Dolby and Harris' X75 interface
Oct 1, 2007 12:00 PM, BY RANDY CONROD AND JEFF NELSON
Harris’ X75 interface is a single video processing device with integrated Dolby E and Dolby Digital decoding and encoding.
During the continuing HDTV rollout, keeping costs down — as well as saving rack space by using less equipment — is key. Audio has become as important as video, and one of today's major challenges is to incorporate both stereo and surround-sound mixes into broadcasting while maintaining video-to-audio timing and levels.
A single point for video and audio processing is considered a definite asset to new system designs. Easy access to the video processing with audio timing, levels and remapping is important when implementing HDTV with surround sound.
A/V processing
The initial design of the Harris X75 multiple-path converter and frame synchronizer included five AES inputs and outputs so that outboard Dolby decoders and encoders could be easily connected for a single video and audio processing point. While the design was being implemented, Dolby created small form factor options for the system that could be easily integrated for Dolby E and Dolby Digital (AC-3) decoding and encoding. The final design enables a single video processing device to be used easily with inboard or outboard Dolby products. (See Figure 1.)
Figure 1. A simple block diagram of the X75 interface. Click on image to enlarge.
The converter/synchronizer processes both video and audio signals. In today's hybrid production environment, both SD and HD baseband video must be accommodated, as well as analog, digital, embedded, and Dolby E or Dolby Digital compressed audio.
Video frame synchronizers for analog, SD-SDI and HD-SDI with processing amplifiers allow the user to level-adjust the video with genlock and timing. Video signals are up-, down- or crossconverted and aspect-ratio-converted as necessary. If required, additional delay is provided for matching to the audio portion of the signal.
Audio may be analog, digital (AES 75Ω or 100Ω), or embedded into SD-SDI and HD-SDI. Audio that has been compressed into Dolby E or Dolby Digital may be passed through untouched and properly aligned with video. At the same time, if required, the compressed stream may be decoded and processed along with the video. Gain may be applied, the audio signal inverted and delay added (or video delay) to match up the video-to-audio timing for lip sync purposes.
Metadata
Dolby audio metadata may be handled in many ways. For upconversion of video with stereo audio, externally generated metadata used to configure the downstream Dolby Digital encoder may be inserted into the vertical ancillary data space (VANC), ultimately controlling the decode process.
The metadata stream also includes parameters that designate the number of audio channels in the program, as well as loudness normalization, dynamic range control and other key functions. A number of additional metadata values are for information purposes only.
When decoding Dolby E content, the metadata may be embedded into the VANC or provided as serial data on the data port of the device. When encoding Dolby E or Dolby Digital, audio metadata may be generated and authored, brought in on the data port as serial data, or taken from the Dolby E decoder to the Dolby E or Dolby Digital encoder.
In addition to video and audio processing control, a single point of monitoring is important. Video and audio signals are monitored for signal presence with many user-settable parameters. Control and monitoring over an IP network is possible using a Web browser or dedicated GUI software.
Conclusion
The X75 multiple-path converter and frame synchronizer with integrated Dolby E and Dolby Digital decoding and encoding provides a single point of total control over the video and audio portions of a program signal. Video and audio can be manipulated — for any application — for ingest into a system or before output from a system into a distribution or contribution channel release.
Randy Conrod is product manager, Digital Products, Harris. Jeff Nelson is broadcast channel manager for Dolby Labs.
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