Orban Maintaining Audio User Manual

Maintaining

Audio Quality

in the Radio Plant

by Bob Orban

Maintaining Audio Quality

Orban is a registered trademark . All trademarks are propert y of t heir respectable companies.

1525 A

Phone: (1) 510/351-3500; Fax: (1) 510/351-0500; E-Mail: custserv@orban.com; Site: www.orban.com

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, S

AN LEANDRO

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in the Radio Plant

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 Tape ..........................................................................................................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 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 ...............................................................................................................35

FM Transmitter........................................................................................................35

FM Antenna............................................................................................................36

AM Transmitter.......................................................................................................36

AM Antenna...........................................................................................................36

Summary...........................................................................................37

Maintaining Audio Quality

Maintaining Audio Quality in the Radio Plant

By Robert Orban, Chief Engineer, Orban Inc.

(revised August 1999)

Author’s 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 identical 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 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. Successful 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 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 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 problems. Competent 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 balances of any other. Multiband limiting, when used properly, can automatically make the segues between sources much more consistent. 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 control 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, and OPTIMOD-DAB have been conceived to meet the special problems and needs of broadcasters while delivering a quality product that most

in the Radio Plant

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 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 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 fidelity” 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 this 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 supplement 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:

Recording Media:

1. data compression, vinyl disk, phonograph equipment selection and maintenance, analog tape, tape recorder maintenance, recording alignment tapes and cart machine maintenance — see page 6.

System Considerations:

2. see page 21.

The Production Studio:

3. room acoustics, loudspeaker equalization, stereo enhancement, other production equipment, and production practices — see page 27.

Equipment Follow ing OP TIMOD:

34.

Note:

Because the state of the art in audio technology is constantly advancing, it is important 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.

compact disc, CD-R and CR-RW, digital tape, magnetic disk and

headroom, voice/music balance, and electronic quality —

choosing monitor loudspeakers, loudspeaker location and

exciters, transmitters, and antennas — see page

Maintaining Audio Quality

Part 1: Recording Media

Compact Disc

The compact disc (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 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 useful. 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 accuracy. 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 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 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 transmission 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. 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 player’s 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 assess 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 important in broadcast. There

in the Radio Plant

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 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 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 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 applications, 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 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 available, and manufacturers disagree on their archival life. However, it has been extrapolated that any competently manufactured CDR 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 can 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 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 further material can be added to it even if the disk is not full. If a CD-R has not been finalized, it can only be played in a CD-R recorder, or in certain CD players that specifically support the playing of unfinalized CD-Rs.

Maintaining Audio Quality

Digital Tape

While DAT was originally designed as a consumer format, it has achieved substantial penetration into the broadcast environment. This 16-bit 48kHz format is theoretically capable of slightly higher quality than CD because of the higher sample rate. In the DAR environment, where 48kHz-sample rate is typical, this improvement can be passed to the consumer. However, because the “sample rate” of the FM stereo system is 38kHz, there is no benefit to the higher sampling rate by the time the sound is aired on FM.

The usual broadcast requirements for ruggedness, reliability, and quick cueing apply to most digital tape applications, and these requirements have proven to be quite difficult to meet in practice. The DAT format packs information on the tape far more tightly than do analog formats. This produces a proportional decrease in the durability of the data. To complicate matters, complete muting of the signal, rather than a momentary loss of level or high frequency content, as in the case of analog, accompanies a major digital dropout.

At this writing, there is still debate over the reliability and longevity of the tape. Some testers have reported deterioration after as little as 10 passes, while others have demonstrated almost 1000 passes without problems. Each demonstration of a tape surviving hundreds of passes shows that it is physically possible for R-DAT to be reliable and durable. Nevertheless, we therefore advise broadcasters not to trust the reliability of DAT tape for mastering or long-term storage. Always make a backup!

Because the cost of recordable CD blanks has dropped to the point where they are almost throwaway items, we advise using CD-R instead of DAT when long-term archivability is important.

Magnetic Disk and Data Compression

Hard disk systems use sealed Winchester hard magnetic discs (originally developed for mass storage in data processing) to store digitized audio. This technology has become increasingly popular as a delivery system for material to be aired. There are many manufacturers offering systems combining proprietary software with a bit of proprietary hardware and a great deal of off-the-shelf hardware.

It is beyond the scope of this monograph to discuss the mechanics of these systems, which relate more to ergonomics and reliability than to audio quality. However, one crucial issue is whether the audio data is stored in uncompressed (linear PCM) form or using some sort of data compression.

There are two forms of compression — lossy, and lossless. Lossless compression provides an output that is bit-for-bit identical to its input. The best known of these systems for audio is MLP (Meridian Lossless Packing), which has been accepted for use with the DVD-Audio standard to increase its data carrying capacity by approximately

1.7x.

