Analog vs. digital

Sound in a room is a continuous wave of air pressure. Light from a scene varies smoothly across space. An analog signal mirrors that continuity directly: a voltage on a wire rises and falls in proportion to the original. A digital signal describes the same thing as a stream of numbers. Almost every AV system you'll work on is digital end to end, except at the very edges — the microphone diaphragm and the loudspeaker cone are still, by nature, analog.

From wave to numbers

Converting analog to digital (an A/D converter) happens in two steps:

1. Sampling — measure the signal at regular instants. How often you measure is the sample rate, in samples per second (Hz).
2. Quantization — round each measurement to the nearest available value. How many values are available is set by the bit depth: n bits give 2ⁿ levels.

Play with both below. Watch how the green reconstruction drifts from the original as you starve it of either.

The Nyquist limit

To capture a frequency faithfully you must sample at more than twice that frequency. Human hearing tops out around 20 kHz, which is why audio sample rates sit just above 40 kHz: 44.1 kHz for CDs and 48 kHz, the standard across professional AV, broadcast and networked audio. Anything above half the sample rate must be filtered out before sampling, or it folds back as a false, lower frequency — an alias.

Bit depth and dynamic range

Every bit roughly doubles the number of levels, which adds about 6 dB of dynamic range (you'll meet decibels in the next lesson). The rounding error that remains sounds like a low hiss, the noise floor.

Why the industry went digital

• Perfect copies. Numbers can be copied, switched and stored without degrading. A tenth-generation copy is identical to the original.
• Noise immunity. An analog signal picks up noise along every meter of cable and the noise is now part of the signal. A digital receiver only has to tell a 1 from a 0.
• Networkable. Once it's data, video and audio can travel over the same IP networks as everything else — the foundation of AV-over-IP.
• Processable. Scaling, mixing, echo cancellation and compression are all mathematics on numbers.

The digital cliff

Analog degrades gracefully: a long or poor cable gives a softer, noisier picture. Digital behaves differently. It stays perfect until errors exceed what the receiver can correct, then it fails abruptly — sparkles, dropouts, or a black screen. This is the digital cliff.