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Next Steps in Surround
To minimize the aforementioned effects, there are several key fundamentals to check for and maintain. Some of the chief areas of concern are:
No. 1 — Implement and maintain optimum system levels, including a standard reference level, headroom and signal-to-noise ratios.
Whether a station has an all-digital infrastructure or one comprised of analog and digital components, implementing and maintaining optimum system levels greatly reduces the possibilities of audible errors. There should be a system-wide established reference level, headroom, signal-to-noise ratio and unified clip level of the system. One standard practice for professional digital and analog equipment is shown in Table 1.
With the proliferation of less expensive professional equipment (e.g. prosumer), it is becoming more common to have equipment that is unable to achieve a +24dBu analog output. Therefore, some accommodations to the audio level standards may be necessary. Regardless, if you do not have established reference levels, not only will the broadcast of surround content be in question, the quality of stereo/mono content will be reduced as well.
No. 2 — Minimizing use of digital audio codecs/prevent use of multiple audio codecs.
Most digital audio codecs utilized, such as AAC or MP3, are lossy. Passing audio through more than one codec creates generation loss, degrading the audio quality. If one or more codecs are used, set the data rate of the available codec at the highest setting feasible to minimize the effects that audio codec will have to the audio. If possible, remove unnecessary use of an audio codec.
No. 3 — Appropriate selection of HD Radio data bit-rates.
Currently, the maximum data bit-rate for HD Radio is 96kb/s. When choosing the data bit-rate on HD Radio for the surround content, make the choice wisely and with careful consideration. The integrity of the surround playback field will be reduced as a lower codec data rate is used. Using 64kb/s or more and no less than 48kb/s is suggested.
No. 4 — Minimizing distortion, including inter-modulation distortion (IMD).
Distortion can occur in a variety of ways. Among others, inadequate headroom in a system component can result in the clipping of audio, producing distortion. One simplistic method to minimize distortion is to avoid reaching the last 2 or 3dB before full scale of an A-to-D converter. With the increased quality of storage and transport protocols, the need to reach digital zero to maximize audio quality is no longer necessary.
Increased distortion also occurs with heavy/aggressive use of broadcast audio processors. In surround, the more aggressive the approach, the more distortion that may have been previously masked in stereo now becomes apparent in surround. A few rules of thumb will assist in reducing IMD and processing artifacts:
Choose a less aggressive limiter/clipper type.
Minimize the use of the limiting/clipping section of the processor.
6; Use look-ahead whenever possible.
Utilize slower release times of the compressor/limiters as much as possible.
No. 5 — Maintain equal balance of all audio channels.
Maintaining equal balance of the audio channels is required for surround. In addition, maintaining phase relationships and unified frequency response of the audio channels is also imperative. An imbalance or improper phase relationship and/or frequency loss of even one channel can result in a dramatic shift in the integrity of surround content.
Now that we have addressed these basic infrastructure requirements, we can move to the other end of the surround broadcast spectrum, choosing and creating the surround content itself.
There are many sources of pre-recorded surround content available. It is possible to pass the surround content through one of the surround sound broadcast technologies and likely have a pleasant result. However, it is unlikely the result will be pleasant at all times. This is where one of the advantages of using prerecorded content comes into play.
With each selection, either the surround content and/or the technology employed can be optimized for optimum playback performance. Much like content provided to many radio stations today, it is possible for prerecorded content to be pre-encoded appropriately in surround in an ideal production environment. Then this content can be shipped and placed on digital audio delivery systems for playback. This is the case for those who are successfully broadcasting surround content heard today on satellite and on several FM radio stations. Other advantages of utilizing prerecorded content include adding metadata for various automation systems, audio processing and/or to provide content information to listeners.
With live content, there are several other variables involved. The variables include how the live content will be captured and delivered to the broadcast station from remote locations and maintain quality. Standard ISDN connections or 128kb/s are barely enough simply due to the limited data bandwidth available as well as the utilization of another digital audio codec in the broadcast chain. Luckily, there is an ongoing development of new technologies and connectivity to assist in this process providing higher data bandwidth and advanced audio codecs to optimize the expanded data throughput. Data rates of 256kb/s begin to permit enough data throughput for surround content with some systems reaching near linear digital audio data throughput, which is the ideal scenario.
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