Data Storage & Compression Flashcards

All 67 cards in this deck

What is a bit?

The smallest unit of data in computing, represented by either 0 or 1.

How many bits are in a nibble?

4 bits.

How many bits are in a byte?

8 bits.

List the units of data storage in order of size from byte upwards.

Byte, kilobyte, megabyte, gigabyte, terabyte, petabyte (each 1,024 times larger than the one before).

What are the steps to convert between two units of data storage?
e.g. convert 3 GB into MB

1. Going to a smaller unit, multiply by 1,024 (going larger, divide).
2. $3 \times 1024 = 3072$.
3. Answer: 3,072 MB.

What is binary?

A number system containing two symbols, 0 and 1, also known as base 2.

Why do computers store and process all data in binary?

They work with electrical signals that are either on or off, which match the two binary digits 1 and 0.

What must happen to text, images and sound before a computer can process them?

They must be converted into binary format.

True or false? A computer processor can work directly with denary numbers.

False. Computer systems can only store and process binary digits.

What is the formula for the file size of a sound file?

sample rate $\times$ duration (s) $\times$ bit depth

What is the formula for the file size of an image?

colour depth $\times$ image height (px) $\times$ image width (px)

What is the formula for the file size of a text file?

bits per character $\times$ number of characters

True or false? The image file size formula gives the answer in bytes.

False. It gives the size in bits; divide by 8 to get bytes.

What are the steps to work out the total storage needed for a set of files and choose a device?
e.g. 100 photos of 2 MB each

1. Multiply file size by number of files: $100 \times 2 = 200$ MB.
2. Add on the totals for any other files.
3. Choose a device whose capacity is greater than the total, e.g. a 1 GB memory stick.

What are the eight place values of an 8-bit binary number?

128, 64, 32, 16, 8, 4, 2, 1.

What is the largest denary number that can be stored in 8 bits?

255 (11111111).

What are the steps to convert a denary number to 8-bit binary?
e.g. convert 20

1. Write the place values 128 64 32 16 8 4 2 1.
2. Working from the left, put a 1 where the value fits and subtract it: 16, then 4.
3. Fill the rest with 0s: 00010100.

What are the steps to convert an 8-bit binary number to denary?
e.g. convert 00001101

1. Write the place values above the bits.
2. Add the place values wherever there is a 1: $8 + 4 + 1$.
3. Answer: 13.

State the four rules of binary addition.

$0+0=0$, $0+1=1$, $1+1=10$ (0 carry 1), $1+1+1=11$ (1 carry 1).

What are the steps to add two binary numbers?
e.g. $0011 + 0110$

1. Line up the numbers and add each column starting from the right (least significant bit).
2. Carry a 1 into the next column whenever a column totals 2 or 3.
3. Answer: 1001.

What is an overflow error?

When the result of a binary addition needs more bits than are available, so the extra carry bit is lost and the answer stored is incorrect.

True or false? An 8-bit addition that produces a carry out of the most significant bit causes an overflow error.

True. There is no ninth bit to store the carry, so the stored result is wrong.

What is hexadecimal?

A base 16 number system using the digits 0–9 and the letters A–F.

What denary values do the hex digits A and F represent?

A = 10 and F = 15.

What is the largest denary number that can be written as a 2-digit hexadecimal number?

255, written as FF.

True or false? Hexadecimal is used because it is shorter and easier for people to read and remember than binary.

True. Two hex digits replace eight binary digits.

What are the steps to convert a denary number to 2-digit hexadecimal?
e.g. convert 60

1. Divide by 16: $60 \div 16 = 3$ remainder 12.
2. The whole number is the first hex digit: 3.
3. The remainder as a hex digit is the second: 12 = C, giving 3C.

What are the steps to convert a 2-digit hexadecimal number to denary?
e.g. convert 2A

1. Multiply the first digit by 16: $2 \times 16 = 32$.
2. Find the value of the second digit: A = 10.
3. Add them: $32 + 10 = 42$.

How many binary digits does one hexadecimal digit represent?

4 binary digits (one nibble).

