1、 Rec. ITU-R BS.1726 1 RECOMMENDATION ITU-R BS.1726 Signal level of digital audio accompanying television in international programme exchange (Question ITU-R 2/6) (2005) Scope This Recommendation gives guidance on the level of digital audio signals accompanying television in international programme e
2、xchange. The Recommendation specifies that one of two specified reference levels (18 dBFS or 20 dBFS) be employed and declared, and that peak audio levels, when measured by a quasi-peak programme meter (PPM), do not exceed a level of 9 dBFS. The ITU Radiocommunication Assembly, considering a) that R
3、ecommendation ITU-R BS.645 Test signals and metering to be used on international sound programme connections, specifies metering characteristics and signal levels for analogue audio programmes; b) that Recommendation ITU-R BR.777 International exchange of two-channel digital audio recordings, contai
4、ns parameters and specifications of signal levels for the international exchange of digital audio recordings; c) that international programme exchange is now generally based on the use of digital techniques; d) that modern digital techniques support a very wide dynamic range of levels for audio sign
5、als; e) that digital media overload abruptly and thus even momentary overload should be avoided; f) that a uniform audio level is highly desirable in the international exchange of television programmes, and this can only be achieved through the use of uniform operating practices; g) that the Europea
6、n Broadcasting Union (EBU) Technical Recommendation R68-2000 and the Society for Motion Picture h) that both reference levels are widely recognized and used in different parts of the world, recommends 1 that broadcasters and common carriers should uniquely use either 18 dBFS or 20 dBFS as the refere
7、nce level (also designated as the alignment level (AL) for digital audio accompanying television in international programme exchange, and they should declare their chosen reference level; 1Decibels with respect to full-scale level. 2 Rec. ITU-R BS.1726 2 that, whichever reference level is chosen and
8、 used, digital audio programme peaks should not be allowed to exceed a level 9 dB below the digital full-scale level (0 dBFS2) when monitored by a quasi-peak programme meter3as specified in International Electrotechnical Commission (IEC) 60268-10. This level is called the permitted maximum level (PM
9、L) (see Appendix 1 for a graphical representation of the recommended relations among these audio levels); 3 that consideration should be given to revising this Recommendation when practical methods of metering become available that objectively measure and indicate true peak levels and perceived loud
10、ness. NOTE 1 Due to the characteristics of quasi-ppm used by broadcasters, the true programme peaks may typically be 3 dB greater than those indicated, and may be even higher for some unusual short-duration transients, reaching up to 5 dB, or more, in exceptional cases. NOTE 2 IEC 60268-10 publicati
11、on is available in electronic form at the following address: http:/www.iec.ch/itu. Appendix 1 (Informative) Graphical representation of the recommended relation among audio levels This Appendix offers a graphical representation of the relation among the maximum audio level (0 dBFS), the permitted ma
12、ximum level as indicated by a quasi-ppm and ALs agreed by the SMPTE and EBU. It illustrates that audio material conforming to this Recommendation may be exchanged and used without any level adjustment being required. However, audio material must still be indicated as using an SMPTE RP 155 or EBU Tec
13、hnical Recommendation R68 alignment level in case operators or automatic equipment use steady tone at this level to line up circuits or recording devices. 20 dBFS is the maximum signal level that a digital audio system is capable of representing. Above this level, abrupt signal clipping occurs with
14、the consequent distortion. 3Quasi-peak programme meters have an integration time of 10 ms, thereby also indicating relatively short audio programme transients. True peak-reading meters will exceed this indication on some programme material, whereas VU meters will typically under-read this indication as they have a long integration time. Rec. ITU-R BS.1726 3 FIGURE 1 Graphical representation of the recommended relation among audio levels
