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doc:appunti:linux:video:glossary

Audio Video Editing Glossary

AAC

Advanced Audio Coding is the audio coding standard defined by the International Organization for Standardization (ISO) as part of the MPEG-2 specification. It is considered to be the actual “state of the art” in general audio coding and the successor of MP3. Compared to MP3, AAC provides higher quality music with approximately 30% less storage space or bandwidth. AAC provides up to 48 full-bandwidth audio channels with sample rates up to 96 kHz, plus 15 low frequency (limited to 120 Hz) channels. Someone wants to limit the use of this format enforcing patents, see www.vialicensing.com.

ABR

Average Bit Rate. For a lossy compression algorithm it would be better to encode the analog signal with more bits when the signal is complex (say a piece of music with many instruments playing together), and few bits when the signal is simple. In this case the bit rate is not constant, we can instead speak of an Average Bit Rate. Seeking a specific time offset into an ABR file is not simple without decoding all the previous samples or without building a time map.

AC-3

Format for audio files also known as Dolby Digital. It can contain from 1 to 5 full-range channels (10 Hz-22 kHz), plus a limited channel (10 Hz-120 Hz, Low Frequency Effect channel) reserved for bass tones to be reproducted by a subwoofer. Generally the audio in a DVD Video Disk is coded in some AC-3 variant. Audio is compressed (with loss) approximately 12:1 compared to PCM.

CBR

Constant Bit Rate. The analog signal will be encoded in the same amount of bits for each unit of time. For example a 128 kbits rate will use (128 * 1000) / 8 = 16000 bytes per second. With CBR files it is easy to seek an exact time offset, because offset = time * bitrate.

CD size

Each manufacturer can impose some differencies on writable optical media capacity, however the canonical sizes measured in minutes are:

Min Sect/min Sectors Bytes Mb Nominal Size
CD 74 min 60 * 75 333.000 681.984.000 650,391 650 Mb
CD 80 min 60 * 75 360.000 737.280.000 703,125 700 Mb
DVD-R 120 min 4.706.074.624 4.488,063 4.7 Gb
DVD-R 120 min Sony 4.707.319.808 4.489,250 4.7 Gb
DVD+R 120 min 4.700.372.992 4.482,063 4.7 Gb

Frame types

Type What they are
I Intra-coded frames, average 7:1 reduction. Like JPEG, every video frame is broken into blocks of 8×8 pixels of Y, R-Y, and B-Y
P P frames are predicted based on prior I or P frames plus the addition of data for changed macroblocks. Average about 20:1 reduction, or about half the size of I frames.
B Bidirectionally predicted frames based on appearance and positions of past and future frames macroblocks. B frames require less data than P frames, averaging about 50:1 reduction. B frames require more decoder buffer memory because 2 frames are compared during the reconstruction process. B frames also require manipulation of the coding order: frames moving from the coder to the decoder are NOT in presentation sequence.

Joint Stereo

Joint stereo coding methods try to increase the coding efficiency when encoding stereo signals by exploiting commonalties between the left and right signal. It can disturb Dolby Surround (Dolby Pro Logic) signal, it is suggested specially for low bit rates (⇐ 128 kbit/s) and when the sound has less stereo effects.

MP3

MPEG-1 Layer 3 audio files. It is a compression technique for audio recording. The compression ratio usually is 12:1 (128..112 kbps for a stereo signal) and there is only very little loss of quality. Basically, this is realized by perceptual coding techniques addressing the perception of sound waves by the human ear. This technique was developed by Fraunhofer IIS. There are some patent issues on the software licensing, which concern Free Software.

MP2

MPEG-1 Layer 2 audio files. Compression technique less powerful than Layer 3 (mp3), but very similar in principles. The compression ratio usually is 8:1 (corresponds to 256..192 kbps for a stereo signal). It seems that no patents are claimed on MP2 compression, but the fact that MP2 and MP3 are derivatives of the same algorithm family, can pose doubts for patent issues on this technique too.

MPEG

The Moving Picture Experts Group (MPEG) is a small group charged with the development of video and audio encoding standards. Since its first meeting in 1988, MPEG has grown to include approximately 350 members from various industries and universities. MPEG has standardized the following compression formats and ancillary standards:

  • MPEG-1

Initial video and audio compression standard. Later used as the standard for Video CD, and includes the popular Layer 3 (MP3) audio compression format.

  • MPEG-2

Transport, video and audio standards for broadcast-quality television. Used for over-the-air digital television ATSC and DVB, digital satellite TV services like DirecTV, digital cable television signals, and (with slight modifications) for DVD video discs.

  • MPEG-3

Originally designed for HDTV, but abandoned when it was discovered that MPEG-2 was sufficient for HDTV.

