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1、Maintaining Audio Quality in the Radio Plant by Bob Orban RadioFans.CN 收音机爱 好者资料库 2 Maintaining Audio Quality Orban is a registered trademark. All trademarks are property of their respectable companies. This manual is part number 95383.000.01 Copyright 1999, 2000 1525 ALVARADO STREET, SAN LEANDRO, C

2、A 94577 USA Phone: (1) 510/351-3500; Fax: (1) 510/351-0500; E-Mail: ; Site: RadioFans.CN 收音机爱 好者资料库 in the Radio Plant3 Maintaining Audio Quality in the Radio Plant Part 1: Recording Media.6 Compact Disc .6 CD-R and CD-RW.7 Digital Tape.8 Magnetic Disk and Data Compression.8 Vinyl Disk.10 Analog Tap

3、e.13 Tape Recorder Maintenance .16 Recording Your Own Alignment Tapes.19 Cartridge Machine Maintenance.20 Part 2: System Considerations .21 Headroom.21 Voice/Music Balance.23 Electronic Quality.23 Part 3: The Production Studio .27 Choosing Monitor Loudspeakers.28 Loudspeaker Location and Room Acoust

4、ics.28 Loudspeaker Equalization.29 Stereo Enhancement.31 Other Production Equipment.31 Production Practices .32 Part 4: Equipment Following OPTIMOD .34 STL.34 FM Exciter .35 FM Transmitter.35 FM Antenna.36 AM Transmitter.36 AM Antenna.36 Summary.37 RadioFans.CN 收音机爱 好者资料库 4 Maintaining Audio Quality

5、 Maintaining Audio Quality in the Radio Plant By Robert Orban, Chief Engineer, Orban Inc. (revised August 1999) Authors Note: This white paper combines and revises two previous Orban papers on maintaining audio quality in the FM and AM plants. In 1999, considerations for both are essentially identic

6、al except at the transmitter because, with modern equipment, there is seldom reason to relax studio quality in AM plants. The text emphasizes FM (and, to a lesser extent, DAR) practice; differences applicable to AM have been edited into the FM text. Audio processors change certain characteristics of

7、 the original program material in the quest for positive benefits such as increased loudness, improved consistency, and absolute peak control. The art of audio processing is based on the idea that such benefits can be achieved without allowing the listener to detect that anything has been changed. S

8、uccessful audio processing performs the desired electrical modifications while presenting a result to the listener that, subjectively, sounds natural and realistic. This sounds impossible, but it is not. Audio processing provides a few benefits that are often unappreciated by the radio or television

9、 listener. For example, the reduction of dynamic range caused by processing makes listening in noisy environments (particularly the car) much less difficult. In music having a wide dynamic range, soft passages are often lost completely in the presence of background noise. Few listeners listen in a p

10、erfectly quiet environment. If the volume is turned up, subsequent louder passages can be uncomfortably loud. In the automobile, dynamic range cannot exceed 20dB without causing these problems. Competent audio processing can reduce the dynamic range of the program without introducing objectionable s

11、ide effects. Further, broadcast program material typically comes from a rapidly changing variety of sources, most of which were not produced with any regard for the spectral balances of any other. Multiband limiting, when used properly, can automatically make the segues between sources much more con

12、sistent. Multiband limiting and consistency are vital to the station that wants to develop a characteristic audio signature and strong positive personality. Each broadcaster also has special operational considerations. First, good broadcast operators are hard to find, making artful automatic gain co

13、ntrol essential for the correction of errors caused by distractions or lack of skill. Second, the regulatory authorities in most countries have little tolerance for excessive modulation, making peak limiting mandatory for signals destined for the regulated public airwaves. OPTIMOD-FM, OPTIMOD-AM, an

14、d OPTIMOD-DAB have been conceived to meet the special problems and needs of broadcasters while delivering a quality product that most in the Radio Plant5 listeners consider highly pleasing. However, every electronic communication medium has technical limits that must be fully heeded if the most plea

15、sing results are to be presented to the audience. For instance, the audio quality delivered by OPTIMOD is highly influenced by the quality of the audio presented to it. If the input audio is very clean, the signal after processing will probably sound excellent even after heavy processing. Distortion

16、 of any kind in the input signal is likely to be exaggerated by processing and, if severe, can end up sounding offensive and unlistenable. AM is limited by poor signal-to-noise ratio and by limited receiver audio bandwidth (typically 2-3kHz). As delivered to the consumer, it can never be “high fidel

17、ity” in any real sense. Consequently, multiband audio processing for AM compresses dynamic range more severely than in typical FM practice. In addition, pre-emphasis (whether NRSC or more aggressive than NRSC) is required to ensure reasonably crisp, intelligible sound from typical AM radios. In AM t

18、his is always provided in the audio processor and never in the transmitter. Achieving consistent state-of-the-art audio quality in broadcast is a challenging task. It begins with a professional attitude, considerable skill, patience, and an unshakable belief that quality is well worth having. This s

19、upplement provides some technical insights and tips on how to achieve immaculate audio, and keep it that way. This paper is organized into four main parts: 1. Recording Media: compact disc, CD-R and CR-RW, digital tape, magnetic disk and data compression, vinyl disk, phonograph equipment selection a