What are the steps to convert an 8-bit binary number to hexadecimal?
e.g. convert 11011010

1. Split into two nibbles: 1101 and 1010.
2. Convert each nibble to denary: 13 and 10.
3. Write each as a hex digit: DA.

What are the steps to convert a 2-digit hexadecimal number to binary?
e.g. convert 4F

1. Take each hex digit separately.
2. Convert each to a 4-bit nibble: 4 = 0100, F = 1111.
3. Join them: 01001111.

What is the most significant bit in a binary number?

The bit with the highest place value, i.e. the leftmost bit.

What is the least significant bit in a binary number?

The bit with the lowest place value (1), i.e. the rightmost bit.

What is the effect on a number's value of a left binary shift of $n$ places?

It multiplies the number by $2^n$.
e.g. a shift left of 2 places multiplies by 4.

What is the effect on a number's value of a right binary shift of $n$ places?

It divides the number by $2^n$ (any remainder is lost).
e.g. a shift right of 1 place halves the number.

True or false? No data is ever lost when a binary shift is carried out.

False. Bits shifted off either end are lost and cannot be recovered.

What are the steps to carry out a left binary shift?
e.g. shift 00001011 two places left

1. Move every bit the given number of places to the left.
2. Fill the empty places on the right with 0s.
3. Result: 00101100 (11 becomes 44).

What is a character set?

A collection of characters a computer recognises, each represented by a unique binary code.

How many bits per character does ASCII use, and how many characters can it represent?

7 bits per character, representing 128 characters.

What is the advantage of Unicode over ASCII?

It uses more bits per character, so it can represent characters from many languages including emojis.

True or false? Storing the same text in Unicode gives a larger file than storing it in ASCII.

True. Unicode uses more bits per character.

How many characters can a character set with $n$ bits per character represent?
e.g. 8 bits per character

$2^n$ characters.
e.g. 8 bits gives 256 characters.

What does it mean to say a character set is logically ordered?
e.g. 'A' has the code 65

Characters are given consecutive codes in order, so 'B' is 66 and 'C' is 67.

How is a bitmap image represented in a computer?

As a grid of pixels, with the colour of each pixel stored as a binary value.

What is colour depth?

The number of bits used to store the colour of each pixel.

What is the resolution of an image?

The number of pixels in the image, given as width $\times$ height.

What is metadata in an image file?

Data stored about the image itself, such as width, height, colour depth and date created.

What is the effect of increasing the colour depth of an image?

More colours can be stored so the quality improves, but the file size increases.

True or false? Increasing the resolution of an image reduces its file size.

False. More pixels are stored, so the file size increases.

What is sampling in the context of sound?

Measuring the amplitude of an analogue sound wave at regular intervals and storing each measurement as a binary value.

What is the sample rate of a sound recording?

The number of samples taken per second, measured in hertz (Hz).

What is bit depth in a sound recording?

The number of bits used to store each sample.

What is the effect of increasing the sample rate of a sound file?

The recording is closer to the original sound so playback quality improves, but the file size increases.

True or false? Doubling the bit depth of a sound recording doubles its file size.

True. File size = sample rate $\times$ duration $\times$ bit depth.

Why must analogue sound be sampled before a computer can store it?

Computers can only store binary, so the analogue wave must be converted into digital (binary) values.

Give two reasons why files are compressed.

They take up less storage space, and they need fewer packets so transmit faster across a network.

Give two scenarios where compression would be needed.

Streaming or downloading high-definition video, and emailing or uploading large audio or image files.

What is lossy compression?

Compression that reduces file size by permanently removing data, so the original cannot be fully recovered.

What is lossless compression?

Compression that reduces file size without losing any detail, so the file can be restored to its original quality.

What is run length encoding?

A lossless compression method that stores a run of repeated data as the data item and the number of times it occurs.

True or false? Lossless compression reduces file size more than lossy compression.

False. Lossy achieves the greater file size reduction; lossless keeps all the data.

Which types of file must be compressed using lossless compression?

Text files, spreadsheets, financial records and emails, where no data may be lost.

Which type of compression does the JPEG format use?

Lossy compression.

Which type of compression does the MP3 audio format use?

Lossy compression.

Which type of compression does the MPEG video format use?

Lossy compression, which makes it suitable for streaming media.

Which type of compression does the GIF image format use?

Lossless compression (an 8-bit format that can also store short animations).

Which type of compression does the PDF format use?

Lossless compression, used for text documents.