Video glossary: bitrate, codec, HDR, latency and other terms

4 min read

When you set up a player or prepare your videos, words come up that aren’t always explained. Here they are, grouped by topic, each in a few lines.

The picture

Resolution: the number of pixels in the picture, width by height. 1280×720 is “720p”; 1920×1080, “1080p” or Full HD; 3840×2160, 4K UHD. More resolution gives more detail if the bitrate keeps up.
Progressive and interlaced: the “p” in 1080p means each frame is complete. The “i” in 1080i means the even and odd lines are drawn alternately, as on old-style TV. If interlacing isn’t handled well, you see comb-shaped edges.
Frames per second (fps): how many images are shown each second. Movies are usually 24; European TV, 25 or 50. More fps means smoother motion.
Aspect ratio: the proportion between width and height: 16:9 on today’s TVs, 4:3 on old ones, wider still in many movies. In Whatplay, “Fit” changes how the picture fills the screen.
HDR: high dynamic range. More difference between the darkest and the brightest parts, and more colors. Its best-known formats are HDR10, HDR10+, Dolby Vision and HLG. It needs a compatible screen.
SDR: standard dynamic range, the classic kind.
Color depth: the bits for each color. With 8 bits there are about 16.7 million colors; with 10, over a billion, and gradients look smooth, without banding. HDR uses 10 bits or more.

Codec: the method that compresses and decompresses the picture or the sound. For video: H.264, HEVC, VP9, AV1. For audio: AAC, AC-3, Opus. They’re explained in video and audio formats.
Container: the file that bundles the video, audio and subtitles: MP4, MKV, TS. The extension tells you the container, not the codec.
Bitrate: the amount of data per second, in megabits per second (Mb/s). It determines both quality and data use.
CBR and VBR: constant or variable bitrate. Variable bitrate gives more data to complex scenes and less to simple ones.
Keyframe: a complete frame. The frames in between only store what changes. To jump to a point, the player has to start from one of these, which is why a jump can take a moment.
Profile and level: variants of the same codec that indicate how demanding a video is. A device may open H.264 but not a high profile or level.
Hardware and software decoding: decompressing with a dedicated circuit in the chip, which uses little power, or with the main processor, which supports more formats but uses more.
Converting (transcoding): decompressing and recompressing into another format. It takes work and costs some quality.
Repackaging (remuxing): changing the container without touching the codecs. It’s fast and loses no quality.

Channels: 2.0 is stereo; 5.1 is five speakers plus a subwoofer (the “.1”); 7.1, seven plus the subwoofer.
Downmixing to stereo: converting 5.1 sound into two channels for headphones or the phone’s speaker.
Sample rate: how many times per second the sound is measured; the usual rate for video is 48 kHz.
Track: each of the audio or subtitle streams in a video, such as a language or a commentary.

Streaming: watching while it downloads, without waiting for the whole file.
Adaptive streaming: the video is offered in several qualities and the player picks the one the network allows. HLS and DASH use it, and we explain them in HLS and DASH.
Manifest: the index of an adaptive video, with its qualities and its chunks: an .m3u8 in HLS or an .mpd in DASH.
Segment: each of the chunks, a few seconds long, that an adaptive video is split into.
Buffer: the seconds the player downloads ahead, to ride out network dips.
Latency: the delay between what happens and what you see. In an internet broadcast it’s usually several seconds, more than over the air.
Bandwidth: what your connection provides, in Mb/s. Don’t confuse megabits (Mb) with megabytes (MB): 8 Mb make 1 MB.
CDN: a network of servers spread around the world that serves the video from one close to you.
CORS: a web rule by which a server says who’s allowed to request content from it. That’s why a video your phone can open may not open on a Chromecast on its own.
DRM: an encryption and licensing system that only lets content be watched in authorized players.

M3U and W3U: list formats. M3U is text, line by line; W3U is JSON with groups within groups.
EPG: the electronic program guide: what’s on now and next.
XMLTV: the most widely used open format for those guides. We explain it in the XMLTV TV guide.
tvg-id: the identifier that links a channel in an M3U list to its schedule in the guide.

Chromecast (Cast): Google’s system with which your phone tells the TV what to play and the TV downloads it itself.
DLNA and UPnP: long-standing standards for sharing content between devices on your home network.
NAS: network-attached storage, a small computer with disks that stores files for the whole household.
Local IP: a device’s address within your network, such as 192.168.1.20. It only works from inside your home.
Turning on over the network: a feature of some TVs, sometimes called “Turn on via Wi-Fi”, that lets another device switch them on remotely.