AQA A Level Computer Science (2025
Edition)
What are the 6 main categories for numbers? What are their symbols? -
ANSWERSReal (ℝ), rational (ℚ), integer (ℤ), natural (ℕ). Irrational. Ordinal.
What are the 3 main number systems? What are their bases? -
ANSWERSDecimal/Denary: Base-10 Binary: Base-2 Hexadecimal: Base-16
How are hexadecimal values shown? - ANSWERSEach hexadecimal represents \
(16^{n-1}\), with values \(10-15\) being shown by A-F.
How do you convert between hexadecimal to denary/binary? - ANSWERSD → H: divide
number by 16, use remainders. If remainder >\(16\), divide again. B → H: split bytes into
nibbles, convert to denary, convert to hex, put values back together
Why is hexadecimal/denary preferred over binary for some information (e.g.
colours/MAC address)? - ANSWERSShorter to type or write and easier to remember
than a long string of binary digits.
Which data storage prefixes denote powers of 10, and which denote powers of 1024?
What are their symbols? - ANSWERSKilo (k), Mega (M), Giga (G), Tera (T) Kibi (Ki),
Mebi (Mi), Gibi (Gi), Tebi (Ti)
What does ASCII stand for? What is it for? - ANSWERSAmerican Standard Code for
Information Interchange. Used to encode characters and symbols so they can be
represented in computers. 8 bits are used for encoding.
Why can you not perform operations between numbers in different formats (string and
integer)? - ANSWERSThe binary values are different, therefore the operation would not
turn out as expected.
What is Unicode? Why would it be preferable to ASCII? What does it have in common
with ASCII? - ANSWERSAnother form of encoding characters. It uses variable
encoding, meaning that it can use different amounts of bits. To make usage easier, first
128 characters are the same as the ASCII set.
What are four main causes of data not being transferred completely/correctly? What can
this lead to? - ANSWERSNormal wear & tear on cables/connectors, synchronisation
issues, electrical interference, or power surges. Can lead to flipped bits in data.
What are the four main ways to check for errors in data transmission? -
ANSWERSParity bits: odd/even number of bits is mandated. Carried out using the
,MSB. Not useful if an even number of bits have flipped. Majority voting: data is sent
over three times, and majority of bits shown will be used. Not useful if the erroneous bits
are in the majority. Check digits: number at the end of a string that is formulated using
the other values. Typically found in ISBNs. Add the numbers, find remainder after
dividing by 10 & subtract from 10 to get the check digit. Checksum: total sum of bytes is
calculated and sent with data in transmission. Recipient re-calculates sum of bytes and
compares to checksum to see if data has changed. If so, data is resent. Used to verify
credit card numbers.
What are the 4 main rules to binary addition? - ANSWERS\(0+0=1\) \(0+1=1\) \(1+1=0\),
carry a 1. \(1+1+1=1\), carry a 1.
How does binary multiplication work? - ANSWERSMultiply starting from LSB to MSB.
Shift one place each time you move to new value in the number (with LSB at zero
shifts). Add up the values found at the end.
What is the maximum range of values that can be shown in signed/unsigned binary by
using n bits? - ANSWERSUnsigned: \(0 \ \ \text{to}\ \ 2^n-1\) Signed: \(-(2^{n-1})\ \ \
text{to}\ \ 2^{n-1}-1\)
When does an overflow error occur in binary? - ANSWERSWhen the decimal value
does not fit within the range representable in binary e.g. greater than 255 for unsigned
binary with 8 bits.
How do you find the negative of a number in binary? - ANSWERSWrite the positive of
the number in binary, then flip the bits. Add 1 to this to get the negative value.
How are fractional values represented in binary? - ANSWERSA fixed/floating point is
used. The bits on the left of this are positive powers of 2, while bits on the right use
negative powers of 2, which are fractions.
What is the difference between a fixed and floating point? Which is better and why? -
ANSWERSA fixed point will keep the number of bits representing positive and negative
powers the same, no matter what. A floating point will move based on the number that
needs to be represented. Both will increase accuracy but decrease maximum range,
although a floating point can change this by moving the point. So, a floating point is
preferred.
What types of data are analogue and digital data? - ANSWERSAnalogue data is
continuous, while digital data is discrete.
How would a camera convert analogue light data to digital data to show an image? -
ANSWERSThe light intensity is measured and shown as an integer value. This is
recorded in an ADC, and is assigned to a pixel. Done as much as possible to have a
high-quality image.
