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-aac_rc cbr|abr (default cbr). ABR runs the nd-target quality solve with the set-point owned by a slow rate servo, and joins VBR as a quality-target mode in the psy model (fixed-reference PE reduction, quality attack map, broadband mask floor). The servo integrates the log rate error but applies it to the nd set-point in rare, discrete, calm-gated steps. The set-point must be quasi-static: any drift on content timescales moves lambda, and with it band/PNS/scalefactor state, tracking sections in antiphase and coarsening exactly what needs bits - measured 2x worse than a fixed target at equal mean rate. Locally this mode IS fixed-target VBR; rate honesty converges on the minutes scale. Bootstrap. The set-point is seeded from the VBR calibration anchor (one constant, NMR_VBR_ANCHOR: the two are required to agree, and moving only one copy made ABR overshoot its rate target by up to 9%). After ~2s of fast rate measurement off a representative window (an all-transient head reads over-target and would coarsen the very content that needs bits), an open-loop jump over the measured loop gain (~ -0.24 log2 rate per target unit) corrects the seed's per-content error, applied at 1.2x - the estimator's own inverse warm-up factor, since the EMA is seeded on target (a cold start must not read as a fake deficit) and so recovers only ~82% of the error. One correction is not enough whatever the gain estimate, because the 0.24 loop gain is only nominal: the boot re-arms while the rate is still off, up to a bounded number of times, re-measuring each time, and re-seeds the estimator on target rather than to zero (a zero estimate pins the integrator at its clip via log2f(0) and reads as a 100% deficit to the fill in the same frame). Its settle window sits below the stepper's hold, or the stepper resets the shared counter first and the re-arm never fires. Every set-point correction is glided in at ~0.5 log2-units/s rather than stepped: a one-frame noise-floor jump at a fixed stream time (the boot lands at ~2s in every stream) is audible against revealing content even at step sizes the corpus cannot see. Bits-fill. Under the long-run target, an easy frame is topped up toward the ask by moving lambda finer only - the quality target is a floor, never traded away. It spends the rate the user asked for on sub-mask margin the nd statistic cannot see (it clips at -4/band); without it, stat-transparent content caps the rate below any ask. The fill runs from frame one and fades continuously as the rate EMA converges (a boot-gated fill flipped regimes at ~2s, an audible quality step), follows the psy PE demand shape, and is slew-bounded, writing the lambda continuity state from long frames only instead of free-diving up to 64x per frame. The un-booted stream head behaves CBR-like until the servo takes over (love.flac: >6k energy had swung 0.61-1.83x vs the source at frame rate while the head ran 25% under the ask). The reachable-range clamps do not track the calibration anchor: their floors are set by the stat's -4/band sub-mask clip. A floor at -1.65 railed the servo on easy content (male_speech at a 128k ask pinned at the clamp with a 10% standing deficit); -3.0 keeps 384k asks reachable without the stall. Easy content can still fall short of very high asks (main_theme: 196k at a 256k ask) - there is no padding. The coarse clamp is a quality guard: set-points past it buy little rate for a lot of quality (2.5 -> 6.0 at a 48k ask: -5% rate, +32% Zimtohrli), so asks below about 56 kbps stereo land above the ask (24k/32k asks: ~40k). The documentation points those rates at CBR. Rate accuracy against the -b:a ask: corpus bias mean|err| worst IgorC-3 -1.3% 2.2% -11.8% IgorC-1 +1.1% 2.0% +5.0% 32 random -0.3% 2.2% -10.0% A 4.5-minute concatenation of the IgorC-3 samples lands at 128.2 kbps on a 128k ask. castanets/velvet ABR beat CBR; ABR corpus 0.00060 at 126k. -aac_rc is documented, and fate-aac-nmr-abr-encode pins the per-packet sizes through the servo's boot.
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides means to alter decoded audio and video through a directed graph of connected filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.