Difference between analogue and digital signals (A-level only): Bits, bytes.

The idea

A bit is the fundamental unit of digital information and has exactly two possible states, conventionally 0 and 1. A byte is a group of 8 bits treated as one unit, so one byte can represent 2⁸ = 256 distinct patterns. In A-level physics, bits and bytes matter because digital signals are encoded as sequences of binary digits, and the number of bits available limits how precisely a quantity such as a voltage sample can be represented.

For example: A 4-bit sample can take only 16 levels, whereas an 8-bit sample can take 256 levels, giving finer resolution.

What earns the marks

• A bit is a binary digit with two possible states, 0 or 1, and is the smallest unit of digital information.
• A byte is a fixed group of 8 bits, so 1 byte = 8 bits and 1 kB = 8 kbit when using the same prefix convention.
• An n-bit binary pattern can represent 2ⁿ distinct values, so 8 bits give 2⁸ = 256 possible patterns.
• Increasing the number of bits per sample increases the number of quantisation levels and reduces the step size between adjacent levels.
• Binary patterns can be converted to decimal by summing place values, for example 1011₂ = 1 × 2³ + 0 × 2² + 1 × 2¹ + 1 × 2⁰ = 11.
• Digital data transmission rates are often quoted in bits per second, so a byte rate must be multiplied by 8 to compare with a bit rate.

Common mistakes and how to put them right

• Confusing bits with bytes and treating them as interchangeable.

  Put it right by remembering that a byte contains 8 bits and by checking the unit symbol b for bit and B for byte.

• Assuming that a larger number of bits automatically means a faster transmission rate.

  Put it right by separating bit depth, which affects resolution, from bit rate, which affects transmission speed.

• Writing binary place values using baseline digits such as 23 instead of raised exponents such as 2³.

  Put it right by using proper index notation throughout.

• Thinking that 8 bits can represent 8 values rather than 256.

  Put it right by using 2ⁿ for the number of distinct patterns.

Exam tips

• State the unit clearly in every numerical answer, for example 16 bits or 2 bytes, so the examiner can see that the bit–byte distinction has been applied.
• When converting a binary pattern to decimal, show the place-value sum explicitly rather than writing only the final number.
• Link bit depth to quantisation in a sentence, for example more bits per sample means more levels and a smaller voltage step.
• Check whether a data rate is given in bit s⁻¹ or byte s⁻¹ before substituting it into a calculation.