,What are bitmapped graphics represented by? What are some bitmap image file types?
- ANSWERSBitmap images are made up of pixels. Common file types
include .jpg, .png, .gif, .bmp, .tif
What determines the file size of a bitmap image? - ANSWERSResolution: height in
pixels * width in pixels Colour depth: number of bits assigned per pixel e.g. 3 bits per
pixel. Resolution * colour depth = file size in bits.
What is metadata? Give 4 examples: - ANSWERSMetadata: data about files/data.
Examples include date created, owner of file, resolution, colour depth.
How is sound converted from analogue to digital, and vice versa? - ANSWERSSample
of the analogue signal is recorded by an amplifier, which quantises the data (giving
wave heights integer values). Data is encoded in binary. Reverse occurs to play sound.
Give an example of what may be an ADC or DAC: - ANSWERSADC: microphone DAC:
loudspeaker
State two things needed for sound quality in recordings: - ANSWERSSample rate: the
number of times per second a sample is measured. Measured in Hertz. Audio bit depth:
number of bits available to determine the amplitude of the wave at the sample point.
How is file size determined in sound files? - ANSWERSSample rate * sample resolution
* length in seconds = file size in bits sample resolution = bit audio depth
Why does using stereo sound result in larger file sizes? - ANSWERSThe sample has to
be recorded for each channel, doubling the file size if two channels are used e.g. left &
right
"What is Nyquist's Theorem? Why is it important?" - ANSWERS"Nyquist's Theorem
dictates that the lowest sampling rate for a recording must be at least twice the
maximum frequency of the sample to replicate the original soundwave. Used because
sounds are usually made up of different frequencies. Having the sampling rate above
twice the highest frequency ensures each peak and trough of the wave is picked up, for
accuracy."
"Provide an example displaying why Nyquist's Theorem is important:" - ANSWERS"If a
sound wave sample has a frequency of \(x\) Hz, it will take \(1/x\) seconds for one full
wave to pass a point (e.g. peak to peak). If the recording's sample rate was \(x\) Hz then
the recording would be picking up the same point of the wave over and over again i.e.
only the amplitude. By doubling the sample rate, the peak and trough of the wave will be
recorded as well, improving accuracy."
"What is the formula for Nyquist's Theorem?" - ANSWERS\(f_s > 2f_{max}\) \(f_s\) is the
lowest sampling rate \(2f_{max}\) is twice the highest frequency of the original sound
wave.
, What is a MIDI? What does it do? - ANSWERSMusical Instrument Digital Interface.
Creates synthesised sounds as requested from an instrument/software.
How does a MIDI play sounds? Why are they synthesised and not recorded? -
ANSWERSThe sounds are triggered by instructions called event messages, which are
2-3 bytes long. Using a synthesised sound means only the instruction is needed to play
the sound, reducing data sent.
What are the advantages of connecting a sound processor to multiple
instruments/devices? - ANSWERSInstruments can be introduced/silenced instantly,
timed messages can be sent to control tempo/pitch/volume.
What are three important factors of data transmission speeds? - ANSWERSBandwidth,
transmission distance, storage space.
What are the benefits of data compression? - ANSWERSThe crucial data is sent much
quicker & less bandwidth is used overall. Reduces the chance of buffering.
What is lossy compression? What are the (dis)advantages? Examples of what is
removed? - ANSWERS"Non-essential data is permanently removed. Significantly
reduces file size (although quality is heavily sacrificed). Examples: quiet sounds played
at the same time as loud sounds, sounds humans can't hear, similar shades of a
colour."
What is lossless compression? When might it be used? - ANSWERSData patterns are
spotted and summarised, encoding data more efficiently. Nothing is deleted. Used to
compress text files/software code.
What is RLE? What is it best used for? - ANSWERSRun-Length Encoding will
compress patterns of consecutive data. Works best with images/sounds, where data
may be repeated many times.
Why is RLE beneficial? - ANSWERSIt encodes consecutive data patterns much more
efficiently, reducing the overall file size by a decent amount, while not sacrificing any
data.
What is dictionary compression? - ANSWERS"Regularly occurring data is stored
separately in a 'dictionary'. Entry references are used to call this data and are stored in
the main file."
Why is dictionary compression beneficial? - ANSWERSInstead of needing multiple
bytes to show a word through ASCII/Unicode, the dictionary stores the word & only a
few bits are used to call the entry instead.
