Choosing file formats
PracticalPresent ▶
What this lesson covers
Topic 5 Representing images
KS3 Computing · A picture is a grid of tiny squares called pixels. Each pixel's colour is stored as a binary number. More pixels and more colours make a better picture, and a bigger file.
Topic 6 Representing sound
KS3 Computing · A computer measures the height of a sound wave thousands of times a second and saves each measurement as a binary number. This is sampling.
Topic 7 Compression
KS3 Computing · Compression stores the same information in fewer bits. Lossless gets every bit back. Lossy throws away detail you probably won't notice.
Computer SystemsCambridge Lower Secondary Computing 0860
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.
Success criteria
- 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.
Key words
Say each word, then read what it means. Every one is in the glossary too.
- 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.
- 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.
- 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.
- 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.
- Compression ratio
- Original size ÷ compressed size. A ratio of 4 means a quarter of the size. Percentage saved = (1 − compressed ÷ original) × 100.
The explanation
Read it here, or open the study page below to go one step at a time with things to try.
Calculating file size
e.g.Example
An MP3 of the same song is about 3–4 MB because it's compressed.
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.
Study it
Open the study page and go one step at a time.
Topic 7 · Year 9
Compression
Compression stores the same information in fewer bits. Lossless gets every bit back. Lossy throws away detail you probably won't notice.
Topic 5 · Year 9Representing images
A picture is a grid of tiny squares called pixels. Each pixel's colour is a binary number. More pixels and more colours = better picture, bigger file.
Topic 6 · Year 9Representing sound
A computer measures the height of a sound wave thousands of times a second and saves each measurement as a binary number. This is sampling.
Check yourself
One question at a time. The reason shows after each answer.
Saved on this device.