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DEFINITIONS Tele-conference: any form of communication between two or several participants in two or several different places and relying on the transmission of one or several audio signals between those places. Video-conference: a tele-conference comprising one or several video signals which convey the images of some or all the participants. Multilingual video-conference: a video-conference in two or several languages with interpretation (consecutive or simultaneous). Tele-interpreting: interpretation of a multilingual video-conference by interpreters who have a direct view of neither the speaker nor their audience. APPLICABLE STANDARDS 1. ISO standards as regards simultaneous interpreting equipment ISO / DIS 2603 (revision of standard ISO 2603) for permanent simultaneous interpretation booths and standard ISO / DIS 4043 (revision of ISO 4043) for mobile booths. These standards describe all the practical conditions with which conference rooms (interpreting booths and equipment) have to comply. They stipulate in particular that the interpreter has to have a direct view of the room as well as of any screen used for projections. As regards the quality of the sound in the interpreters’ headsets, it is explicitly stated that the waveband between 125 and 12500 Hz has to be accurately reproduced 2. Standards applicable to digitalisation and compression of audio and video signals Digitalisation is carried out by sampling an (analogue) audio or video signal. For a telephone conversation to be digitised, for instance, the audio signal is sampled 8000 times a second and each sample is encoded on 8 bits, which gives a rate of 64000 bits a second or 64 Kbps. The frequency is, however, limited to 3-4 Khz. By way of comparison, the audio signal from a CD player is usually sampled 44100 times a second, equivalent to a frequency of 0 - 20 Khz: it is encoded on 16 bits, thus generating a bit rate of approximately 711 Kbps before compression. Standard ITU-R 601 («Studio-Quality TV») applies to digital transmission of TV pictures. The three components of the video signal: R (red), B (blue) and G (green) are first converted into a luminance signal Y = R + B + G and two signals for colour difference R - Y and B - Y. For each image or frame comprising 486 lines in the NTSC system and 576 in PAL /SECAM, 720 samples are taken for luminance, but only half that number (360) for colour differences, each sample being encoded on 8 bits. The resulting bit rate is approximately 165 Mbps (1 Mbp = 1000 Kbps) before compression. Before being transmitted by digital connection, the audio and video signals have to be digitised and then compressed so as to reduce the high rate of data to be carried. This compression is achieved by means of a CODEC (encoder - decoder). Digital systems are characterised by a difference in transmission time (including the time needed for encoding/decoding) depending on whether the signal is audio or video. The result is a fluctuating time-lag between the sound and picture, which has effects on the synchronisation. 3. ISO-MPEG standards MPEG (Moving Picture Experts’ Group) is the name given to a family of international standards used to codify audio-visual data in a compressed digitised format. The MPEG family is made up of standards MPEG-1, MPEG-2 and MPEG-4, identified respectively as ISO / IEC 11172, ISO / IEC 13818 and ISO / IEC 14496. Standard MPEG-2 (ISO 13818) is what is generally used for digital transmission of TV pictures. Transmission of such pictures according to standard ITU-R 601 would require 4 or even 25 to 34 Mbps for HDTV pictures (1920 x1080 pixels and 60 frames/second). Standards MPEG-audio Layer 1,.2 or 3, in order of increasing complexity and performance, apply to the audio part. 4. Standards 320x of the International Telecommunications Union A family of standards applicable to the video-conferences, as regards audio and video transmission by ordinary telephone line (H324), ISDN / ATM (H320 - H321 - H310) or local networks (H322). H-320 is based on a video compression algorithm, J.261, with two types of resolution: CIF (Common Interchange Format)
QCIF (Quarter Common Interchange Format)
Standards H.320 make it possible to obtain a maximum bit rate of 30 frames a second. The H.320 family covers three audio standards applicable to codecs: G.711, with a bit rate of 64 Kbps, offers an audio quality of 3-Khz, telephone level; G.722, with a higher quality algorithm, reaches 7,5 kHz at a bit rate of 64 Kbps; G.728 provides an audio quality, not far from telephone level (3.4 Khz), at only 16 Kbps. H.310 and H321 adapt the H.320 family to new transport protocols such as ATM and ISDN broadband. H.310, for example, uses the video compression algorithm ISO MPEG-2, which would provide a high-definition picture. ITU has drawn up the T-120 series of recommendations, defining the standards applying to H.32X video-conferences as regards document exchanges.
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