Edition)
What are the 6 main categories for numbers? What are their symbols? -
ANSWERSReal (ℝ), rational (ℚ), integer (ℤ), natural (ℕ). Irrational. Ordinal.
What are the 3 main number systems? What are their bases? -
ANSWERSDecimal/Denary: Base-10 Binary: Base-2 Hexadecimal: Base-16
How are hexadecimal values shown? - ANSWERSEach hexadecimal represents \
(16^{n-1}\), with values \(10-15\) being shown by A-F.
How do you convert between hexadecimal to denary/binary? - ANSWERSD → H: divide
number by 16, use remainders. If remainder >\(16\), divide again. B → H: split bytes into
nibbles, convert to denary, convert to hex, put values back together
Why is hexadecimal/denary preferred over binary for some information (e.g.
colours/MAC address)? - ANSWERSShorter to type or write and easier to remember
than a long string of binary digits.
Which data storage prefixes denote powers of 10, and which denote powers of 1024?
What are their symbols? - ANSWERSKilo (k), Mega (M), Giga (G), Tera (T) Kibi (Ki),
Mebi (Mi), Gibi (Gi), Tebi (Ti)
What does ASCII stand for? What is it for? - ANSWERSAmerican Standard Code for
Information Interchange. Used to encode characters and symbols so they can be
represented in computers. 8 bits are used for encoding.
Why can you not perform operations between numbers in different formats (string and
integer)? - ANSWERSThe binary values are different, therefore the operation would not
turn out as expected.
What is Unicode? Why would it be preferable to ASCII? What does it have in common
with ASCII? - ANSWERSAnother form of encoding characters. It uses variable
encoding, meaning that it can use different amounts of bits. To make usage easier, first
128 characters are the same as the ASCII set.
What are four main causes of data not being transferred completely/correctly? What can
this lead to? - ANSWERSNormal wear & tear on cables/connectors, synchronisation
issues, electrical interference, or power surges. Can lead to flipped bits in data.
What are the four main ways to check for errors in data transmission? -
ANSWERSParity bits: odd/even number of bits is mandated. Carried out using the
,MSB. Not useful if an even number of bits have flipped. Majority voting: data is sent
over three times, and majority of bits shown will be used. Not useful if the erroneous bits
are in the majority. Check digits: number at the end of a string that is formulated using
the other values. Typically found in ISBNs. Add the numbers, find remainder after
dividing by 10 & subtract from 10 to get the check digit. Checksum: total sum of bytes is
calculated and sent with data in transmission. Recipient re-calculates sum of bytes and
compares to checksum to see if data has changed. If so, data is resent. Used to verify
credit card numbers.
What are the 4 main rules to binary addition? - ANSWERS\(0+0=1\) \(0+1=1\) \(1+1=0\),
carry a 1. \(1+1+1=1\), carry a 1.
How does binary multiplication work? - ANSWERSMultiply starting from LSB to MSB.
Shift one place each time you move to new value in the number (with LSB at zero
shifts). Add up the values found at the end.
What is the maximum range of values that can be shown in signed/unsigned binary by
using n bits? - ANSWERSUnsigned: \(0 \ \ \text{to}\ \ 2^n-1\) Signed: \(-(2^{n-1})\ \ \
text{to}\ \ 2^{n-1}-1\)
When does an overflow error occur in binary? - ANSWERSWhen the decimal value
does not fit within the range representable in binary e.g. greater than 255 for unsigned
binary with 8 bits.
How do you find the negative of a number in binary? - ANSWERSWrite the positive of
the number in binary, then flip the bits. Add 1 to this to get the negative value.
How are fractional values represented in binary? - ANSWERSA fixed/floating point is
used. The bits on the left of this are positive powers of 2, while bits on the right use
negative powers of 2, which are fractions.
What is the difference between a fixed and floating point? Which is better and why? -
ANSWERSA fixed point will keep the number of bits representing positive and negative
powers the same, no matter what. A floating point will move based on the number that
needs to be represented. Both will increase accuracy but decrease maximum range,
although a floating point can change this by moving the point. So, a floating point is
preferred.
What types of data are analogue and digital data? - ANSWERSAnalogue data is
continuous, while digital data is discrete.
How would a camera convert analogue light data to digital data to show an image? -
ANSWERSThe light intensity is measured and shown as an integer value. This is
recorded in an ADC, and is assigned to a pixel. Done as much as possible to have a
high-quality image.
