SMPTE EG 23-2005 Transfer of Two-Channel Stereo Audio from Audio Magnetic Film or Tape to Video Tape《将双通道立体声音频从音频磁性胶片或磁带转移至录像磁带》.pdf

1、 1 Scope 1.1 This guideline specifies the handling of volume range issues when transferring two-channel stereo audio from 35-mm or 16-mm magnetic film, or other audio tape formats utilizing time-code based methods of synchronization, to any video recording media. 1.2 Among the volume range issues ar

2、e both objective and subjective ones, including: most end users will usually adjust loudness so that dialog is reproduced at a normalized level (despite how loud or soft other parts of the program may be) for best intelligibility; dubbing stages and monitor theaters are often operated at higher refe

3、rence sound pressure levels than the end video consumer will use; commercials may well be intercut with program material, creating the potential for abrupt level changes among video sources; many of the video formats have reduced audio dynamic range capability when compared to magnetic audio-only ma

4、sters; video transmission involves at least a 75-ms preemphasis network in the transmitter, and often much more nonstandardized audio signal processing, which has an impact on the volume range relationships within the program material. 1.3 The guideline also specifies certain labeling requirements f

5、or audio masters. 2 Definitions 2.1 dynamic range: The range along an intensity scale of which a medium is capable; measured from a maximum point of stated distortion to a background noise level. Dynamic range varies frequency-by-frequency, but is often expressed as a single number from the mid-band

6、 defined distortion level to the psycho-acoustically weighted noise level, e.g., “the dynamic range was 72 dB (3% THD to CCIR-ARM noise).“ 2.2 volume range: The range along an intensity scale occupied by program material; volume range is taken to be from the highest peaks of the program to the lowes

7、t signal intended to be audible. Page 1 of 4 pages EG 23-2005 Revision of EG 23-1996 SMPTE ENGINEERING GUIDELINE Transfer of Two-Channel Stereo Audio from Audio Magnetic Film or Tape to Video Tape Copyright 2005 by THE SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 3 Barker Avenue, White Plains,

8、 NY 10601 (914) 761-1100 Approved December 16, 2005EG 23-2005 Page 2 of 4 pages 3 Identification 3.1 The audio film or tape shall be clearly marked with the title and reel number of the program and identified as a two-channel stereo recording. If the medium is capable of more than two audio records,

9、 then the channels containing the left and right audio master information shall be indicated. 3.2 If proprietary noise-reduction companding is employed on the master, the type of noise reduction, and, if required, the reference fluxivity for setting the operating point of the noise reduction system

10、shall be indicated. 4 Use of noise reduction 4.1 If the program material has been encoded for a proprietary companding noise-reduction system, then corresponding noise reduction decoders must be engaged. If external to the video recorder, such decoders shall be included in the signal path immediatel

11、y after the audio film or tape playback preamplifier, and before any other audio signal processing. 4.2 If the video master is to employ external noise-reduction encoding, then the noise-reduction encoder shall be included in the signal path after all other audio processing, immediately before the v

12、ideo recorder. 5 Reference levels and their usage 5.1 Reference fluxivity for film masters is given in SMPTE EG 9 as 185 nWb/m. Reference fluxivity for 1-in C-format video tape is given in SMPTE 20M as 100 nWb/m. Reference fluxivity on tape masters may range from 185 to 320 nWb/m, depending on the p

13、ractice of the studio making the master and so must be indicated on the label. Even though reference fluxivity on tape masters may range from 185 to 320 nWb/m, the peak level used for program material is usually within a smaller range, since metering practice (vu vs. peak) dictates that vu meters ar

14、e used with lower reference levels, and peak meters with higher ones. 5.2 Normal practice is to set the audio reproduction and recording chain gain structure such that the reference fluxivity used in the source medium is copied to the reference fluxivity of the recording medium. For media wherein re

15、ference fluxivity does not apply as a level-determining mechanism, such as 1/2-in “hi-fi“ recording by way of audio on FM carriers on the video tracks, a combination of setting the level using meter readings on the recorder while playing the source material, and sample recordings, are the best way o

16、f determining the correct recording level. 5.3 If the video recorder has a compressor or limiter as a part of its circuitry, the compressor or limiter should be switched off, if possible. 6 Need for volume-range reduction 6.1 The combination of the subjective and objective factors given in the scope

