Audio & video

How an encoder orders I, P and B frames into groups of pictures: the knob behind seekability, compression, latency and stream switching.

Per-block QP offsets driven by local texture energy: spend bits on flat areas where blocking shows, save them where texture masks the error.

The client fetches short HTTP segments and picks each segment's rendition from throughput estimates, buffer level, or both, trading stalls against quality and switches.

ISO/IEC 23001-7 fixes how MP4 samples are encrypted and where key ID, IVs and clear/protected byte ranges are stored, so one file serves many DRM systems.

Turn one source into resolution x bitrate x codec renditions chosen from measured rate-quality points, cut on aligned keyframes so players can switch mid-stream.

How audio and video stay lip-synced: per-stream timebases, PTS/DTS, PCR and RTCP clock mapping, and a master clock that slave streams chase by dropping, repeating or resampling.

Full-reference scores that say how bad a compressed picture looks: PSNR (pixel error), SSIM (structure), VMAF (learned fusion), and when each one lies.

RTP stamps each media packet with sequence number, media-clock timestamp and source ID; RTCP reports loss, jitter, round trip and clock mapping within a bandwidth cap.

A receiver-side buffer holds packets just long enough, a delay tracked from recent arrivals, to reorder them and hand out media at a steady rate.

A server receives each participant's media once and forwards chosen packets to every other participant, picking the quality layer per receiver without decoding anything.

Learn your public address with STUN, swap candidate addresses over signalling, test pairs with ICE checks, and fall back to a TURN relay if no direct path works.

Per-packet encryption, authentication and replay protection for RTP and RTCP that keeps the headers readable, with the packet index derived from the 16-bit sequence number.

Predict each sample from the previous p; code only the small prediction error plus p coefficients. Core of FLAC, speech codecs and the Opus SILK layer.