Computer Science 9618/13 — October/November 2022

Background Concept

Digital sound is stored by taking many measurements of the original analogue sound wave.

Two important factors affect the accuracy of the recording:

• Sampling rate: how many times per second the sound is measured. A higher sampling rate means the computer captures more detail from the original wave.
• Sampling resolution (bit depth): how many bits are used for each sample. More bits allow more possible amplitude values, so the stored sound is closer to the original.

A third factor, duration, affects how long the recording lasts and therefore its file size, but it does not make each sample more or less accurate.

Understanding the Question

You are given three changes to a digital audio recording and must decide whether each one makes the recording more accurate, less accurate, or leaves the accuracy unchanged.

The key idea is to separate:

• factors that improve or reduce quality/accuracy
• factors that only change length/file size

This is a classification question, so each row needs exactly one choice.

Approach

For each action, ask:

1. Does it change how often the sound is sampled?
2. Does it change how precisely each sample is stored?
3. Or does it only change how long the recording is?

If it increases the amount of detail captured, accuracy increases. If it reduces detail, accuracy decreases. If it only affects length, accuracy does not change.

Step-by-Step Reasoning

1. Sampling rate from 40 kHz to 60 kHz

A sampling rate of 60 kHz means more samples are taken each second than at 40 kHz.

• More samples per second means the waveform is recorded more closely.
• So the digital version is a more accurate representation of the original sound.

Therefore: accuracy increases.

2. Duration from 20 minutes to 40 minutes

Changing the duration means the recording lasts longer.

• The sound is still sampled at the same rate.
• Each sample is still stored with the same resolution.
• So the quality of each part of the recording does not change.

Only the total amount of data changes.

Therefore: accuracy does not change.

3. Sampling resolution from 24 bits to 16 bits

Sampling resolution tells us how many possible values each sample can have.

• 24 bits allows many more possible amplitude levels than 16 bits.
• Reducing from 24 bits to 16 bits means each sample is stored less precisely.
• This increases quantisation error.

Therefore: accuracy decreases.

Key Takeaways

• Higher sampling rate improves sound accuracy.
• Higher sampling resolution improves sound accuracy.
• Longer duration increases file size, not recording quality.
• Always separate factors affecting quality from factors affecting amount of data.

Common Mistakes

• Saying that a longer recording is more accurate. It is only longer, not better quality.
• Confusing sampling rate with sampling resolution. Rate is how often samples are taken; resolution is how many bits store each sample.
• Thinking that reducing bit depth has no effect. It reduces the number of levels available, so accuracy falls.

Things to Be Careful About

• The question asks about accuracy of the recording, not file size.
• Do not choose more than one box per row.
• A higher number is not always better in every sense: longer duration means more storage, but not better sound quality.
• Make sure you read the direction of the change carefully, especially when the number goes down, such as 24 bits to 16 bits.

Background Concept

For uncompressed digital sound, file size depends mainly on:

• sampling rate: samples per second
• sampling resolution: bits per sample
• duration: total recording time
• number of channels: for example mono or stereo

Here, only one channel is implied, so we use:

$$\text{file size in bits} = \text{sampling rate} \times \text{sampling resolution} \times \text{time in seconds}$$

Then convert:

• bits to bytes: divide by 8
• bytes to megabytes: divide by 1 000 000 when using decimal prefixes for MB

Understanding the Question

The question gives:

• sampling rate = 50 kHz = 50 000 samples per second
• sampling resolution = 16 bits
• duration = 20 minutes

It asks for the final answer in megabytes, and it specifically says to show working. So you must not jump straight to the final value.

Approach

Use a clear sequence:

1. Convert the recording time from minutes to seconds.
2. Calculate total number of bits.
3. Convert bits to bytes.
4. Convert bytes to megabytes.

Because no stereo information is given, do not multiply by 2.