,What are bitmapped graphics represented by? What are some bitmap image file types?
- ANSWERSBitmap images are made up of pixels. Common file types
include .jpg, .png, .gif, .bmp, .tif
What determines the file size of a bitmap image? - ANSWERSResolution: height in
pixels * width in pixels Colour depth: number of bits assigned per pixel e.g. 3 bits per
pixel. Resolution * colour depth = file size in bits.
What is metadata? Give 4 examples: - ANSWERSMetadata: data about files/data.
Examples include date created, owner of file, resolution, colour depth.
How is sound converted from analogue to digital, and vice versa? - ANSWERSSample
of the analogue signal is recorded by an amplifier, which quantises the data (giving
wave heights integer values). Data is encoded in binary. Reverse occurs to play sound.
Give an example of what may be an ADC or DAC: - ANSWERSADC: microphone DAC:
loudspeaker
State two things needed for sound quality in recordings: - ANSWERSSample rate: the
number of times per second a sample is measured. Measured in Hertz. Audio bit depth:
number of bits available to determine the amplitude of the wave at the sample point.
How is file size determined in sound files? - ANSWERSSample rate * sample resolution
* length in seconds = file size in bits sample resolution = bit audio depth
Why does using stereo sound result in larger file sizes? - ANSWERSThe sample has to
be recorded for each channel, doubling the file size if two channels are used e.g. left &
right
"What is Nyquist's Theorem? Why is it important?" - ANSWERS"Nyquist's Theorem
dictates that the lowest sampling rate for a recording must be at least twice the
maximum frequency of the sample to replicate the original soundwave. Used because
sounds are usually made up of different frequencies. Having the sampling rate above
twice the highest frequency ensures each peak and trough of the wave is picked up, for
accuracy."
"Provide an example displaying why Nyquist's Theorem is important:" - ANSWERS"If a
sound wave sample has a frequency of \(x\) Hz, it will take \(1/x\) seconds for one full
wave to pass a point (e.g. peak to peak). If the recording's sample rate was \(x\) Hz then
the recording would be picking up the same point of the wave over and over again i.e.
only the amplitude. By doubling the sample rate, the peak and trough of the wave will be
recorded as well, improving accuracy."
"What is the formula for Nyquist's Theorem?" - ANSWERS\(f_s > 2f_{max}\) \(f_s\) is the
lowest sampling rate \(2f_{max}\) is twice the highest frequency of the original sound
wave.
, What is a MIDI? What does it do? - ANSWERSMusical Instrument Digital Interface.
Creates synthesised sounds as requested from an instrument/software.
How does a MIDI play sounds? Why are they synthesised and not recorded? -
ANSWERSThe sounds are triggered by instructions called event messages, which are
2-3 bytes long. Using a synthesised sound means only the instruction is needed to play
the sound, reducing data sent.
What are the advantages of connecting a sound processor to multiple
instruments/devices? - ANSWERSInstruments can be introduced/silenced instantly,
timed messages can be sent to control tempo/pitch/volume.
What are three important factors of data transmission speeds? - ANSWERSBandwidth,
transmission distance, storage space.
What are the benefits of data compression? - ANSWERSThe crucial data is sent much
quicker & less bandwidth is used overall. Reduces the chance of buffering.
What is lossy compression? What are the (dis)advantages? Examples of what is
removed? - ANSWERS"Non-essential data is permanently removed. Significantly
reduces file size (although quality is heavily sacrificed). Examples: quiet sounds played
at the same time as loud sounds, sounds humans can't hear, similar shades of a
colour."
What is lossless compression? When might it be used? - ANSWERSData patterns are
spotted and summarised, encoding data more efficiently. Nothing is deleted. Used to
compress text files/software code.
What is RLE? What is it best used for? - ANSWERSRun-Length Encoding will
compress patterns of consecutive data. Works best with images/sounds, where data
may be repeated many times.
Why is RLE beneficial? - ANSWERSIt encodes consecutive data patterns much more
efficiently, reducing the overall file size by a decent amount, while not sacrificing any
data.
What is dictionary compression? - ANSWERS"Regularly occurring data is stored
separately in a 'dictionary'. Entry references are used to call this data and are stored in
the main file."
Why is dictionary compression beneficial? - ANSWERSInstead of needing multiple
bytes to show a word through ASCII/Unicode, the dictionary stores the word & only a
few bits are used to call the entry instead.
