Representing images, sound and other data common mistakes

Common mistakes

• Assuming bits have a fixed meaning

  Saying that a bit pattern must always represent one particular type of data.

  Fix itExplain that its meaning depends on the agreed representation used to interpret it.

• Confusing data with signals

  Describing digital data when the question asks about how a digital signal changes.

  Fix itCheck whether the question concerns data or signals. State that digital data uses distinct values, while a digital signal uses distinct levels.

• Reversing ADC and DAC

  Stating that an ADC converts digital data into an analogue signal or that a DAC converts analogue data into digital data.

  Fix itRemember: ADC means analogue to digital, while DAC means digital to analogue. Link ADCs with analogue sensors and DACs with digital audio output.

• Confusing resolution with size in pixels

  Treating width × height as the resolution expressed in dots per inch.

  Fix itWidth × height gives the size of the image in pixels. Resolution is expressed as dots per inch, where a dot is a pixel.

• Describing pixels instead of objects

  Saying that a vector graphic stores the colour or brightness of every individual pixel.

  Fix itExplain that a vector graphic stores a list of geometric objects and the properties of each object.

• Treating one approach as universally better

  Claiming that vector graphics are always better because they can be resized, or that bitmaps are always better because they can show detail.

  Fix itChoose the approach according to the image and its intended use: vectors are often suitable for logos and diagrams, while bitmaps are often suitable for photographs.

• Forgetting to convert bits to bytes

  Multiplying the number of samples by the sample resolution and giving the result in bits when the question asks for bytes.

  Fix itAfter calculating the total number of bits, divide by 8 to convert the result to bytes.

• Confusing MIDI with recorded audio

  Saying that a MIDI file stores the actual sound waveform of the performance.

  Fix itExplain that MIDI stores event messages and performance instructions; the playback system produces the sound.

• Confusing lossless and lossy compression

  Stating that lossless compression permanently removes data, or that lossy compression allows the original file to be reconstructed exactly.

  Fix itLossless compression preserves all data and permits exact reconstruction. Lossy compression permanently removes some data, so the exact original cannot be recovered.

• Reusing a one-time pad

  Assuming that the key for a Vernam cipher can be reused for several messages.

  Fix itState that the key must be chosen uniformly at random and used once only. Reusing it means it no longer meets the one-time-pad condition for perfect security.