20、nd maintenance, analog tape, tape recorder maintenance, recording alignment tapes and cart machine maintenance see page 6. 2. System Considerations: headroom, voice/music balance, and electronic quality see page 21. 3. The Production Studio: choosing monitor loudspeakers, loudspeaker location and ro

21、om acoustics, loudspeaker equalization, stereo enhancement, other production equipment, and production practices see page 27. 4. Equipment Following OPTIMOD: exciters, transmitters, and antennas see page 34. Note: Because the state of the art in audio technology is constantly advancing, it is import

22、ant to know that this material was last revised in 1999. Our comments and recommendations obviously cannot take into account later developments. We have tried to anticipate technological trends when that seemed useful. 6 Maintaining Audio Quality Part 1: Recording Media Compact Disc The compact disc

23、 (CD), with 16-bit resolution and 44.1kHz sample rate, represents the reference standard source quality for radio, although it may be superceded in the future by DVD-Audio, with 24-bit resolution and 96kHz sample rate. Further, many stations broadcast digital sources to which various forms of lossy

24、data compression have been applied. While we had expected the black vinyl disk to be obsolete by this revision, it is still used on-air in specialized applications like live “club-style” D.J. mixing. Although CD technology is constantly improving, we believe that some general observations could be u

25、seful. In attempting to reproduce CDs with the highest possible quality, the industry has settled into technology using “delta-sigma” digital-to-analog converters (DACs) with extreme over-sampling. These converters use pulse width modulation or pulse-duration modulation techniques to achieve high ac

26、curacy. Instead of being dependent on the precise switching of voltages or currents to achieve accurate conversion, the new designs depend on precise timing, which is far easier to achieve in production. Over-sampling simultaneously increases the theoretical signal-to-noise ratio and produces (prior

27、 to the reconstruction filter within the CD player) a signal that has no significant out-of-band power near the audio range. This power can be readily removed with a simple, phase-linear analog filter to ensure the most accurate phase response through the system. We recommend that CD players used in

28、 broadcast employ technology of at least this quality. However, the engineer should be aware that these units might emit substantial amounts of supersonic noise, so that low-pass filtering in the transmission audio processor must be sufficient to reject this to prevent aliasing in digital transmissi

29、on processors or STLs. The radio station environment demands ruggedness, reliability, and quick cueing from audio source equipment. The CD player must also be chosen for its ability to track even dirty or scratched CDs with minimum audible artifacts, and on its ability to resist external vibration.

30、There are dramatic differences between players in these areas! We suggest careful comparative tests between players using imperfect CDs to determine which players click, mute, skip, or otherwise mistrack. Striking the top and sides of the player with varying degrees of force while listening to the o

31、utput can give a “feel” for the players vibration resistance. Fortunately, some of the players with the best sound also track best. The depressing trade-off between quality and ruggedness that is inevitable in vinyl disk reproduction is unnecessary when CDs are used. Reliability is not easy to asses

32、s without experience. The experience of your fellow broadcasters can be valuable here ask around during local broadcast engineers meetings. Be skeptical if examination of the “insides” of the machine reveals evidence of poor construction. Cueing and interface to the rest of the station are uniquely

33、important in broadcast. There in the Radio Plant7 are, at this writing, relatively few players that are specifically designed for broadcast use players that can be cued by ear to the start of a desired selection, paused, and then started by a contact closure. The practical operation of the CD player

34、 in your studio should be carefully considered. Relatively few listeners will notice the finest sound, but all listeners will notice miscues, dead air and other obvious embarrassments! Some innovative designs that have already been introduced include jukebox-like CD players that can hold 100 or more

35、 CDs. These players feature musical selections that can be chosen through computer-controlled commands. An alternative design, which also tries to minimize CD damage caused by careless handling, places each CD in a protective plastic “caddy.” The importance of handling CDs with care and keeping the

36、playing surface clean cannot be over-emphasized. Contrary to initial marketing claims of invulnerability, CDs have proven to require handling comparable to that used with vinyl disks in order to avoid on-air disasters. Except for those few CD players specifically designed for professional applicatio

37、ns, CD players usually have unbalanced 10dBV outputs. In many cases, it is possible to interface such outputs directly to the console (by trimming input gains) without RFI or ground loop problems. If these problems do appear, several manufacturers produce low-cost 10dBV to +4dBu adapters for raising

38、 the output level of a CD player to professional standards. CD-R and CD-RW The cost of CD-R (compact disk-recordable) has now dropped to the point where it is a very attractive solution as an on-air source and for archiving. The quality is equivalent to CD. There are several dye formulations availab

39、le, and manufacturers disagree on their archival life. However, it has been extrapolated that any competently manufactured CD- R should last at least 30 years if it is stored at moderate temperatures (below 75 degrees F) and away from very bright light like sunlight. On the other hand, these disks c

40、an literally be destroyed in a few hours if they are left in a locked automobile, exposed to direct sunlight. CD-RW (compact disk-rewritable) is not a true random-access medium. You cannot randomly erase cuts and replace them because the cuts have to be unfragmented and sequential. However, you can

41、erase blocks of cuts, always starting backwards with the last one previously recorded. You can then re-record over the space you have freed up. The disadvantage of CD-RW is that most common CD payers cannot read them, unlike CD-R, which can be read by almost any conventional CD player, provided that the disk has been “finalized” to record a final Table of Contents track on it. A finalized CD-R looks to any CD player like an ordinary CD. Once a CD-R has been finalized, no fu