Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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in the Broadcast/Netcast Facility 7 though not always) mastered without the quality-degrading peak limiting and clipping that have become ubiquitous on contemporary CD releases. Quality Control in CD Transfers When one builds a music library on a digital delivery system, it is important to validate all audio sources. A track’s being available on CD does not guarantee good audio quality. For the best audio quality results, do not accept MP3 audio from record companies or download services. MP4/AAC audio may be used on a case-by-case basis if nothing better is available. Many original record labels are defunct and have transferred licenses to other labels. It was formerly safe to assume that the audio from the original record/CD label or authorized licensee is as good as it gets, but tasteless remastering has ruined many recent major label re-releases, even within the same labels. Many major labels produce collections for other well-known marketing groups. Many of these sources are acceptable, although they require careful auditioning and quality validation. Some smaller and obscure labels have acquired licenses from the original labels. While some of this work has proven to be excellent, some of these reissues should probably be avoided. Syndicated programming can be another source of audio quality problems, especially if the audio source material has been preprocessed in any way. Such programming should be very carefully validated. Another pitfall in CD reissues is mono compatibility. This is important because most clock radios and mobile phone speakers are mono and every FM car radio implements blend-to-mono as a function of signal strength and/or multipath. Each CD that is transferred should be checked by ear to ensure that the left and right channels sum to mono without artifacts. CDs that sound fine in stereo may suffer from high frequency loss or “flanging” caused by uncorrected relative time delays between the left and right channels. Some computer audio editing software, such as Adobe Audition 3.0, contains restoration tools like Automatic Phase Correction. With careful adjustment, possibly even in manual mode, good results are achievable. Some CDs sourced from full-track mono masters have been transferred using a stereo playback tape head. The slight delay and azimuth differences between the two gaps can cause high frequency loss when summed to mono, which can be completely eliminated by choosing the best channel from the CD and using it as a source for both channels of the file in your playout system. Some music was poorly recorded and mastered. There are many reasons for this, including poor equipment or monitoring environments. Some cases will require careful equalization to achieve consistent results. Many tracks, even from “desirable” labels, have been recently re-mastered and may sound quite different from the original transfer to CD. Regardless of source, it is wise to use the original performance even if its audio quality is worse than alterna-
8 Maintaining Audio Quality tive versions. Sometimes the original performance has been remixed for CD release, which often improves the quality. However, beware of remixes so radical that they no longer sound like the hit version as remembered by audiences. Furthermore, many of these original performances may be available on many different albums, possibly from the same label but from different sources with varying quality. A valuable on-line resource for this information is Top 40 Music on Compact Disc: http://www.top40musiconcd.com/ Some of the more recent remasterings may contain additional signal processing beyond simple click and pop elimination. Newly remastered tracks should be validated very carefully, as the newer tracks may suffer from excessive digital limiting that reduces transient impact and punch. Therefore, the older, less-processed sources may stand up better to Optimod transmission processing. Some computer audio editing software, such as Adobe Audition, Steinberg WaveLab, iZotope RX, and Diamond Cut DC8, contains restoration tools that attempt to undo clipping. This is a challenging task, so these tools should be qualified and used very carefully. One size does not fit all. Unfortunately, there is no reliable formula for choosing old or new CDs. For example, some original CD releases were simply transfers of a vinyl pressing. The limitations of vinyl are usually audible and subsequent remasters were a dramatic improvement, either because the original master tapes were discovered and used or because improved vinyl restoration software and techniques were employed. On the other hand, many remasters were subject to additional dynamic range compression and peak limiting and do not sound as good as their original releases even though the newer remasters may claim higher resolution. We believe that the best-sounding CDs are probably those mastered from about 1990 to 1995. Before 1990, many mastering engineers used the Sony PCM1610’s converters because the standard medium for transmitting a mastered CD to a replication house was then a 3/4" “U-Matic” video recorder and the Sony formatted the audio so that it could be recorded as video on such a machine. However, the PCM1610’s converters were widely criticized for their sound. There were too many low-slewrate opamps and electrolytic capacitors in the signal path, not to mention high-order analog anti-aliasing filters. Things changed around 1990 with the introduction of analog to digital converters based on the then-new Analog Devices AD1879 delta/sigma A/D converter chip. This chip uses oversampling to eliminate the need for high-order analog anti-aliasing filters and has very good low-level linearity. The resulting signal path has very little group delay distortion. If properly designed, converters based on the AD1879 sound very good. Meanwhile, the loudness wars were still at least five years in the future. Starting around 1995, average levels on CDs started to increase. The availability of early digital look-ahead limiters like the WAVES L1 enabled CD mastering engineers to limit peaks without obvious side effects. However, like most anything else, the availability of the tools led to their abuse. Digital limiting, which was a bit like crack to some mastering engineers, started to suck the life and punch out of material for the sake of loudness. Certain engineers developed a reputation as “go-to guys” if you wanted a “loud” CD. Look-ahead limiting by itself was no longer enough —
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