Data Representation | AQA GCSE Computer Science (8525)

Data Representation

143 questions
9 subtopics
Paper 2
Paper 2

Data Representation is examined in Paper 2, Computing concepts.

It covers number bases and why hexadecimal is used, converting between binary, decimal and hexadecimal, units of information: bits, bytes and prefixes, binary addition and binary shifts, character encoding: ASCII and Unicode, pixels, bitmaps and colour depth, bitmap image file size calculations, digital sound: sampling rate and sample resolution and data compression: Huffman coding and run-length encoding.

Sample questions from Data Representation

Answer each one closed book first, then open the answer.

Number bases and why hexadecimal is used

State the decimal values of the hexadecimal digits A to F.

Show the answer
A is 10, B is 11 and C is 12. D is 13, E is 14 and F is 15.
Converting between binary, decimal and hexadecimal

How can you check a decimal-to-binary conversion?

Show the answer
Add up the place values of every column that holds a 1. The total should equal the decimal number you started with.
Units of information: bits, bytes and prefixes

How many megabytes make one gigabyte?

Show the answer
One gigabyte, written 1 GB, is 1,000 megabytes. A gigabyte is therefore 1,000,000,000 bytes.
Binary addition and binary shifts

How can you check the answer to a binary addition?

Show the answer
Convert each number to decimal and add them. The decimal total should match the binary answer converted to decimal.
Character encoding: ASCII and Unicode

The ASCII code for a lower-case a is 97. Which character has the code 100?

Show the answer
The code 100 stands for the lower-case letter d. The lower-case letters run in order from 97 upwards.
Pixels, bitmaps and colour depth

How many colours can an image with a colour depth of 1 bit show?

Show the answer
A colour depth of 1 bit gives two colours. Each pixel is either 0 or 1, usually white or black.
Bitmap image file size calculations

Calculate the file size in megabytes of a 1,920 × 1,080 image with a colour depth of 24 bits.

Show the answer
The file size is about 6.22 MB. The image needs 1,920 × 1,080 × 24 = 49,766,400 bits, which is 6,220,800 bytes.
Digital sound: sampling rate and sample resolution

What does a sampling rate of 44,100 Hz mean?

Show the answer
The sound is measured 44,100 times every second. One hertz means one sample per second.

The 9 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Number bases and why hexadecimal is used	Decimal, binary and hexadecimal, the values of A to F, place values in binary and hex, why computers work in binary, and why programmers read data in hexadecimal.	16
Converting between binary, decimal and hexadecimal	Step-by-step methods and worked conversions between binary, decimal and hexadecimal, checking an answer, the largest value one byte holds, and a common conversion mistake explained.	16
Units of information: bits, bytes and prefixes	Bits and bytes, the prefixes kilo, mega, giga and tera, why some count a kilobyte as 1,024 bytes, and conversions between units including how many photos fit on a card.	16
Binary addition and binary shifts	The rules for adding binary digits, worked additions of two and three numbers, overflow beyond 255, and left and right logical shifts as multiplication and division by powers of two.	16
Character encoding: ASCII and Unicode	Character sets and codes, 7-bit ASCII and the order of its codes, working out a letter's code from A = 65, why '5' differs from 5, and why Unicode was needed.	16
Pixels, bitmaps and colour depth	Pixels, image size and colour depth, how many colours a given depth can show, how more pixels or bits change an image, and decoding short rows of bitmap data.	16
Bitmap image file size calculations	The file size formula for bitmaps, worked calculations in bits, bytes, kilobytes and megabytes, working backwards to a width or colour depth, and how resizing changes storage.	15
Digital sound: sampling rate and sample resolution	Analogue and digital data, how sound is sampled, sampling rate and sample resolution and their effect on quality, the file size formula, and worked sound file calculations.	16
Data compression: Huffman coding and run-length encoding	Why data is compressed, lossy and lossless compression, building and reading Huffman codes, bits saved against 7-bit ASCII, and run-length encoding with worked pairs.	16

Data Representation is 143 of the 664 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.