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Audio FOR HD: Common problems, simple cures

Sep 1, 2008 12:00 PM, By Michel Proulx

Stop multichannel audio variance and inconsistency dead in its tracks

Maintaining 5.1 channel continuity in master control

Figure 5. Switching from stereo to 5.1 results in a corresponding change at the home.
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Let's now consider the second issue of integrating legacy stereo material and 5.1 surround-sound material. Early HDTV broadcasters initially elected to air a mixture of stereo and 5.1, depending on the original material. This resulted in viewer confusion and dissatisfaction as the program switched in and out of 5.1 surround, depending on the show segment. (See Figure 5.)

An an alternative, the broadcaster can send 5.1 surround sound to viewers by employing a technique often referred to as upmixing. This allows stereo material to be converted to 5.1 by synthesizing the center, surround and LFE channels. (See Figure 6.)

Figure 6. Audio upmixing in master control ensures that the outgoing audio stream is always 5.1, allowing the home receiver to always fill all speakers regardless of the source.
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The criteria for a good upmixer include:

Dynamic adaptation

The upmixer automatically adjusts the synthesis algorithm depending on the program content (dialog, action and music).
Auto-sensing

It automatically detects whether the input is 5.1 or stereo, and it seamlessly and silently switches the synthesis in and out as required.
Downmix-compatible

The upmixing is designed for homes with surround-sound listening configurations, as well as those with stereo televisions. In homes with stereo audio, the incoming 5.1 signal is converted back to stereo, or downmixed, by the decoder or receiver. Hence, it is imperative that the synthesized 5.1 is compatible with an eventual downmix in stereo homes.

Conclusion

Figure 7. Integrated solutions that combine technology from multiple vendors result in simpler systems.
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The main reason that multichannel audio has proved so demanding for facilities is not necessarily because of the increased channel count from two channels for stereo to six channels for 5.1. It is mainly because of all the additional technology required to receive, process and deliver 5.1 audio throughout the broadcast chain. Using industry standards, such as Dolby Digital (AC-3) and SMPTE 2020, has helped to improve interconnection and operation across multiple audio devices from multiple vendors. However, a more effective solution is to use fewer devices, thus reducing costs and simplifying systems. (See Figure 7.)

Looking ahead, we can be optimistic that there will be fewer troublesome HD audio issues, as facilities start to deploy more advanced audio processing technologies. It should all add up to fewer headaches for engineers and fewer complaints. Now that's progress!

Michel Proulx is the chief technology officer for Miranda Technologies.