Text, images and sound

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Notes de leçon Diapos Questions d'entraînement

Notes de leçon

Text

Character sets map characters to codes. Original ASCII defines 128 codes using seven bits; text files often store each ASCII character in a full byte. A = 65, a = 97 and 0 = 48.
Unicode covers many writing systems and emoji. Its encodings differ: UTF-8 uses one to four bytes per code point and preserves ASCII-range byte values; UTF-16 uses one or two 16-bit code units.
Text size depends on the encoding and actual characters. A fixed characters × bits-per-character calculation needs an explicit fixed-width assumption and excludes headers or compression.

Unicode character representation

Images

A bitmap is a grid of pixels. Pixel dimensions are width × height; colour depth is bits per pixel, allowing up to 2^depth possible pixel values.
Raw pixel-data size = width × height × colour depth. At 100 × 100 and eight bits per pixel, this is 80,000 bits = 10,000 bytes.
Headers, metadata, padding and compression can change the stored file size. More bits allow more possible colours but cannot recreate absent source detail.

Colour depth and available shades

Sound

Sampling measures an analogue signal at regular intervals; quantisation represents each measured amplitude using an available discrete level. Sample rate is samples per second, and bit depth is bits per sample.
Raw PCM size in bits = sample rate × bit depth × duration × channels, excluding headers and compression. Mono audio at 44,100 Hz, 16 bits and ten seconds needs 7,056,000 bits = 882,000 bytes.
Higher sample rate can capture higher-frequency detail, and greater bit depth permits finer amplitude precision. Neither guarantees audible improvement for every source.

Illustrative 2-bit samples

Trade-offs

Quality versus file size, storage and transmission.

Diapos

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Questions d'entraînement

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1.What is a pixel in a bitmap image?
Easy
- Athe smallest element of an image
- Ba sound sample
- Ca character
- Da byte
2.What is ASCII?
Easy
- Aa character set using 7 bits
- Ban image format
- Ca sound format
- Da compression type
3.What do bitmap pixel dimensions specify?
Easy
- Athe number of pixels in width by height
- Bthe bits per pixel
- Cthe number of colours
- Dthe sample rate
4.What is colour depth?
Easy
- Anumber of bits used per pixel
- Bthe width
- Cthe file size
- Dthe sample rate
5.What is sample rate?
Easy
- Athe number of samples per second
- Bbits per sample
- Cnumber of channels
- Dlength
6.What is the maximum number of distinct pixel values at eight bits per pixel?
Easy
- A256
- B8
- C128
- D65,536
7.Ignoring compression and headers, how many bytes of pixel data are in a 100 × 100 bitmap at eight bits per pixel?
Easy
- A10,000
- B80,000
- C1,000
- D100
8.What is the ASCII code for A?
Easy
- A65
- B97
- C1
- D48

Plus de questions

9.At fixed pixel dimensions and without compression, what is the effect of increasing colour depth?
Easy
- Amore colours but larger file
- Bfewer colours
- Csmaller file
- Dno effect
10.What does sampling do?
Easy
- Ameasures the sound wave at regular intervals
- Bcompresses the sound
- Cencrypts it
- Ddeletes it
11.What does metadata store?
Easy
- Ainformation about a file such as size and date
- Bthe image pixels
- Cthe sound
- Dthe text only
12.Assuming codes are packed using exactly seven bits each and no overhead, how many bits represent ten ASCII characters?
Medium
- A70
- B10
- C17
- D7
13.Ignoring headers, padding and compression, how many bits of pixel data are in a 50 × 40 bitmap at one bit per pixel?
Medium
- A2,000
- B200
- C8,000
- D50
14.Uncompressed mono audio is sampled at 8,000 samples per second, 16 bits per sample, for five seconds. Excluding headers, what is its size in bits?
Medium
- A640,000
- B80,000
- C128,000
- D40,000
15.How does UTF-8 encode an ASCII-range character such as A?
Medium
- Aone byte with the same value as byte-stored ASCII
- Balways four bytes
- Calways two bytes
- Dno binary data
16.With all other recording settings unchanged, which bit depth allows finer representation of sample amplitude?
Medium
- A16-bit
- B8-bit
- Cequal
- Dneither
17.At fixed duration, bit depth and channel count, what can reducing sample rate do?
Medium
- Areduce raw size while limiting representable frequency detail
- Bguarantee better sound quality
- Cincrease the number of samples
- Dleave the raw data size unchanged
18.How many bits per pixel are needed for 16 colours?
Medium
- A4
- B2
- C8
- D16
19.Ignoring headers and compression, how many bytes of pixel data are in a 200 × 100 bitmap at 24 bits per pixel?
Medium
- A60,000
- B20,000
- C480,000
- D600
20.Excluding headers and compression, how many bytes are needed for mono audio at 44,100 samples/s, 16 bits/sample, for ten seconds?
Medium
- A882,000
- B441,000
- C7,056,000
- D88,200
21.Without compression, why do greater pixel dimensions and colour depth require more storage?
Medium
- Amore pixels and more bits per pixel
- Bfewer pixels
- Cfewer colours
- Dno reason
22.What is the ASCII code for a (lowercase)?
Easy
- A97
- B65
- C1
- D32
23.What is analogue data?
Easy
- Acontinuous data such as sound waves
- Bbinary only
- Ctext only
- Ddigital pictures
24.What does 'bit depth' mean for sound?
Medium
- Anumber of bits per sample
- Bsamples per second
- Clength
- Dchannels
25.What is a colour palette?
Medium
- Athe set of colours available
- Ba pixel
- Ca sound
- Da file
26.ASCII uses 7 bits per character.
Easy
True or false?
27.Unicode can represent many languages.
Easy
True or false?
28.Increasing sample rate necessarily reduces the quality of a recording.
Easy
True or false?
29.A pixel is one picture element in a bitmap image.
Easy
True or false?
30.Colour depth is the number of pixels.
Medium
True or false?
31.Which can change the bytes stored in an image file, rather than merely renaming it?
Medium
- Aresolution
- Bcolour depth
- Cmetadata
- Dcompression
- Ethe name of the file
32.Which affect sound file size?
Medium
- Asample rate
- Bbit depth
- Cduration
- Dnumber of channels
- Efile extension
33.Ignoring headers and compression, calculate the raw pixel-data size in bytes for a bitmap 80 pixels wide and 60 high at eight bits per pixel.
Medium

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