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1、Robert Orban Greg Ogonowski Maintaining Audio Quality in the Broadcast Facility 2008 Edition RadioFans.CN 收音机爱 好者资料库 Table of Contents MAINTAINING AUDIO QUALITY IN THE BROADCAST FACILITY.1 Introduction.1 PART 1: RECORDING MEDIA.4 Compact Disc.4 Quality Control in CD Transfers.5 CD-R and CD-RW, DVDR,
2、 DVDRW, DVD-A, HD-DVD, Blu-ray.6 Digital Tape.7 Hard Disk Systems.8 Data Compression .8 PART 2: SYSTEM CONSIDERATIONS.15 Headroom .15 Measuring and Controlling Loudness.18 Speech/Music Balance.20 Electronic Quality.21 PART 3: THE PRODUCTION STUDIO.27 Choosing Monitor Loudspeakers.28 Loudspeaker Loca
3、tion and Room Acoustics.28 Loudspeaker Equalization .29 Stereo Enhancement.31 Other Production Equipment.31 Production Practices.32 PART 4: EQUIPMENT FOLLOWING OPTIMOD .34 STL.34 FM Exciter.34 FM Transmitter.35 FM Antenna .36 AM Transmitter.36 AM Antenna .36 DAB/ HD Radio / Netcasting Encoders.37 SU
4、MMARY.38 APPENDIX: ANALOG MEDIA.39 Vinyl Disk.39 Analog Tape.43 Tape Recorder Maintenance:.46 Recording Your Own Alignment Tapes.50 “Sticky Shed Syndrome”.50 Cartridge Machine Maintenance:.51 RadioFans.CN 收音机爱 好者资料库 Orban, Optimod, and Opticodec are registered trademarks. All trademarks are property
5、 of their respective companies. This document is part number 95383.000.03 Copyright 1982-2008 Orban. Revision 4-10-2008 970 South Kyrene, Tempe, AZ 85284 USA Phone: +1 480 403-8300; Fax: +1 480 403-8301; E-Mail: ; Web: RadioFans.CN 收音机爱 好者资料库 1 Maintaining Audio Quality in the Broadcast Facility By
6、Robert Orban and Greg Ogonowski, Orban/CRL Authors Note In 1999, we combined and revised two previous Orban papers on maintaining audio quality in the FM and AM plants, with further revisions occurring in 2003 and 2008. In 2008, considerations for both AM and FM are essentially identical except at t
7、he 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. Introduction Audio processors change certain characteristics of
8、the original program material in the quest for positive benefits such as increased loudness, improved consistency, and absolute peak control to prevent distortion in the following signal path and/or to comply with government regulations. The art of audio processing is based on the idea that such ben
9、efits can be achieved while giving the listener the illusion that nothing has been changed. Successful au- dio 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. A
10、udio processing provides a few benefits that are often unappreciated by the radio or television listener. For example, the reduction of dynamic range caused by proc- essing makes listening in noisy environments (particularly the car) much less difficult. In music having a wide dynamic range, soft pa
11、ssages are often lost completely in the presence of background noise. Few listeners listen in a perfectly 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 prob- lems. Compete
12、nt audio processing can reduce the dynamic range of the program without introducing objectionable side 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 bal- ances of any other. Mu
13、ltiband limiting, when used properly, can automatically make the segues between sources much more consistent. Multiband limiting and consis- tency are vital to the station that wants to develop a characteristic audio signature and strong positive personality, just as feature films are produced to ma
14、intain a con- sistent look. Ultimately, it is all about the listener experience Each broadcaster also has special operational considerations. First, good broadcast operators are hard to find, making artful automatic gain control essential for the correction of errors caused by distractions or lack o
15、f skill. Second, the regulatory au- thorities in most countries have little tolerance for excessive modulation, making peak limiting mandatory for signals destined for the regulated public airwaves. 2 Maintaining Audio Quality OPTIMOD-FM, OPTIMOD-AM, OPTIMOD-DAB, OPTIMOD-TV, and OPTIMOD-PC have been
16、 conceived to meet the special problems and needs of broadcasters while deliv- ering a quality product that most listeners consider highly pleasing. However, every electronic communication medium has technical limits that must be fully heeded if the most pleasing results are to be presented to the a
17、udience. For instance, the au- dio 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 excellenteven after heavy processing. Distortion of any kind in the input signal is like
18、ly 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 2-3 kHz). As delivered to the consumer, it can never be truly “high fidel- ity.” Consequently, multiband a
19、udio processing for AM compresses dynamic range more severely than in typical FM practice. In addition, pre-emphasis (whether NRSC or more extreme than NRSC) is required to ensure reasonably crisp, intelligible sound from typical AM radios. In AM, this is always provided in the audio processor and n
