Year 9 · Spring 2

Representations: going audio-visual · Lesson 5 of 5

Choosing file formats

PracticalTopic 5 Representing imagesTopic 6 Representing soundTopic 7 Compression

Year 9 · Representations: going audio-visual · Lesson 5 of 5

You will learn to

  • Compare one image saved as PNG and as JPEG at three quality levels.
  • Identify the damage lossy compression does by zooming in: blocks and blur near sharp edges.
  • Compare one sound saved as WAV and as MP3, by size and by quality.
  • Choose a format for a job and give a reason, using a format table.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Success criteria

You have got it when you can say:

  • I can fill in the table: format, lossy or lossless, use it when.
  • I can point to the damage in a low-quality JPEG.
  • I can choose PNG, JPEG, WAV or MP3 for a job and say why.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Retrieval starter

Questions from before. Write your answers, then show them.

  1. Last lesson

    In Huffman coding, the most common letter gets…

    A An 8-bit codeB The longest codeC The shortest code

    C. The shortest code

  2. Last unit

    In Spanish Excel, =IF(B2>=25,"Pass","Retry") becomes…

    A =IF(B2>=25;"Pass";"Retry")B =SI(B2>=25,"Aprobado","Repetir")C =SI(B2>=25;"Aprobado";"Repetir")

    C. =SI(B2>=25;"Aprobado";"Repetir")

  3. Earlier this year

    Ransomware mainly breaks which part of the CIA triad?

    A ConfidentialityB IntegrityC Availability

    C. Availability

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Key words (1 of 5)

Pixel
The smallest single-colour dot of an image.
Bitmap
An image stored as a grid of pixels.
Resolution
Number of pixels: width × height.
Colour depth
Bits per pixel. Sets how many colours are possible.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Key words (2 of 5)

Metadata
Data about data, like an image's width and height.
Vector
An image stored as shapes and instructions.
Analogue
Smooth and continuous, with any value possible.
Digital
Made of separate, countable steps.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Key words (3 of 5)

ADC / DAC
Analogue-to-digital converter: turns a smooth signal into numbers.
Sample rate
Sound samples taken per second, in hertz.
Bit depth
Bits per sound sample. Sets how precisely each is measured.
Lossless
A reversible way of writing the same data in fewer bits. Every bit comes back exactly. ZIP, PNG, FLAC. Needed for text, code and spreadsheets.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Key words (4 of 5)

Lossy
Throw away detail that people barely notice. The original cannot be recovered. JPEG, MP3, MP4. Fine for photos, music and video.
RLE
Run-length encoding: store how many and what, for repeats.
Huffman coding
Lossless compression giving common characters short codes.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Key words (5 of 5)

Compression ratio
Original size ÷ compressed size. A ratio of 4 means a quarter of the size. Percentage saved = (1 − compressed ÷ original) × 100.

The explanation · step 1 of 3

The maths: Image file size

The explanation · step 2 of 3

Calculating file size

e.g.Example

An MP3 of the same song is about 3–4 MB because it's compressed.

The explanation · step 2 of 3

The maths: Sound file size

The explanation · step 3 of 3

Choosing

★Remember

Ask: would losing a tiny bit of detail matter? A holiday photo: no, lossy. A medical scan, contract or game code: yes, lossless.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Quick quiz · 1 of 3

Turning a microphone's voltage into numbers is done by…

  1. A An ADC
  2. B A DAC

A. An ADC

Analogue to Digital Converter.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Quick quiz · 2 of 3

The highest pitch you want to record is 10,000 Hz. The smallest sample rate that works is…

  1. A 5,000 Hz
  2. B 20,000 Hz
  3. C 10,000 Hz

B. 20,000 Hz

Nyquist: at least 2 × 10,000 = 20,000 Hz.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Quick quiz · 3 of 3

Mono → stereo, everything else the same. The file…

  1. A doubles
  2. B halves
  3. C stays the same

A. doubles

Two channels = two full lists of samples.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. 10 seconds, 1,000 Hz, 8 bits, mono. The size is…

    A 8,000 bytesB 80,000 bitsC 10,000 bits

    B. 80,000 bits 1,000 × 8 × 10 × 1 = 80,000 bits (= 10,000 bytes).

  2. An Arduino reads 1023 from analogRead. The voltage is about…

    A 0 VB 5 VC 2.5 V

    B. 5 V 1023 is the top step: 1023 × 5 ÷ 1024 ≈ 4.995 V, almost 5 V.

Year 9 · Representations: going audio-visual · Lesson 5 of 5

Homework and the challenge

Homework

Study pages 5, 6 and 7 to each quiz.

End-of-unit challenge

File-size calculations plus a justified format choice for three scenarios.