Managing HDTV sound
Jun 1, 2009 12:00 PM, Randy Conrod
5.1 audio implementation requires comprehensive understanding.
Proper management of 5.1 surround-sound audio is the latest challenge facing broadcasters as they continue to enhance the overall HDTV experience in the home. In many ways, managing the rollout of 5.1 audio is proving to be more difficult for engineers than the HDTV rollout itself.
Defining audio
Figure 1. Monitoring setups for various audio types.
Select image to enlarge.
To help define the terms used today in audio processing, Figure 1 lists typical audio types (including a future 7.1 surround-sound scenario), and Figure 2 lists the typical audio conversion types (encoding, decoding, upmixing and downmixing).
The process of compressing audio into a non-PCM compressed signal is typically called encoding; when it is decompressed back to PCM, it is called decoding. A downmixed signal may or may not have additional inaudible information that can be used for a downstream upmix. Downmixed signals are monitored in stereo unless the signal needs to be upmixed for monitoring purposes (Pro Logic Iand II, DTS Neural Surround), and non-PCM signals sound like white noise when monitored.
Monitoring audio
The majority of “true” 5.1 audio is provided by tapes or ingested via satellite in today's HDTV systems. One of the first challenges is the monitoring of the audio. Options include downmixing the 5.1 surround sound or setting up a 5.1 amplifier and speakers. There is also the issue of how to monitor a mono mix of the stereo signal, as well as Dolby E, Dolby Digital AC-3 and Pro Logic II signals as they are encountered. All of these scenarios have to be carefully considered.
Figure 2. Typical audio types detailed.
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Audio provided as three PCM channels, either as AES or embedded into the SDI signal, can be downmixed for monitoring in stereo or in surround sound. A downmix will be provided in most cases (5.1 + 2.0) and is required in the monitoring chain. Audio that has been compressed using Dolby E technology must be decompressed. However, this can lead to lip-sync issues because the compressed non-PCM audio may or may not be prealigned for the one frame of delay apiece incurred during compression and decompression. The picture monitor also can impart its own delay, which may or may not help the lip-sync timing.
If the audio was compressed into Dolby Digital AC-3, decompression is not recommended, because this signal is meant to be decompressed once in the home environment. If recompressed, artifacts will result in lower quality.
Pro Logic also may need to be upmixed for monitoring. Figure 3 shows the various types of monitoring required.
Lip sync
Lip sync is proving to be another issue in 5.1 audio delivery. It has been said that today's larger screen sizes make it easier to see lip-sync issues. If this is true, lip sync becomes even more apparent in HDTV and surround sound. A broadcaster or cable/satellite distributor can control lip sync up to the point it is decoded in the home environment, and many lip-sync errors can be predicted, measured and repaired using today's technology. There are methods of placing a known video and audio test signal, passing it through a system and then offsetting to the measurement.
Processing audio
Another pressing issue is what to do if 5.1 surround sound is not provided within the content. There are two methods with which to address this issue. The purist's method is to continue to move stereo audio into the home environment. However, dynamic audio metadata is used to signal the receiver in the home for the 2.0 stereo or 5.1 surround-sound content. This is known to cause clicks, pops, muting and other issues in the home, because not all receivers react to audio metadata in an inaudible fashion.
Figure 3. Typical audio conversion types.
Select image to enlarge.
The other method is to upmix the audio from stereo to 5.1 surround sound. It is important to perform listening tests to determine whether the upmix sounds natural in the surround-sound domain. Also, listen for any audible artifacts when the input transitions between 2.0 and 5.1. Upmixing provides a constant surround-sound experience in the home environment through the use of static audio metadata.
Moving audio in today's facilities can be simple if the audio is embedded. It becomes more complicated if AES interfaces are used. Today, 75V unbalanced interfaces are mostly used, with occasional use of balanced 110V interfaces. Existing embedded audio equipment handles all four groups of embedded audio (16 total channels, four per group), but interfacing to older embedded audio equipment can cause problems because all four groups may not be handled.
Even if an older-generation embedder/de-embedder handles two groups, the other two may not be identified properly. This can cause issues with newer embedders/de-embedders that can handle four groups of audio. A guard band for embedded Dolby E must be implemented so there is an alignment between the video and Dolby E header for proper de-embedding downstream relative to the video. Also be aware of old de-embedders in a system because they may not de-embed the audio phase aligned across the three PCM channels for surround sound. This will affect the surround-sound audio.
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