Integrating HD and SD video and audio
Feb 1, 2006 12:00 PM, By Jean-Claude Krelic
Preserving audio metadata
The audio-processing challenge for multiformat, HD/SD installations is significant. Broadcasters need to provide a stereo output (LoRo) or a Pro-Logic (LtRt) output for the SD signal, along with a 5.1 or 2.0 signal fed to the AC-3 encoder for HD broadcast as Dolby E, AC-3 or simply as discreet audio as part native HD content. The key challenge here is to maintain and apply audio metadata across the broadcast plant.
Working with 5.1, Dolby E or AC-3 audio always involves handling associated metadata, which includes such information as the type (e.g., 5.1, 2.0, 2+2+2+2), dialnorm values, dynamic range and other details defining the signal. Metadata must accompany the video signal because this information allows the home viewer's AC-3 decoder to react properly and provide the right sound effect at the right moment.
Until recently there were no effective means of maintaining metadata inside a plant without a Dolby E or RS422 interconnect. Miranda Technologies has worked with customers and other manufacturers to develop a standard that will enable carrying of metadata in the ancillary space of the HD-SDI signal. It was logical to embed the audio into the HD-SDI signal, so why not embed the audio data as well? By allowing broadcasters to carry this metadata throughout their facilities, this solution expands the opportunities for producing material with 5.1 audio.
Downmixing and upmixing audio
The preservation of audio metadata is critical to up- or downmixing audio signals. Although more material is produced with 5.1 audio, broadcasters still need to provide the SD output with a stereo or stereo-coded signal (Dolby Pro Logic I or Pro-Logic II). Some broadcasters can carry both a 5.1 and a stereo signal, but others simply don't have enough channels available — particularly given the need to provide 5.1, SAP and descriptive video. In an eight-channel server system, for example, there is no room for a stereo pair.
To overcome this obstacle, broadcasters can implement a stereo-coded downmix from the 5.1 audio. (See Figure 4.) The stereo-coded output allows home users with a Dolby surround decoder to enjoy sound separation for multiple audio channels. Downmixing is also used throughout a broadcast facility for dubbing purposes and to provide an audio sample similar to the sound experienced by home viewers watching the broadcast in SD.
 Figure 4. Creating an audio downmix. Click here to see an enlarged diagram. |
While a basic stereo downmix is relatively easy, a stereo-coded output gets a little bit complicated. Using only the 5.1 audio with the metadata simplifies the audio scheme. When the audio is still 2.0, the device performing the downmix must auto-detect that the signal is 2.0 and deliver a stereo-encoded signal with the appropriate metadata. When no metadata is available, the device must be able to generate a default value for 2.0 metadata to be sent with the audio.
With the move toward 5.1 audio, some broadcasters would rather not carry 2.0, but they still want to have a 5.1 signal created from the 2.0 (stereo) signals. Transforming a stereo 2.0 signal into a 5.1 audio signal is called an upmix, and this function helps to create the sound effects desired. It simplifies the audio path and functions in a facility and delivers a higher-quality experience to the home viewer. Hence, the ability to create and manage high-quality up- and downmixes is also fundamental to successful HD plant operations.
Jean-Claude Krelic is interfaces project manager for Miranda Technologies.
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