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diff --git a/DOCS/OUTDATED-tech/general.txt b/DOCS/OUTDATED-tech/general.txt new file mode 100644 index 0000000000..36a584b746 --- /dev/null +++ b/DOCS/OUTDATED-tech/general.txt @@ -0,0 +1,228 @@ +So, I'll describe how this stuff works. + +The main modules: + +1. stream.c: this is the input layer, this reads the input media (file, stdin, + vcd, dvd, network etc). what it has to know: appropriate buffering by + sector, seek, skip functions, reading by bytes, or blocks with any size. + The stream_t (stream.h) structure describes the input stream, file/device. + + There is a stream cache layer (cache2.c), it's a wrapper for the stream + API. It does fork(), then emulates stream driver in the parent process, + and stream user in the child process, while proxying between them using + preallocated big memory chunk for FIFO buffer. + +2. demuxer.c: this does the demultiplexing (separating) of the input to + audio, video or dvdsub channels, and their reading by buffered packages. + The demuxer.c is basically a framework, which is the same for all the + input formats, and there are parsers for each of them (mpeg-es, + mpeg-ps, avi, avi-ni, asf), these are in the demux_*.c files. + The structure is the demuxer_t. There is only one demuxer. + +2.a. demux_packet_t, that is DP. + Contains one chunk (avi) or packet (asf,mpg). They are stored in memory as + in linked list, cause of their different size. + +2.b. demuxer stream, that is DS. + Struct: demux_stream_t + Every channel (a/v/s) has one. This contains the packets for the stream + (see 2.a). For now, there can be 3 for each demuxer : + - audio (d_audio) + - video (d_video) + - DVD subtitle (d_dvdsub) + +2.c. stream header. There are 2 types (for now): sh_audio_t and sh_video_t + This contains every parameter essential for decoding, such as input/output + buffers, chosen codec, fps, etc. There are each for every stream in + the file. At least one for video, if sound is present then another, + but if there are more, then there'll be one structure for each. + These are filled according to the header (avi/asf), or demux_mpg.c + does it (mpg) if it founds a new stream. If a new stream is found, + the ====> Found audio/video stream: <id> messages is displayed. + + The chosen stream header and its demuxer are connected together + (ds->sh and sh->ds) to simplify the usage. So it's enough to pass the + ds or the sh, depending on the function. + + For example: we have an asf file, 6 streams inside it, 1 audio, 5 + video. During the reading of the header, 6 sh structs are created, 1 + audio and 5 video. When it starts reading the packet, it chooses the + stream for the first found audio & video packet, and sets the sh + pointers of d_audio and d_video according to them. So later it reads + only these streams. Of course the user can force choosing a specific + stream with + -vid and -aid switches. + A good example for this is the DVD, where the english stream is not + always the first, so every VOB has different language :) + That's when we have to use for example the -aid 128 switch. + + Now, how this reading works? + - demuxer.c/demux_read_data() is called, it gets how many bytes, + and where (memory address), would we like to read, and from which + DS. The codecs call this. + - this checks if the given DS's buffer contains something, if so, it + reads from there as much as needed. If there isn't enough, it calls + ds_fill_buffer(), which: + - checks if the given DS has buffered packages (DP's), if so, it moves + the oldest to the buffer, and reads on. If the list is empty, it + calls demux_fill_buffer() : + - this calls the parser for the input format, which reads the file + onward, and moves the found packages to their buffers. + Well it we'd like an audio package, but only a bunch of video + packages are available, then sooner or later the: + DEMUXER: Too many (%d in %d bytes) audio packets in the buffer + error shows up. + +2.d. video.c: this file/function handle the reading and assembling of the + video frames. each call to video_read_frame() should read and return a + single video frame, and it's duration in seconds (float). + The implementation is splitted to 2 big parts - reading from mpeg-like + streams and reading from one-frame-per-chunk files (avi, asf, mov). + Then it calculates duration, either from fixed FPS value, or from the + PTS difference between and after reading the frame. + +2.e. other utility functions: there are some useful code there, like + AVI muxer, or mp3 header parser, but leave them for now. + +So everything is ok 'till now. It can be found in libmpdemux/ library. +It should compile outside of mplayer tree, you just have to implement few +simple functions, like mp_msg() to print messages, etc. +See libmpdemux/test.c for example. + +See also formats.txt, for description of common media file formats and their +implementation details in libmpdemux. + +Now, go on: + +3. mplayer.c - ooh, he's the boss :) + Its main purpose is connecting the other modules, and maintaining A/V + sync. + + The given stream's actual position is in the 'timer' field of the + corresponding stream header (sh_audio / sh_video). + + The structure of the playing loop : + while(not EOF) { + fill audio buffer (read & decode audio) + increase a_frame + read & decode a single video frame + increase v_frame + sleep (wait until a_frame>=v_frame) + display the frame + apply A-V PTS correction to a_frame + handle events (keys,lirc etc) -> pause,seek,... + } + + When playing (a/v), it increases the variables