17、 often leads to the need for reducing the volume range of source material, when copying from audio masters to video tape. 6.2 Measurements of contemporary magnetic film and 1-in video tape on recorders set to all the relevant standards show video tape to have 8 dB less headroom for equal distortion

18、to magnetic film at 200 Hz, but equal headroom at 10 kHz. In addition, video tape is some 10 dB noisier in the psycho-acoustically important 2-3 kHz region. 6.3 Differing conditions outlined in the scope lead to a range of solutions, so that no one set of rules can be developed. On the other hand, e

19、xamples can be given for various transfer conditions, and are shown in the next clause. General principles are: EG 23-2005 Page 3 of 4 pages it is important to monitor the program under conditions as like those as encountered by the final user as possible, so that volume range judgments are not obsc

20、ured by other factors. These conditions include reference sound pressure level and frequency response of monitoring, background noise level of the listening room, choice of stereo vs. monaural monitoring, and the like; it is important to monitor the output from the recording media, rather than simpl

21、y its input, to check the generation loss due to the transfer process. If the recorder does not provide playback off the tape simultaneously while recording, then a sample recording should be synchronized to the audio master, and A/B compared; limiting may be preferred to compression as producing th

22、e better artistic representation of the program material on the recording medium (i.e., limiting can have fewer audible side effects than compression). The amount of limiting is usually kept to the range of about 6 dB on the peaks of program. 7 Examples 7.1 Problem: A 35-mm magnetic master intended

23、for transfer to a stereo optical track of a theatrical motion picture is to be transferred to 1-in C-format video tape for playback on commercial television. The picture is action-oriented, containing loud sound effects and music. Discussion: There are a number of factors stated in the problem that

24、influence the amount of volume range modification necessary: The fact that the master has been prepared for transfer to optical film means that the volume range has probably already been limited, since conventional optical tracks have less dynamic range than do magnetic ones. The statement that the

25、picture contains loud sound effects and music implies that the dialog level is relatively low, compared to the maximum levels, for this example. The reference sound pressure level in motion-picture theaters has been observed to be about 6 dB greater than that used by the majority of listeners in the

26、 home environment. The statement that the program will be played over commercial television means that low dialog level will be much more noticeable than if there were no commercials, since listeners will adjust the volume for a normalized level of dialog during the program, and if the program dialo

27、g level is low, will find the commercials too loud. The first factor implies little volume-range modification is necessary, but, on the other hand, the other three factors, especially taken together, call for a reduction of the volume range. Solution: Use a limiter set so that the program peaks caus

28、e 6 dB of limiting. 7.2 Problem: The same master as in 7.1 is to be transferred to a wide dynamic range digital recording for use as a running master in duplicating 1/2-in “hi-fi“ format video tapes. Discussion: The emphasis in maintaining the full dynamics of the master are more important for the “

29、hi-fi“ part of the market than they are for commercial television, due to the rising popularity of home use of VCRs with attached stereo audio systems. The use of a wide dynamic range running master is essential since the dynamic range performance of the “hi-fi“ recording is greater than that of 1-i

30、n C-format video tape, even if noise reducing companding is employed on the video tape. EG 23-2005 Page 4 of 4 pages Solution: Use no limiting or compression to make the running master. (Note: It may be necessary for the duplicator to employ compression or limiting of the signal sent to the conventi

31、onal longitudinal track, if both the “hi-fi“ and conventional tracks are to be optimized simultaneously. 7.3 Problem: A monaural DME (having dialog, music and effects on separate channels of a 3-track magnetic film) is to be transferred to 3/4-in video tape. The program material is a documentary in

32、which narration is the loudest part of the program. Discussion: Despite the probable low volume range of the original, some limiting may be necessary due to the low audio dynamic range of the 3/4-in video medium compared to the audio film source. Solution: Use a limiter with a maximum gain reduction

33、 of about 6 dB. Annex A (informative) Bibliography SMPTE 20M-2003, Television Analog Recording 1-in Type C Recorders and Reproducers Longitudinal Audio Characteristics SMPTE RP 150-2000 (Archived 2005), Channel Assignments and Test Leader for Magnetic Film Masters Intended for Transfer to Video Media Having Stereo Audio SMPTE EG 9-2005, Audio Recording Reference Level for Post-Production of Motion-Picture Related Materials