Digital Audio and Video
What this domain tests
Conversion of analogue sound and moving images into digital files: sampling, quantisation, bitrate, codecs, containers and delivery.
Core rules and concepts
- Sampling rate is how many times per second the waveform is measured; bit depth is how many levels each sample can take.
- The Nyquist rule: sample at more than twice the highest frequency you want to capture. At 44.1 kHz the usable limit is about 22.05 kHz, just above human hearing.
- PCM bitrate = sample rate × bit depth × channels. For CD stereo: 44,100 × 16 × 2 = 1,411,200 bits per second, about 1.41 Mbps.
- Audio formats: WAV/AIFF uncompressed; FLAC lossless compressed; MP3/AAC lossy.
- Video: frame rate (24, 25, 30, 60 fps), resolution, aspect ratio (16:9), bitrate. H.264, H.265 and VP9 are codecs; MP4 and WebM are containers. A container can hold several streams.
- Streaming adapts quality to bandwidth; progressive download saves the file while playing.
Worked example
Three minutes of CD-quality stereo PCM. Reasoning: 1,411,200 bits/s × 180 s = 254,016,000 bits. Divide by 8 for bytes: 31,752,000 bytes, roughly 31.7 MB. Encoding at 128 kbps lossy gives 128,000 × 180 ÷ 8 ≈ 2.88 MB, a large saving with some quality loss.
Traps that cost marks
Confusing a codec with a container ("MP4" is not an encoder); claiming a higher sample rate adds frequencies that were never recorded; re-encoding lossy audio repeatedly; assuming higher resolution alone improves a badly lit video.