by the duration of the + played a/v. + - with audio this is played bytes / sh_audio->o_bps + Note: i_bps = number of compressed bytes for one second of audio + o_bps = number of uncompressed bytes for one second of audio + (this is = bps*samplerate*channels) + - with video this is usually == 1.0/fps, but I have to note that + fps doesn't really matters at video, for example asf doesn't have that, + instead there is "duration" and it can change per frame. + MPEG2 has "repeat_count" which delays the frame by 1-2.5 ... + Maybe only AVI and MPEG1 has fixed fps. + + So everything works right until the audio and video are in perfect + synchronity, since the audio goes, it gives the timing, and if the + time of a frame passed, the next frame is displayed. + But what if these two aren't synchronized in the input file? + PTS correction kicks in. The input demuxers read the PTS (presentation + timestamp) of the packages, and with it we can see if the streams + are synchronized. Then MPlayer can correct the a_frame, within + a given maximal bounder (see -mc option). The summary of the + corrections can be found in c_total . + + Of course this is not everything, several things suck. + For example the soundcards delay, which has to be corrected by + MPlayer! The audio delay is the sum of all these: + - bytes read since the last timestamp: + t1 = d_audio->pts_bytes/sh_audio->i_bps + - if Win32/ACM then the bytes stored in audio input buffer + t2 = a_in_buffer_len/sh_audio->i_bps + - uncompressed bytes in audio out buffer + t3 = a_buffer_len/sh_audio->o_bps + - not yet played bytes stored in the soundcard's (or DMA's) buffer + t4 = get_audio_delay()/sh_audio->o_bps + + From this we can calculate what PTS we need for the just played + audio, then after we compare this with the video's PTS, we have + the difference! + + Life didn't get simpler with AVI. There's the "official" timing + method, the BPS-based, so the header contains how many compressed + audio bytes or chunks belong to one second of frames. + In the AVI stream header there are 2 important fields, the + dwSampleSize, and dwRate/dwScale pairs: + - If the dwSampleSize is 0, then it's VBR stream, so its bitrate + isn't constant. It means that 1 chunk stores 1 sample, and + dwRate/dwScale gives the chunks/sec value. + - If the dwSampleSize is >0, then it's constant bitrate, and the + time can be measured this way: time = (bytepos/dwSampleSize) / + (dwRate/dwScale) (so the sample's number is divided with the + samplerate). Now the audio can be handled as a stream, which can + be cut to chunks, but can be one chunk also. + + The other method can be used only for interleaved files: from + the order of the chunks, a timestamp (PTS) value can be calculated. + The PTS of the video chunks are simple: chunk number * fps + The audio is the same as the previous video chunk was. + We have to pay attention to the so called "audio preload", that is, + there is a delay between the audio and video streams. This is + usually 0.5-1.0 sec, but can be totally different. + The exact value was measured until now, but now the demux_avi.c + handles it: at the audio chunk after the first video, it calculates + the A/V difference, and take this as a measure for audio preload. + +3.a. audio playback: + Some words on audio playback: + Not the playing is hard, but: + 1. knowing when to write into the buffer, without blocking + 2. knowing how much was played of what we wrote into + The first is needed for audio decoding, and to keep the buffer + full (so the audio will never skip). And the second is needed for + correct timing, because some soundcards delay even 3-7 seconds, + which can't be forgotten about. + To solve this, the OSS gives several possibilities: + - ioctl(SNDCTL_DSP_GETODELAY): tells how many unplayed bytes are in + the soundcard's buffer -> perfect for timing, but not all drivers + support it :( + - ioctl(SNDCTL_DSP_GETOSPACE): tells how much can we write into the + soundcard's buffer, without blocking. If the driver doesn't + support GETODELAY, we can use this to know how much the delay is. + - select(): should tell if we can write into the buffer without + blocking. Unfortunately it doesn't say how much we could :(( + Also, doesn't/badly works with some drivers. + Only used if none of the above works. + +4. Codecs. Consists of libmpcodecs/* and separate files or libs, + for example libmpeg2, loader, mp3lib. + + mplayer.c doesn't call them directly, but through the dec_audio.c and + dec_video.c files, so the mplayer.c doesn't have to know anything about + the codecs. + + libmpcodecs contains wrapper for every codecs, some of them include the + codec function implementation, some calls functions from other files + included with mplayer, some calls optional external libraries. + file naming convention in libmpcodecs: + ad_*.c - audio decoder (called through dec_audio.c) + vd_*.c - video decoder (called through dec_video.c) + ve_*.c - video encoder (used by mencoder) + vf_*.c - video filter (see option -vf) + + On this topic, see also: + libmpcodecs.txt - The structure of the codec-filter path, with explanation + dr-methods.txt - Direct rendering, MPI buffer management for video codecs + codecs.conf.txt - How to write/edit codec configuration file (codecs.conf) + codec-devel.txt - Mike's hints about codec development - a bit OUTDATED + hwac3.txt - about SP/DIF audio passthrough + +5. libvo: this displays the frame. + + for details on this, read libvo.txt + +6. libao2: this control audio playing +6.a audio plugins + + for details on this, read libao2.txt |