20、ever in the transmitter. Audio quality in TV viewing is usually limited by small speakers in the receivers, al- though the increasing popularity of DTV, HDTV and home theatre is changing this, increasing consumer demand for high audio quality. In everyday television viewing, it is important to avoid
21、 listener irritation by maintaining consistent subjective loud- ness from source to source. A CBS Loudness Controller or multi-band processing, both included in OPTIMOD-TV, can achieve this. Netcasting (also known as webcasting), DAB, and HD Radio almost always require low bit-rate codecs. Processin
22、g for such codecs should not use clipping limiting, and should instead use a look-ahead type limiter. OPTIMOD-DAB, OPTIMOD-HD FM, and OPTIMOD-PC provide the correct form of peak limiting for these applications and other low bite rate services. Just as the motion picture industry creates a consistent
23、, professional look to their product by applying exposure and color correction to every scene in a movie, audio processing should be used as part of the audio broadcast product to give it that final professional edge. Achieving consistent state-of-the-art audio quality in broadcast is a challenging
24、task. It begins with a professional attitude, considerable skill, patience, and an unshak- able belief that quality is well worth having. This supplement provides some techni- cal insights and tips on how to achieve immaculate audio, and keep it that way. Re- member, successful audio processing resu
25、lts all starts at the source. This publication is organized into four main parts: 1. Recording media: compact disc, CD-R and CR-RW, DVDR, DVDRW, DVD-A, HD-DVD, Blu-ray, digital tape, magnetic disk and data compression, vinyl disk, phonograph equipment selection and maintenance, analog tape, tape rec
26、order maintenance, recording alignment tapes and cart machine maintenancesee page 4. in the Broadcast Facility 3 2. System considerations: headroom, voice/music balance, and electronic qual- itysee page 15. 3. The production studio: choosing monitor loudspeakers, loudspeaker location and room acoust
27、ics, loudspeaker equalization, stereo enhancement, other pro- duction equipment, and production practicessee page 27. 4. Equipment following OPTIMOD: encoders, exciters, transmitters, and anten- nassee page 34. NOTE: Because the state of the art in audio technology is constantly advancing, it is imp
28、ortant to know that this material was last revised in 2008. Our comments and recommendations obviously cannot take into account later developments. We have tried to anticipate technological trends when that seemed useful. 4 Maintaining Audio Quality Part 1: Recording Media Compact Disc The compact d
29、isc (CD) is currently the primary source of most recorded music. With 16-bit resolution and 44.1 kHz sample rate, it represents the reference standard source quality for radio, although it may be superceded in the future by DVD-Audio, with 24-bit resolution and 96 kHz sample rate, or by SACD, which
30、uses “bitstream” coding instead of the CDs PCM (Pulse Code Modulation). Because most audio is sourced at a 44.1 kHz sample rate, upsampling to 48 kHz does not improve audio quality. Further, many broadcast digital sources have received various forms of lossy data compression. While we had expected t
31、he black vinyl disk to be obsolete by this revision, it is still used in specialized applications like live “club-style” D.J. mixing. Although CD technology is constantly improving, we believe that some general ob- servations could be useful. In attempting to reproduce CDs with the highest possible
32、quality, the industry has settled into technology using “delta-sigma” digi- tal-to-analog converters (DACs) with extreme oversampling. These converters use pulse width modulation or pulse-duration modulation techniques to achieve high accuracy. Instead of being dependent on the precise switching of
33、voltages or cur- rents to achieve accurate conversion, the new designs depend on precise timing, which is far easier to achieve in production. Oversampling simultaneously increases the theoretical signal-to-noise ratio and pro- duces (prior to the reconstruction filter within the CD player) a signal
34、 that has no significant out-of-band power near the audio range. A simple, phase-linear analog filter can readily remove this power, ensuring the most accurate phase response through the system. We recommend that CD players used in broadcast employ tech- nology of at least this quality. However, the
35、 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 alias- ing in digital transmission processors or STLs. The radio station environment demands rugg
36、edness, 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. 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 output can give a “feel” for the playe
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