Lossy compression eliminates data that its designer has determined to be “irrelevant” to human perception. This exploits the phenomenon of

psychoacoustic masking,

which

in the Radio Plant

basically means that quiet sounds coexisting with louder sounds will sometimes be drowned out by the louder sounds so that the quieter sounds are not heard at all. The closer in frequency a quiet sound is to a loud sound, the more efficiently the louder sound can mask it. There are also laws having to do with the time relationship between the quieter and louder sounds. A good psychoacoustic model that predicts whether or not an existing sound will be masked is complicated. The interested reader is referred to the various papers on perceptual coders that have appeared in the professional literature (mostly in the Preprints) since the late 1980s.

Journal of the Audio Engineering Society

and in various AES Convention

There are two general classes of lossy compression systems. The first is exemplified by APT-X have a psychoacoustic model built into it. In exchange for this relative simplicity it has a very short delay time (less than 4ms), which is beneficial for applications requiring foldback monitoring, for example.

The second class contains built-in psychoacoustic models, which are used in the encoder to determine what parts of the signal will be thrown away. These codecs can achieve higher quality for a given bit rate than codecs of the first class, but at the expense of much larger time delays. Examples include the MPEG family of encoders, including Layer 2, Layer 3, and AAC. The Dolby category. The large time delays of these codecs make them unsuitable for any application where they are processing live microphone signals, which are then fed back into the announcer’s headphones. In these applications, it is sometimes possible to design the system to bypass the codec, feeding the undelayed signal into the headphones.

In 1999, the best overall quality for a given data rate appears to be achieved by the MPEG AAC codec, which is about 30% more efficient than MPEG1 Layer 3 and about twice as efficient as MPEG1 Layer 2. The AAC codec can achieve “contribution quality” at a stereo bit rate of 128kb/sec, while the Layer 2 codec requires about 256kb/sec for the same quality. The technology of lossy audio compression appears to be maturing, so we expect that advances beyond AAC will take considerable time to develop and will offer only incremental improvements in data rate.

, which, while designed with full awareness of psychoacoustic laws, does not

AC-2 and AC-3 codecs also fall in this

Lossy compression is one area where AM practice might diverge from FM and DAB practice. Because of the lower audio resolution of AM at the typical receiver, an AM station trying to economize on storage might want to use a lower data rate than an FM or DAR station. However this is likely to be false economy if the owner of this library ever wants to use it on FM or DAR in the future. In general, increasing the quality reduces the likelihood that the library will cause problems in future.

Any library recorded for general-purpose applications should use at least 44.1kHzsample rate so that it is compatible with DAR systems having 20kHz bandwidth. If the library will only be used on FM and AM, 32kHz is adequate and will save considerable storage. However, given the rise of digital radio, we cannot recommend that any futurelooking station use 32kHz for storage.

At this writing, the cost of hard disks is declining so rapidly that there is progressively less argument for storing programming using lossy compression. The highest quality

Maintaining Audio Quality

will, of course, be achieved by either no compression or by lossless compression. (There should be no quality difference between these.) Cascading stages of lossy compression can cause noise and distortion to become unmasked. Multiband audio processing can also cause noise and distortion to become unmasked, because multiband processing “automatically re-equalizes” the program material so that the frequency balance is not the same as the frequency balance seen by the psychoacoustic model in the encoder.

Sony’s MiniDisk format is a technology that combines data compression and randomaccess disk storage. While not offering the same level of audio quality as CD-R or CDRW, these disks are useful for field acquisition or other applications where open-reel or cassette tape had been previously used. They offer notably higher quality than the analog media they replace, along with convenient editing.

Vinyl Disk

Author’s Note for the 1999 Edition:

The next sections devote considerable space to the vagaries of analog media — vinyl disk and analog tape — that are becoming less and less important in broadcast production. However, given that they are still in use, we have chosen to retain this material in the current revision. Because these media are analog, they require far more tweaking and tender loving care than do the digital media discussed above. For this reason, the following sections are long and detailed. They have only been slightly revised for 1999, and therefore represent 1990 practice.

Some radio programming still comes from phonograph records — either directly, or through dubs. Not only are some club DJs mixing directly on-air from vinyl, but also some oldies have not been re-released on CD. This section discusses how to accurately retrieve as much information as possible from the grooves of any record.

Vinyl disk is capable of very high-quality audio reproduction. Consumer equipment manufacturers have developed high-fidelity cartridges, pick-up arms, turntables, and phono preamps of the highest quality. Unfortunately, much of this equipment has insufficient mechanical ruggedness for the pounding that it would typically receive in day-to-day broadcast operations.

