1、 STD-SMPTE RP 84-ENGL L77b 8357403 0002bb5 8T7 SMPTE RECOMMENDED PRACTICE RP 84-1996 Revision of RP 84-1992 Reference Carrier Frea uencies and Preemphasis Characteristics for 1-in Type B Helical-Scan Television Analog Recording 1 Scope Page 1 of 2 pages 2.2 Characteristic frequencies This practice s
2、pecifies the video reference frequen- cies to which the carrier is deviated and the associ- ated video preemphasis for 1 -in type B helical-scan television tape recording. (The video deemphacis to be used in reproduction is specified indirectly by re- quiring a flat input-to-output video response al
3、ong with a specified preemphasis in recording.) The inStantane0US freqUenCieS Of the rf signai COrre- sponding to characteristic kVek Of the video Signal Shall be as specified in table I. 2.3 PreemPhasis and deemPhasis The time constants of the video emphasis networks shall be as defined in table 2.
4、 Table 1 - Video levels Video levels MHz Synchronization tip 7.06 nom 2.1 Modulation system Blanking 7.90 f 0.05 Peak white 10.00 f 0.05 The video information shall be recorded in the form of an rf signal frequency modulated by the video signal. The instantaneous frequencies of the rf signal shall v
5、ary linearly with respect to the amplitude of the modulating signal. 2 Electrical parameters Table 2 - Time constants Time constants ns ti 240 ta 600 Annex A (informative) Transmission characteristics of the signal chain The transmission characteristics of the signal chain of a television tape recor
6、der may be defined by one of two methods which are in agreement: A.l Definition of the recording chain For reference purposes, an ideal recording chain is defined as consisting of (a) a modulator having a flat frequency response with respect to the modulating video frequencies, (b) an rf section hav
7、ing a transfer characteristic that pro- duces constant amplitude alternating magnetic flux in the video head pole tips when driven by an alternating signal from the modulator having constant amplitude, and (c) a video preemphasis network inserted before the modulation stage. The preemphasis is then
8、defined by the frequency and phase characteristic of a network, such as that shown in figure 1, fed from a low-impedance source and feeding a high-impedance load. The ideal recording chain described above is intended to be taken as a basis for producing reference tapes to be used for the alignment o
9、f television tape recorders. When using present-day recording chains, the following points should be considered: Copyright O 1996 by the SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 595 W. Hartsdale Ave., White Plains, NY 10607 (91 4) 761 -1 1 O0 Approved November 18,1996 STD-SMPTE RP 84-ENGL
10、3776 8357403 0002bbb 735 H R P 84-1 996 An approximately linear relationship exists between the magnetic flux emanating from the video head pole tips and the rf current flowing through the video head windings. L t1=- Ri+R L R t2 = - Figure A.2 - Deemphasis Figure A.l - Preemphasis A.2 where tl and t
11、z are time constants in microseconds, R is resistance in ohms, and C is capacitance in microfarads. The amplitude of the recording current in the video heads should be such as to produce maximum rf output in replay at the frequency corresponding to mid-gray level. A.2 Dennition of the playback chain
12、 The deemphasis characteristic is introduced following the demodulator in the signal playback circuitry. (To obtain a flat The deernphasis network is introduced following the de- modulator in the signal playback circuitry. (To obtain a flat input-to-output video response over the passband of inter-
13、est, a complementary preemphasis network is introduced ahead of the frequency modulator stage during recording.) This definition assumes that all preemphasis and deempha- sis are placed in the video portion of the signal path and that the response of the rf portion of the signal path is flat over in
14、put-to-output video response Over the passband Of inter- estg a complementary video preemphasis characteristic is introduced ahead Of the frequency modulator stage during the passband of interest. Ideally, the magnitude of the remanent flux on a recorded tape be independent of frequency over the fre
15、quency range of interest, but since there is no practical way of measuring it, the most practical approach is to ensure that the recording current in the video heads is independent of frequency over the passband of interest. recording.) The video deemphasis curves are defined as the normalized impedance of the two-terminal network, as shown in figure Annex B (informative) Bibliography ANSVSMPTE 15M-1992, Television Analog Recording - 1 -in Type B Helical Scan - Basic System Parameters Page 2 of 2 pages
