It’s all in the mix

zoom

Implementing surround sound audio into HDTV workflows is not without its challenges. We explore this issue and look at how it can be overcome.

As HDTV continues to roll out worldwide, the deployment of surround sound audio is the latest challenge facing broadcasters when upgrading an existing system or building a new one. In today’s HD broadcast environments, new audio processing tasks such as upmixing, loudness control and management of audio metadata are important considerations for broadcasters working to enhance the overall HDTV experience in the home.

Because considerable audio content exists in archives today, the challenge of supporting surround sound prompts a basic question: What can be done with stereo audio?

The answer can be found in a technique called audio upmixing. Upmixing is a process that “converts” stereo audio — whether it is LoRo (true stereo) or some form of matrixed stereo that contains surround sound information (LtRt) — into a 5.1 surround sound signal. As is the case in any conversion process, a number of upmixing technologies are available, which vary greatly in terms of quality.

Some upmixers present the same surround information on both the left surround and right surround channels, which results in an unnatural sound. An upmix should “steer” the correct audio information — which represents the right and left surround channels — into the correct speakers. As dialogue is upmixed, it should be steered into the centre channel, with no dialogue leakage into the surround channels. For optimum surround sound quality in the home, it is also important that the low-frequency effect (LFE) channel sound natural. Quality downmixing is another factor for consideration, as a proficient downmix and upmix can be utilised in a system to carry 5.1 surround sound in a 2.0 stereo infrastructure.

When implementing surround sound audio in their HDTV workflows, broadcasters must consider two key factors in the home listening environment:

1. The perceived loudness differences between stereo and surround sound.

2. The fact that the home receiver requires audio metadata to switch between stereo and surround sound content.

Perceived loudness differences occur not only between stereo and surround sound, but also result from the differing mixes in television from movies and music through drama, comedy, news/weather/sports and, of course, commercials — which are notorious for sounding louder than programming content. Although loudness measurement is standardised, it does not yet solve all of the problems related to providing measurements across all of the types of mixes. The key when choosing a loudness control device is to understand which standards are adhered to and what is done beyond the standards to produce a natural-sounding, well-controlled audio signal.

Today, the ITU-R BS.1770 algorithm is good for dialogue measurement over medium and long-term time intervals, but will not account for all of the varying mixes that can be encountered in today’s television programming. Controlling short-term loudness changes is also important. In addition, as the loudness changes are applied, the process should match the manner in which a human perceives loudness at varying levels in order to produce a natural-sounding loudness control. For example, automatic gain control, which operates in a linear fashion, does not sound natural, as human hearing at varying loudness levels is not linear.

The second factor to consider is that audio metadata switching in the home may result in clicks and pops and muting dependent on the implementation of the home receiver. It is recommended today to consider an upmix when the signal is stereo before encoding into Dolby Digital AC-3 and setting the encoder to 5.1 (3/2 coding mode). It is also necessary to choose an upmix device that can transition between 2.0 and 5.1 in a seamless fashion with a 5.1 constant output.

In conclusion, when considering surround sound deployment as part of today’s HDTV broadcast environments, choosing the most natural-sounding upmix that offers seamless transitions for constant 5.1 output, as well as a loudness control based on the latest standards and featuring the latest enhancements, will ensure a more enjoyable surround sound experience in the home.

Randy Conrod is product manager for Digital Products, Harris Broadcast Communications.

Print | Email | Discuss this article

| Share |

READERS' COMMENTS

Click here to post a comment

Add your Comment All posts are sent to the administrator for review and are published only after approval. ArabianBusiness.com reserves the right to remove any comment at any time for any reason. Please keep your responses appropriate and on topic. Arabian Business would like to point out that only comments relevant to the story will be published. Any containing personal insults or inappropriate language will not be approved.
Name *
Remember me on this computer
Email *	(Your email address will not be published)
City
Country
Subject *
Comment *
Notify me of further comments
Enter the words above *
Please click post only once - your comment will not be published immediately.

MORE FROM ARABIANBUSINESS.COM