Live Streaming
Definition
Live streaming compresses a video signal into a data stream and sends it over a network to a platform. Compression takes place in an encoder that applies a codec. H.264 (AVC) is supported by almost every platform and player. HEVC (H.265) reaches comparable subjective quality at roughly half the bitrate, according to the verification tests of the JCT-VC, but requires more processing power. The bitrate is the main setting, and it has to stay within what the uplink can sustain. Above that capacity the buffer empties and the stream stalls; at a bitrate too low for the content, compression artefacts such as blocking appear. Common practice sets the bitrate well below the measured upload capacity, leaving a margin (headroom) for variations in the connection.
The guide
A transport protocol carries the stream from the encoder to the platform. RTMP is the long-established ingest protocol, and platforms such as YouTube recommend RTMPS, its TLS-encrypted form. SRT (Secure Reliable Transport), released as open source by Haivision in 2017, runs over UDP, retransmits lost packets (ARQ) and supports AES encryption, which suits connections of uncertain quality. The keyframe interval also matters: YouTube recommends 2 seconds and no more than 4. A shorter interval lets the stream recover faster after a loss, at some cost in compression efficiency.
A stalled stream interrupts the audience, while a slightly lower bitrate only softens the picture, so production practice gives stability priority over maximum quality. The bitrate is capped conservatively, the uplink is wired wherever possible, and a bonded or backup connection is kept ready for important events.
The rule
The bitrate is set with a margin below the measured uplink capacity so that the stream stays stable.
Glossary terms
Sources
- Fraunhofer HHI, HEVC/H.265 : environ 50 % de débit en moins que H.264 à qualité subjective égale · hhi.fraunhofer.de
- YouTube Help, réglages d'encodeur : image clé toutes les 2 s, pas plus de 4 s · support.google.com
- YouTube Help : chiffrement du flux par RTMPS · support.google.com
- Ohm, Sullivan et al., IEEE TCSVT (2016), tests de vérification HEVC : environ la moitié du débit de H.264/AVC à qualité subjective égale · microsoft.com
- Haivision/srt (GitHub), SRT open source : UDP, retransmission ARQ, chiffrement AES · github.com
- Secure Reliable Transport (Wikipédia, pointeur secondaire) : publication en open source par Haivision en 2017 · en.wikipedia.org