Minn\Media\Av

An AC-3 (Dolby Digital) frame's header as the reference's media reader reads it (probe media-meta): the rate, bitrate and channels, the low frequency channel counted as ".1". The centre mix level is read for every odd coding mode, mono included, so a mono frame reads its next bits as the low frequency channel; the channel modes follow the coding mode.

A media file's analysis, keyed as the reference's media reader keys the one it hands to the wp_read_audio_metadata and wp_read_video_metadata filters (probe media-meta): the file's size, format and type, its audio and video streams, its playing time and bitrate, and its tags. The format is found by the file's first bytes; a file named .mp3 with none is still typed as MPEG audio.

Advanced Systems Format files (WMV, WMA, ASF) as the reference's media reader measures them (probe media-meta). The file properties give the playing time (less the preroll) and creation date; each stream's properties its size or wave format; the codec list the names; the stream bitrates the bitrate, the file's being their sum. A video file with no audio still describes an empty ASF audio stream.

A bit reader over a byte string for the exponential-Golomb codes of a video parameter set: single bits, unsigned and signed codes. Reading past the end reads zeros, and a code longer than 32 bits stops at 32.

FLAC, on its own or as the first packet of an Ogg stream, as the reference's media reader measures it (probe media-meta): the stream information block gives the rate, channels, depth and samples, the comment block names the encoder, and a picture block is the cover.

Flash video (FLV) as the reference's media reader measures it (probe media-meta). The onMetaData script tag gives the playing time, frame rate, data rates and codecs; the first audio tag gives the rate, sample size and channels; the first video tag's Sorenson picture header the size, or for H.264 its sequence parameter set, read as the reference reads it. The video's data rate is kept as written (kilobits), the audio's made bits; a file with no audio and no video rate gives the video the file's own rate.

The genre names an ID3 tag refers to by number: the ID3v1 list with the Winamp additions (0 to 191) and a later run from 240. An ID3v2 genre names an unknown number as an empty string, and 255 as "Unknown"; an ID3v1 tag names neither (probe media-meta).

ID3 tags, as the reference's audio reader names what they say (probe media-meta): an ID3v2 tag (versions 2.2 to 2.4) at the start of a file and an ID3v1 tag in its last 128 bytes. Each frame the reader knows becomes a named list of values in the order the frames come; one it does not know (a play count, a private frame, a podcast flag) is left out. ID3v2 text is made UTF-8 whatever its encoding; ID3v1 text is kept as its bytes.

MPEG-4 and QuickTime files (MP4, M4A, M4V, MOV) as the reference's media reader measures them (probe media-meta). The movie header gives the playing time and the time it was made; each track's first sample entry gives its codec. Only AAC ("mp4a") is read as audio, and it alone makes a file audio/mp4 (or video/mp4 beside a video track); anything else is video/quicktime. A video entry with a vendor is read the QuickTime way (its compressor's name, lossless false); one without, the ISO way (a named codec, and "mp4" as the format for the codecs it knows). Only H.264 and MPEG-4 video have the bitrate the audio leaves.

The LAME tag an encoder writes after the Xing or Info counts, and what the reference's audio reader makes of it (probe media-meta): the bitrate method names the bitrate mode, and the encoder options are guessed from the preset number, or failing that from the mode, then the minimum bitrate, gapless encoding, a lowpass other than the preset's own and a resampled rate follow.

Matroska and WebM files (MKV, MKA, WebM) as the reference's media reader measures them (probe media-meta). The segment information gives the playing time and the date; the first video and audio tracks give the streams, named by their codec where the reader knows it (and by the codec identifier where it does not). MP3 and MP2 audio is described from its first frame, FLAC from its stream information, AC-3 from its first frame; the bitrate is the file's over its playing time, the video keeping what a known audio bitrate leaves.

MPEG audio (MP3, and layers 1 and 2) as the reference's audio reader measures it (probe media-meta). The first frame gives the version, layer, sample rate and channels. An Xing or Info frame (written by LAME and most encoders) gives the frame and byte counts: a variable bitrate is the bytes over the frames' duration, and the playing time is the audio's bytes less one average frame, at that bitrate. Without one the bitrate is the first frame's and the playing time is every byte at it.

Ogg files as the reference's media reader measures them (probe media-meta): Vorbis, Opus, or FLAC in its first packet. The audio is counted from the end of the comment header to the start of the last page, and the last page's granule position over the rate is the playing time (Opus's pre-skip included). A Vorbis stream's options are the quality its nominal bitrate stands for, when libVorbis made it in stereo at 44.1 or 48 kHz, and otherwise the nominal bitrate itself.

RIFF files, WAV audio and AVI video, as the reference's media reader measures them (probe media-meta). A WAV's format chunk describes its stream (the codec named by its format tag, lossless only for PCM) and its data chunk times it. An AVI's headers give the frames and their rate, each stream's format, and the movie list that holds the data; its video has the bitrate the audio's leaves. MP3 audio in an AVI (or in a WAV that states its rate) is described from its first frame.

A media file read in parts, so a large video is never loaded whole: its size, a run of bytes at an offset, and the integers the containers write. The file's first block is kept, since every reader starts there.