There are only two reasonably high-quality cartridges currently made in the USA that are generally accepted to be sufficiently durable for professional use: the Stanton 681 series, and the Shure Professional series. Although rugged and reliable, neither has the clean, transparent operation of the best high-fidelity cartridges. This phono cartridge dilemma is the prime argument for transferring all vinyl disk material to tape in the production studio, and playing only tape on the air. In this way, it is possible (with care) to use state-of-the-art cartridges, arms, and turntables in the dubbing process, which should not require the mechanical ruggedness needed for on-air equipment. This reduces the problem of record wear as well. However, maintaining tape equipment such that it causes no noticeable quality degradation is by no means easy, and the smaller station (particularly one without a full-time engineer) may well be able to achieve superior quality by playing vinyl disks directly on the air.

in the Radio Plant

The following should be carefully considered when choosing and installing vinyl disk playback equipment:

1. Align the cartridge with great care.

When viewed from the front, the stylus must be absolutely perpendicular to the disc, to sustain a good separation. The cartridge must be parallel to the headshell, to prevent a fixed tracking error. Overhang should be set as accurately as possible

1/16-inch (0.16 cm), and the vertical tracking angle should be set at 20° (by

adjusting arm height).

2. Adjust the tracking force correctly.

Usually, better sound results from tracking close to the maximum force recommended by the cartridge manufacturer. If the cartridge has a built-in brush, do not forget to compensate for it by adding more tracking force according to the manufacturer’s recommendations. Note that brushes usually make it impossible to “back-cue.”

3. Adjust the anti-skating force correctly.

The accuracy of the anti-skating force calibration on many pick-up arms is questionable. The best way to adjust anti-skating force is to obtain a test record with an extremely high-level lateral cut (some IM test records are suitable). Connect the left channel output of the turntable preamp to the horizontal input of an oscilloscope and the fight channel preamp output to the vertical input. Operate the scope in the X/Y mode, such that a straight line at a 45-degree angle is visible. If the cartridge mistracks asymmetrically (indicating incorrect anti-skating compensation), then the scope trace will be “bent” at its ends. If this happens, adjust the anti-skating until the trace is a straight line (indicating symmetrical clipping).

It is important to note that in live-disk operations, use of anti-skating compensation may increase the chance of the phono arm sticking in damaged grooves instead of jumping over the bad spots. Increasing tracking force by approximately 15% has the same effect on distortion as applying anti-skating compensation. This alternative is recommended in live-disk operations.

4. Use a modern, direct-drive turntable.

None of the older types of professional broadcast turntables have low enough rumble to be inaudible on the air. These old puck-, belt-, or gear-driven turntables might as well be thrown away! Multiband audio processing can exaggerate rumble to extremely offensive levels.

5. Mount the turntable properly.

Proper turntable mounting is crucial — an improperly mounted turntable can pick up footsteps or other building vibrations, as well as acoustic feedback from monitor speakers (which will cause muddiness and severe loss of definition). The turntable is

Maintaining Audio Quality

best mounted on a vibration isolator placed on a non-resonant pedestal anchored as solidly as possible to the building (or, preferably, to a concrete slab).

6. Use a properly adjusted, high-quality phono preamp.

Until recently, most professional phono preamps were seriously deficient compared to the best “high-end” consumer preamps. Fortunately, this situation has changed, and a small number of high-quality professional preamps are now available (mostly from small domestic manufacturers). A good preamp is characterized by extremely accurate RIAA equalization, high input overload point (better than 100mV at 1kHz), low noise (optimized for the reactive source impedance of a real cartridge), low distortion (particularly CCIF difference-frequency IM), load resistance and capacitance that can be adjusted for a given cartridge and cable capacitance, and effective RFI suppression.

After the preamp has been chosen and installed, the entire vinyl disk playback system should be checked with a reliable test record for compliance with the RIAA equalization curve. (If you wish to equalize the station’s air sound to produce a certain “sound signature,” the phono preamp is better preamps have adjustable equalizers to compensate for frequency response irregularities in phono cartridges. Since critical listeners can detect deviations of

0.5dB, ultra-accurate equalization of the entire cartridge/preamp worthwhile.

not

the place to do it.) Some of the

system

is most

The load capacitance and resistance should be adjusted according to the cartridge manufacturer’s recommendations, taking into account the capacitance of cables. If a separate equalizer control is not available, load capacitance and resistance may be trimmed to obtain the flattest frequency response. Failure to do this can result in frequency response errors as great as 10dB in the 10-15kHz region!

The final step in adjusting the preamp is to accurately set the channel balance with a test record, and to set gain such that output clipping is avoided on any record. If you need to operate the preamp close to its maximum output level due to the system gain structure, then observe the output of the preamp with an oscilloscope, and play a loud passage. Set the gain so that at least 6dB peak headroom is left between the loudest part of the record and peak-clipping in the preamp.

7. Routinely and regularly replace styli.

We believe that the single most significant cause of distorted on-air sound from vinyl disk reproduction is a worn phono stylus. (Excessive audio processing is, alas, a close second.) Styli deteriorate sonically before any visible degradation can be detected even under a microscope, because the cause of the degradation is usually deterioration of the mechanical damping and centering system in the stylus (or actual bending of the stylus shank), rather than diamond wear. This deterioration is primarily caused by back-cueing, although rough handling will always make a stylus die before its time.

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