Loading...
A character set maps numeric codes to characters. ASCII and Unicode support different ranges; Unicode is a broad standard rather than a rule that every character always uses exactly one byte. Text interpretation requires the correct encoding. A bitmap stores a grid of pixels, each encoded using a colour depth. More bits per pixel allow more possible colours, but the meaning of the bit pattern depends on the encoding.
Video by Craig'n'Dave. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate raw image and sound sizes
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does image colour depth determine?
The number of representable colours. b bits can encode up to 2^b colour values.
After watching 2: Which compression reconstructs the original exactly?
Lossless. No original information is discarded.
What you'll be able to do
A character set maps numeric codes to characters. ASCII and Unicode support different ranges; Unicode is a broad standard rather than a rule that every character always uses exactly one byte. Text interpretation requires the correct encoding. A bitmap stores a grid of pixels, each encoded using a colour depth. More bits per pixel allow more possible colours, but the meaning of the bit pattern depends on the encoding.
Raw bitmap size in bits is width × height × bits per pixel, excluding metadata and compression. Sampled sound records amplitude at regular intervals. Raw sound size is sample rate × duration × sample depth × channels. Higher sample rate captures time detail, while greater bit depth increases amplitude precision. State whether a question includes stereo, metadata or compression; adding an unexplained factor can make an otherwise correct formula wrong.
Lossless compression permits exact reconstruction, useful for text and source code. Lossy compression removes some information to reduce size, often useful for media when a quality trade-off is acceptable. Compression ratio depends on content and method; not every file shrinks equally. Run-length encoding suits long runs of repeated values but can perform poorly for rapidly changing data. Explain what information must survive for the intended application.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A mono recording has 8000 samples/s, 8-bit depth and 10 s duration. Find raw size.
80000 bytes.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.