Analog and digital signals – sending through a noisy channel

What it shows

Every communication link adds noise. An analog signal can take any value, so a relay can only amplify what arrives, noise included, and the noise builds up link after link. A digital signal uses just two levels: a regenerator decides whether each pulse is above or below a threshold and sends a clean pulse on, so the message survives until the noise is large enough to cross the threshold. To go digital, a signal is sampled and quantised; more samples and more bits per sample mean less error but bigger files.

How to use

On the Noisy channel tab pick Waveform or Text message, set Noise per link and Relays, then press Send and drag Look at to compare the analog and digital copies after each link. On the Sampling tab move Sampling rate and Bit depth and watch the error and file size. On the Light & sound pulses tab type a message, choose the medium and press Send; press New Noise for another noise pattern.

Parameters you can change

• Screen Noisy channel with relays, Sampling and quantisation, Text as light or sound pulses
• Signal sent Waveform, Text message
• Message (up to 8 ASCII characters)
• Noise per link (σ relative to the signal amplitude) 0–100 %
• Number of relays along the path 0–8 relays
• Sampling rate 1000–48000 Hz
• Bit depth 1–16 bits
• Recording length (for the file size) 1–600 s
• Medium for the pulses Light pulses in an optical fibre, Sound pulses in air
• Distance 1–1000 m

Questions to explore

1. Why does the analog copy get worse at every relay while the digital copy stays perfect at low noise?
2. How does the file size change when you double both the sampling rate and the bit depth?
3. Why can one flipped bit change a whole received character?