The service requirements for visual telephone services are presented in Recommendation H.200/AV.120-Series; video and audio coding systems and other technical set aspects common to audiovisual services are covered in other Recommendations in the H.200/AV.200- Series.
This standard is intended for carrying video over ISDN - in particular for face-to- face videophone applications and for videoconferencing. Videotelephony is less demanding of image quality, and can be achieved for n=1 or 2. For videoconferencing applications (where there are more than one person in the field of view) higher picture quality is required and n must be at least 6.
There are actually two picture formats defined within H.261; CIF (Common Intermediate Format) has 288 lines by 360 pixels/line of luminance information and 144 x 180 of chrominance information; and QCIF (Quarter Common Intermediate Format) which is 144 lines by 180 pixels/line of luminance and 72 x 90 of chrominance. The choice of CIF or QCIF depends on available channel capacity - eg QCIF is normally used if n<3.
The actual encoding algorithm is similar to (but incompatible with) that of MPEG. Another difference is that H.261 needs substantially less CPU power for real-time encoding than MPEG. The algorithm includes a mechanism which optimises bandwidth usage by trading picture quality against motion, so that a quickly-changing picture will have a lower quality than a relatively static picture. H.261 used in this way is thus a constant-bit-rate encoding rather than a constant-quality, variable- bit-rate encoding.
To provide these services, a scheme is recommended in which a channel accommodates speech, and optionally video and/or data at several rates, in a number of different modes. Signalling procedures are required to establish a compatible mode upon call set-up, to switch between modes during a call and to allow for call transfer.
All audio and audiovisual terminals using G.722 audio coding and/or G.711 speech coding or other standardised audio codings at lower bit rates should be compatible to permit connection between any two terminals. This implied that a common mode of operation has to be established for the call. The initial mode might be the only one used during a call or, alternatively, switching to another mode can occur as needed depending on the capabilities of the terminals. Products complying to this recommendation include codecs from BT, GPT, PictureTel, VideoTel & others.
JPEG is "lossy", meaning that the image you get out of decompression isn't quite identical to what you originally put in. The algorithm achieves much of its compression by exploiting known limitations of the human eye, notably the fact that small colour details aren't perceived as well as small details of light- and-dark. Thus, JPEG is intended for compressing images that will be looked at by humans. If you plan to machine-analyse your images, the small errors introduced by JPEG may well be a problem for you, even if they are invisible to the eye.
MPEG 1 has been devised and is in three parts: video, audio and systems, where the last part provides for the integration of the audio and video streams with the proper time stamping to allow synchronisation of the two. The MPEG 1 standard is available as ISO CD 11172. MPEG has the potential for higher compression rates than H.261 (because of support for frame interpolation between frames, not just extrapolation) and it will also provide a standard for higher resolution images.
Supporters of the new Video on CD MPEG 1 standard include Philips (co-developer of the original audio CD); JVC (developer of the VHS video-tape standard); PC maker Commodore International; and Korean consumer electronics giants Samsung and Goldstar.
MPEG 2 is similar to MPEG 1, but includes extensions to cover a wider range of applications. It is designed to handle applications (e.g. video on CD) that need higher-quality playback. MPEG 2 supports CCIR 601 resolutions, but it requires transmission rates above 4 Mbps, as opposed to the 1 to 3 Mbps that MPEG 1 requires. It also requires a lot more processing time to encode the source video signal.
The primary application targeted during the MPEG 2 definition process was the all- digital transmission of broadcast TV quality video at coded bit-rates between 4 and 9 Mbit/sec. However, the MPEG-2 syntax has been found to be efficient for other applications such as those at higher bit rates and sample rages (e.g. HDTV). The most significant enhancement over MPEG-1 is the addition of syntax for efficient coding of interlaced video (e.g. 16x8 block size motion compensation, Dual Prime, etc.).
The MPEG 2 Audio Standard supports low bit-rate coding of multichannel audio, supplying up to five full bandwidth channels (left, right, centre, and two surround channels), plus an additional low frequency enhancement channel, and/or up to seven commentary/multilingual channels. It extends the stereo and mono coding of the MPEG-1 Audio Standard to half sampling-rates (16 kHz, 22.05 kHz, and 24 kHz), for improved quality for bit- rates at or below 64 Kbits/s per channel. The MPEG 2 Systems Standard specifies coding formats for multiplexing audio, video and other data into a form suitable for transmission or storage.
MPEG 3 targeted HDTV applications with sampling dimensions up to 1920 x 1080 x 3 Hz and coded bit-rates between 20 and 40 Mbit/sec. It was later discovered that with some (compatible) fine tuning, MPEG 2 and MPEG 1 syntax worked very well for HDTV rate video.
MPEG 4 Work on a new initiative for very low bit-rate coding of audio-visual programs has been approved by unanimous ballot of all national bodies of ISO/IEC JTC1. This work began in September 1993 and is now in the application identification phase. It is scheduled to result in a draft specification in 1997. When completed, the MPEG-4 standard will enable a whole spectrum of new applications, including interactive mobile multimedia communications, videophone, mobile audio-visual communication, multimedia electronic mail, remote sensing, electronic newspapers, interactive multimedia databases, multimedia videotex, games, interactive computer imagery, sign language captioning. Since the primary target for these applications is a bit-rate of up to 64 kbit/s at good quality, it is anticipated that new coding techniques allowing higher compression than traditional techniques may be necessary.
Message handling technology is complex; as well as the sheer technical difficulties involved, as a global service it has had to take account of political, commercial, legal, and historical realities. Some issues which are dependent on national telecommunications regulation are not covered by the International Standards and are addressed by national standards.
The relatively poor penetration of X.400 messaging has been caused by a variety of factors. The heavy investment in developing 1984 products has lead to considerable resistance to change, regardless that global interconnectivity is severely constrained in 1984 products, and that 1984-1988 interworking degrades the quality of service offered. Paradoxically it is the attempt to recoup the investment in 1984 products which is impeding the introduction of 1988 products that are essential for a highly functional global messaging service.
Services typically required by multimedia conferences are playout synchronisation, demultiplexing, media identification and active-party identification. RTP is not restricted to multimedia conferences, however, and other real-time services such as data acquisition and control may use its services.
A QuickTime movie contains time based data which may represent sound, video or other time-sequenced information such as financial data or lab results. A movie is constructed of one or more tracks, each track being a single data stream.
A QuickTime movie file on an Apple Macintosh consists of a "resource fork" containing the movie resources and a "data fork" containing the actual movie data or references to external data sources such as video tape. To facilitate the exchange of data with systems which use single fork files, it is possible to combine these into a file which uses only the data fork . It is possible that QuickTime could become a computer-industry standard for the interchange of video/audio sequences.
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