Step-by-Step Reasoning

1. Convert time to seconds

The recording lasts 20 minutes.

$$20 \times 60 = 1200 \text{ seconds}$$

2. Calculate the number of bits per second

The sound is sampled 50 000 times each second, and each sample uses 16 bits.

$$50\,000 \times 16 = 800\,000 \text{ bits per second}$$

3. Calculate the total number of bits

Now multiply by the total number of seconds:

$$800\,000 \times 1200 = 960\,000\,000 \text{ bits}$$

You may also combine the calculation in one line:

$$50\,000 \times 16 \times 1200 = 960\,000\,000 \text{ bits}$$

4. Convert bits to bytes

There are 8 bits in 1 byte.

$$\frac{960\,000\,000}{8} = 120\,000\,000 \text{ bytes}$$

5. Convert bytes to megabytes

Using decimal megabytes:

$$\frac{120\,000\,000}{1\,000\,000} = 120 \text{ MB}$$

So the file size is 120 megabytes.

Key Takeaways

• For sound files, size depends on sampling rate, bit depth and duration.
• Always convert time to seconds before using the formula.
• Always convert bits to bytes before converting to MB.
• If stereo were stated, you would multiply by 2 channels.

Common Mistakes

• Forgetting to convert 20 minutes into 1200 seconds.
• Using 50 instead of 50 000 for 50 kHz.
• Forgetting to divide by 8 when converting bits to bytes.
• Multiplying by 2 for stereo when the question does not say stereo.
• Giving the answer in bits or bytes instead of megabytes.

Things to Be Careful About

• kHz means thousand samples per second, so 50 kHz = 50 000, not 50.
• The question asks for megabytes, so your final unit must be MB.
• Show the conversion stages clearly, because method marks are often awarded even if arithmetic slips occur later.
• Read whether the syllabus context expects decimal MB or binary MiB; in exam questions like this, MB is normally taken as 1 000 000 bytes unless stated otherwise.

Background Concept

A buffer is a temporary storage area in memory. It is used when data is moving between two parts of a computer system that do not work at exactly the same speed.

Buffers are common because many devices and processes produce or consume data at uneven rates. For example:

• a processor may supply data very quickly
• an output device may use that data more slowly
• incoming data may arrive in bursts rather than steadily

The buffer smooths this out by storing data temporarily until it is needed.

Understanding the Question

The question mentions a computer using a buffer when playing an audio message. It asks for:

1. the purpose of a buffer in a computer system
2. one other example of buffering

So a complete answer needs both a general definition and an applied example. Just naming an example is not enough; you must explain why the buffer is needed.

Approach

A strong answer should include three ideas:

• what a buffer is: temporary storage
• why it is used: to deal with different data transfer speeds or irregular data flow
• one example: such as printing, keyboard input, streaming or disk transfer

Since the question already gives audio playback, it is good to explain smooth playback first, then add a different example such as a printer buffer.

Step-by-Step Reasoning

1. Define the buffer

A buffer is not permanent storage. It is a temporary holding area, usually in RAM.

This gives the first mark-worthy point: temporary storage for data during transfer or processing.

2. Explain why it is needed for audio

When audio is played, the sound card or playback process needs a steady stream of data.

If data were taken directly from the source with no temporary store, any slight delay could cause:

• pauses
• gaps
• jerky playback

A buffer stores a small amount of upcoming audio data so that playback can continue smoothly even if the data source is not perfectly steady.

This gives the second idea: matching different speeds / preventing interruptions.

3. Give one other example

A common example is a printer buffer.

• The computer sends data much faster than the printer can physically print it.
• The buffer stores the waiting print data.
• The printer then takes the data from the buffer at its own slower rate.

This gives the third mark-worthy element: a correct example with explanation.

Other valid examples could include:

• a keyboard buffer storing keystrokes until the CPU processes them
• buffering during video streaming
• a disk buffer used during reading or writing

Key Takeaways

• A buffer is temporary memory storage.
• It is used when two devices or processes work at different speeds.
• In audio, buffering helps maintain continuous playback.
• Good examples explain both where the buffer is used and why it is needed.

Common Mistakes

• Saying a buffer is permanent storage. It is temporary.
• Describing a buffer only as “stores data” without saying it is used during transfer between parts running at different speeds.
• Giving an example such as “printer” without explaining how the speed difference makes the buffer necessary.
• Confusing a buffer with a cache. A cache stores frequently used data to speed access; a buffer mainly smooths transfer timing.

Things to Be Careful About

• The question says using one other example, so do not only talk about audio playback.
• Make sure your example is clearly different from the one in the question.
• Use the phrase temporary storage because that is the key definition.
• Link the explanation to speed difference or irregular data flow, since that is the main purpose of buffering in exam answers.