1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 * EXG0#-44 G9TELECOMMUNICATION (02/78)STANDARDIZATION SECTOROF ITU4%,%6)3)/.G0G0!.$G0G03/5.$G0G042!.3-)33)/.42!.3-)33)/.G0G0/(b) that one method of solving this problem would be to transmit both signals over the same circuit bymultiplexing;(c) that it is
2、desirable when sound and video signals are transmitted by time division multiplex to standardize on aparticular solution,UNANIMOUSLY RECOMMENDSthat for the transmission of one sound-programme signal together with an analogue television signal bytime-division multiplex, a pulse-code modulation system
3、 should be used, in which the most important principles are asfollows:1. the sound-programme signal should be sampled at twice the rate of the video-line frequency (see Note 3);2. the samples should be encoded in binary PCM form suitably companded into 10 bit words;3. alternate coded samples should
4、be delayed by half a line period, one 10 bit word from each pair of samplesbeing complemented, and the digits of the two samples should be interleaved so that similarly numbered digits from thetwo samples appear consecutively;4. the individual digit pulses should be shaped into pulses of sine-square
5、d form, of duration and spacing such that21 pulses can be inserted into the line synchronizing pulse. In television systems employing equalizing pulses in the fieldblanking interval, the duration of the appropriate equalizing pulses should be lengthened at the coder, and restored tonormal at the dec
6、oder;5. the combined and shaped pulse groups (20 bits) together with a marker pulse (similarly shaped) should beinserted in the line synchronizing pulses so that the marker pulse followed by the least significant digits are nearest to theleading edge of the synchronizing pulse, and the most signific
7、ant digits are nearest to the trailing edge of thesynchronizing pulse. The pulse amplitude should extend from sync bottom to peak white;6. pre- and de-emphasis should be used, the curve in accordance with CCITT Recommendation J.17 beingsuitable for this purpose. The low-frequency gain should be fixe
8、d in conjunction with the compandor to minimize anyimpairment due to programme-modulated noise which may occur with critical programme material;7. protection should be provided so that in the event of the decoded samples being in error due to either the videowaveform or the audio pulses being non-st
9、andard, any resulting audio disturbance is minimized. For example, this maybe achieved by means of holds, i.e. by the repetition of previous samples for a short duration of errors, and by muting theaudio output for a longer duration of errors, or if too many holds occur in a short time;_1)Formerly R
10、ecommendation ITU-R CMTT.572.2 Recommendation J.66 (02/78)8. for three codecs in tandem, the characteristics should conform to Recommendation 505 (see Note 3);9. the video performance of the system should be such that a single coder and decoder introduces negligibledistortion, compared with the limi
11、ting values for the television hypothetical reference circuit, given in Recommenda-tion 567;10. the quality of the decoded sound obtained from one codec should not be adversely affected if the televisioncircuit between the coder and the decoder is such that its performance is equivalent to a long in
12、ternational televisionconnection (see Recommendation 603); equalized if necessary.Note 1 Circuits using sync bottom clamps are not suitable for use with this system.Note 2 For use over satellite circuits which involve 17.5 MHz bandwidth operation at IF, it is desirable to reduce thepulse amplitude by 6 dB.Note 3 It may be difficult to obtain a response extending to 15 kHz as the sampling rate is twice line-frequency (see 1above). Hence the highest frequency at which the hypothetical reference circuit amplitude/frequency response is specified may bemodified to 14 kHz.
