. Adding the numbers (100 + 8) is a common mistake. Bit rate = sample rate x sample resolution. If one of the numbers gets bigger and the other stays the same, the answer gets bigger. Recording B stores 32 000 bits every second, which is more than the 8000 bits of Recording A. So it needs more storage per second.

Common mistakes: A Python function definition must end with a colon. The line should be def bit_rate(rate, bits): so the function body can follow. Bit rate is sample rate times sample resolution, not plus. The line should be bit_rate = rate * bits.

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This module's key terms are also in a printable crossword (Modules 153–172) and word search (Modules 153–188).

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Learning
Duo
Which formula gives the bit rate of mono digital audio?
Sample rate + sample resolutionSample rate x sample resolution
Matches
bitRate = sampleRate + resolution
↓
The sample rate and sample resolution are added instead of multiplied. Bit rate is sample rate x sample resolution.
Applying
Meanings
Term
bit rate
Meaning
The number of bits used to store each second of sound: sample rate x sample resolution. A higher bit rate means better accuracy but more storage per second.
Practice
sampleRate ← 44100
Testing
Gaps
bitRate ← sampleRate resolution
Check Answer
Spots
1def bit_rate(rate, bits)
2 return rate * bits

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Quick Questions

1. Which formula gives the bit rate of mono digital audio?
A. Sample rate x sample resolution ✓
B. Sample rate + sample resolution
Bit rate is the sample rate (samples per second) multiplied by the sample resolution (bits per sample). This gives the bits used for each second of sound.
2. What does bit rate measure?
A. The number of bits used to store each second of sound ✓
B. The number of seconds a sound lasts
Bit rate is about bits per second. It does not measure how long the sound is.
3. A sound is sampled 100 times per second, and each sample uses 8 bits. What is the bit rate?
A. 800 bits per second ✓
B. 108 bits per second
Multiply: 100 x 8 = 800 bits per second. Adding the numbers (100 + 8) is a common mistake.
4. A sound has 1000 samples per second and 16 bits per sample. What is its bit rate?
A. 1016 bits per second
B. 16 000 bits per second ✓
1000 x 16 = 16 000 bits per second. You multiply the two numbers, you do not add them.
5. What happens to the bit rate if you increase the sample rate but keep the sample resolution the same?
A. The bit rate stays the same
B. The bit rate goes up ✓
Bit rate = sample rate x sample resolution. If one of the numbers gets bigger and the other stays the same, the answer gets bigger.

Match the Pairs

bit rate = 8000 * 8 → 64000 bits are stored for each second of sound
bit rate = 44100 * 16 → 705600 bits are stored for each second of sound
Bit rate is found by multiplying the sample rate by the sample resolution → bitRate = sampleRate * resolution
Doubling the sample rate doubles the bit rate, so each second needs twice the storage → (2 * 8000) * 8 = 128000
bitRate = sampleRate + resolution → The sample rate and sample resolution are added instead of multiplied. Bit rate is sample rate x sample resolution.
if bitRate goes up: storage per second goes down → A higher bit rate means MORE bits are stored for each second, so it needs more storage, not less.

Key Term

bit rate
The number of bits used to store each second of sound: sample rate x sample resolution. A higher bit rate means better accuracy but more storage per second.

Fill in the Blanks

1. bitRate ← sampleRate * resolution
Hint: Type the operator that multiplies sample rate by sample resolution to get the bit rate.
2. sampleRate ← 8000 resolution ← 8 bitRate ← sampleRate * resolution
Hint: Type the variable name that stores the number of bits used for each second of sound.
3. sampleRate ← 44100 resolution ← 16 bitRate ← sampleRate * resolution
Hint: Type the variable that holds the number of bits in each sample.
4. IF bitRate1 > bitRate2 THEN accuracy1 ← better ENDIF
Hint: Type the comparison operator for 'greater than'. A higher bit rate gives better accuracy.
5. fileBits ← bitRate * seconds
Hint: Type the operator that multiplies the bits per second by the length of the recording to show how storage grows.

Spot the Error

Question 1
A def bit_rate(rate, bits)
B return rate * bits
C print(bit_rate(8000, 16))
Line A — A Python function definition must end with a colon. The line should be def bit_rate(rate, bits): so the function body can follow.
Question 2
A rate = 44100
B bits = 16
C bit_rate = rate + bits
D print(bit_rate)
Line C — Bit rate is sample rate times sample resolution, not plus. The line should be bit_rate = rate * bits.
Question 3
A rate = 1000
B bits = 8
C bit_rate = rate * bits
D print(bit_rate, 'bytes/s')
Line D — Bit rate is measured in bits per second, not bytes. The unit label should be 'bits/s'.
Question 4
A SET low TO 8000 * 8
B SET high TO 44100 - 16
C IF high > low THEN
D DISPLAY 'Better accuracy'
E END IF
Line B — The bit rate is found by multiplying sample rate by sample resolution. The line should be SET high TO 44100 * 16.
Question 5
A bit_rate = 16000
B total_bits = bit_rate / 10
C print(total_bits)
Line B — Bit rate is the bits stored for each second, so 10 seconds needs bit_rate multiplied by 10, not divided. It should be total_bits = bit_rate * 10.

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