  • MPEG-4

Expands MPEG-1 to support video/audio “objects”, 3D content, low bitrate encoding and support for Digital Rights Management. A new (newer than MPEG-2 Video) higher efficiency video codec is included (an alternative to MPEG-2 Video), see H.264

Ogg Vorbis

Ogg Vorbis is a fully open, non-proprietary, patent-and-royalty-free, general-purpose compressed audio format for audio and music at fixed and variable bitrates from 16 to 128 kbps/channel. Ogg Vorbis has been designed to completely replace all proprietary, patented audio formats. That means that you can encode all your music or audio content in Vorbis and never look back. The Home Page for the project is http://www.vorbis.com/.

PAL

Formato video televisivo adottato in Italia ed altri paesi (Phase Alternating Line), formato video 4:3, 625 linee nominali, 25 frame al secondo (sistema B, G, H, I, e N). In realtà il formato è interlaced, cioè si trasmettono 50 mezzi frame al secondo, alternativamente le linee pari e quelle dispari. Essendo 625 un numero dispari, le linee pari e quelle dispari sono esattamente 312.5, cioè l'immagine inizia a metà della prima riga e termina a metà dell'ultima. Inoltre alcune delle prime linee ed alcune delle ultime non trasportano segnale video, in definitiva si hanno un massimo di 576 linee utili.

Non ha senso parlare di larghezza in pixel, in quanto si tratta di linee analogiche, tuttavia essendo un formato 4:3, assumendo il pixel quadrato e considerando solo le linee utili, si può dire che la conversione in digitale di un video in formato PAL deve avere una risoluzione di 768 x 576 pixel. Per approfondire: The 625/50 PAL Video Signal and TV Compatible Graphics Modes.

NTSC

Composto da 525 linee orizzontali

QPEL

Quarter Pixel Motion Search Precision. Basically most MPEG-4 codecs by default detect motion between two frames down to half a pixel (HalfPel). with QuarterPel you can detect motion that is only a quarter of a pixel per frame, effectively doubling precision! Practically this means that you will get a much sharper image with QPEL.

Region Codes

Region Codes are part of the DVD Standard. There is a Region number located within one or all of the components required for DVD-VIDEO playback. The Region number defines the region of the DVD-ROM drive and its playback hardware/software. DVD-Video discs may also contain a Region number in the shape of a world globe. Unless the Region number on both the DVD-Video disc and DVD-ROM drive and its playback components match, playback is not possible. When the word “ALL” is located in the world globe on a DVD-Video disc, that particular DVD-Video disc can be played on all DVD-ROM drives and its playback components, regardless of its region.

Region Codes
1 USA, Canada
2 Japan, Europe, South Africa, Middle East, Greenland
3 S.Korea, Taiwan, Hong Kong, Parts of South East Asia
4 Australia, New Zealand, Latin America (including Mexico)
5 Eastern Europe, Russia, India, Africa
6 China
7 Reserved for Unspecified Special Use
8 Resevered for Cruise Ships, Airlines, etc…
0 or ALL Discs are uncoded and can be played Worldwide

RPC mode 2 protection

All DVD drives shipped today have RPC mode 2 protection - that means that you can only switch your region code five times before it is locked. This time the region protection is hardware. No software will get around RPC-2 DVD-ROM drives hardware region protection with the exception of the library libdvdcss. It doesn't require the region of your drive to be set and will try its best to read from the disc even in the case of a region mismatch: it uses a crypto attack, so it can fail. Most Linux software (mplayer, dvdbackup, …) are linked against this library.

Some DVD drives are “downgradeable” to RPC-1 (software based) protection with a pirate firmware flash.

On Linux you can use the regionset software to see the actual setting of the DVD drive and the number of remaining changes. The utility can also change the region code.

regionset version 0.1 -- reads/sets region code on DVD drives
Current Region Code settings:
RPC Phase: II
type: NONE
vendor resets available: 4
user controlled changes resets available: 5
drive plays discs from region(s):, mask=0xFF
Would you like to change the region setting of your drive? [y/n]:n

VBR

Variable Bit Rate. The same considerations made for ABR applies here. For the Lame encoder, the VBR compression is also called Extreme because it works computing the actual encoding/quantization error and using more bits to minimize the error.

VobSub

VobSub is a well known subtitle format that saves subtitles nearly in the same format as it appears in DVD subtitle streams. From a technical point of view, VobSub saves subtitles as little images. Subtitles in this format are generally saved as a set of two files; one .sub file, which actually contains the images, and one .idx file with the timeline information.

XviD

XviD is an ISO MPEG-4 compliant video codec. It's no product, it's an open source project which is developed and maintained by lots of people from all over the world. See http://www.xvid.org/. Some features of XviD codec are believed to be covered by software patents in a number of countries (notably US or Japan). Because of this, XviD 0.9.x versions were not licensed in countries where these type of patents are enforcible. With the 1.0.x releases, a plain GNU GPLv2 license is used with no explicit geographical restriction. However the legal usage of XviD may still be restricted by local laws.

doc/appunti/linux/video/glossary.txt · Last modified: 2006/11/06 14:54 (external edit)