5.1 Audio for HD: YES Case Study, Part I
June 27, 2007
When designing and building an HD facility, the video part can be relatively simple compared to audio.
With 5.1 audio, there are so many permutations. Embedded or not? Discrete or multiplexed/compressed? Besides the six surround channels, discrete or derived stereo may be needed. And what kind of stereo—left only/right only or left total/right total? What about other downmixes for monitoring? And don’t forget the SAP channel or channels, mono, stereo or 5.1. If using embedded audio, do all recording, routing and processing devices support it? And how many channels do they support? Is the embedded audio disturbed during any of the video processes? There will likely be locations like the audio control room where de-embedders will be needed to extract the discrete audio channels. And don’t forget the metadata and lip-sync timing with the video.
(click thumbnail) YES Network’s Dolby control center NO MONO NO MORE Going back to mono isn’t an option, and organizations facing these kinds of audio issues for their HD facilities have developed solutions to meet their unique operating and technical requirements. From time to time, “Audio by Design” will share some of these mini case studies. This month, we’ll look start with the Yankees Entertainment & Sports, or YES, Network, which recently completed a new HD production and master control facility in Stamford, Conn. As YES Chief Engineer John McKenna said, “audio was and remains the biggest problem we have.” As McKenna explained, all the audio at the YES Broadcast Center is embedded because there’s only one level on the Nvision NV5128 HD digital production router. Not only that, YES uses Dolby E on one of the embedded AES channels to distribute 5.1, where available, within the new facility. The Nvision NV5128 can switch embedded Dolby E with out loss of audio or pop and clicks, according to Chuck Diehl, northeast account manager for Nvision. MASTER CONTROL PATH Since the New York Yankees baseball games play such a major part of YES Network’s broadcast schedule, let’s start with the audio signal from Yankee Stadium in the Bronx. YES employs Dolby E for its 5.1 audio feeds from Yankee Stadium in the Bronx to the YES Broadcast Center in Stamford. The stadium feed is actually an ASI feed decoded in Stamford through a Harris LinkPlus. Dolby E channel assignments for the stadium feed are as follows: Stadium center is the announcer/commentator dialog channel. AES1/2—stadium front left and right; AES3/4—stadium center/low frequency effects (LFE); AES5/6—stadium rear left and right; AES7/8—stadium natural sound left and right. The signal from the game is fed to both master control and the production control room at the YES Broadcast Center in Stamford. This month we’ll concentrate on at the master control audio path. One copy of the inbound Dolby E stream is fed into a Dolby DP 572 decoder and turned into its three component AES 5.1 channels, which are then fed into a Dolby DP 563 ProLogic encoder to generate Lt/Rt (left total/right total), according to McKenna. The fourth channel, natural sound from the game, is recovered and distributed via a separate AES stream where needed around the plant. A copy of the 5.1 discrete components are used in production.
The Lt/Rt output from the Dolby DP 563 is then fed into the AES-1 input of a Harris Leitch MXA-3901 audio multiplexer where it is re-embedded into the HD-SDI video.
(click thumbnail) YES Network master control (Each AES channel contains two audio channels, and AES designations are indicated on the connector panel of each piece of gear. Some companies may label the first AES input as AES-1, while others may use AES-1/2.) A copy of the Lt/Rt signal is also fed to destinations requiring a stereo signal, for example, the history recordings for Major League Baseball. BACK TO THE GAME Now back to the other inputs of the MXA-3901 audio embedder. The incoming Dolby E stream is fed to the AES-2 input and the Spanish radio game call (SAP, or secondary audio program) is placed on AES-3. This is how the HD video from the game is married to its embedded audio so that it can be switched by the Harris Leitch Opus-HD master control switcher. SD programs and commercials are upconverted to HD before hitting the production HD routing switcher and master control switcher. In addition, all audio associated with video is embedded into the HD video signal. Not all sources contain Dolby E data as one of the AES channels of the embedded audio stream, for example those that are stereo only. Dolby E is used for remotes and YES HD studio 5.1 productions. The master control switcher is fed some audio-only sources as well, such as voice-over or music for the slide/character generator. All master control switching is performed on the embedded HD inputs, producing an embedded HD output. This output of the master control switcher feeds a Harris Leitch LogoMotion and EEG closed caption encoders, and finally the resulting HD-SDI embedded signal is fed into a Harris Leitch DXM-3901audio de-embedder (demultiplexer) where the AES-1 (Lt/Rt), AES-2 (Dolby E) and AES-3 (SAP) channels are recovered. The video is fed directly into the Motorola DigiCipher II broadcast encoder, but audio takes a less direct route. The AES-1 and AES-2 audio outputs from the DMX-3901 audio de-embedder are fed into the PCM and main inputs of another Dolby DP 572 Dolby E decoder and post-master control. The decoder produces the three component AES 5.1 channels from the output of the master control switcher. Why is an Lt/Rt signal fed into the PCM input of the post-master control Dolby E decoder? When the Dolby E decoder detects a Dolby E signal on the main input, the Dolby E signal is decoded into discrete 5.1 channel audio on three AES pairs. If a Dolby E signal is not present on the main input, then the DP 572 decoder automatically switches to the PCM input (Lt/Rt) and sends this out on the AES-1/2 output. McKenna noted that when the DP 572 switches to the PCM (Lt/Rt), input it tells the DP 569 (AC-3 encoder), via the metadata ribbon cable interconnection, that it should switch to the Lt/Rt mode and modify the metadata in the AC-3 stream to tell the home receivers that it’s now a stereo, not surround, signal. As McKenna put it, “this scheme allows relatively automatic switching between ‘E’ and stereo but can cause a glitch if the master control room operator accidentally dissolves rather than cuts.” From the post-master control Dolby E decoder, the three discrete AES 5.1 signals are then passed through a Norpak NAVEII (Nielsen) encoder to a Dolby DP 569 AC-3 encoder. The output of the AC-3 encoder is the audio that is ultimately sent to the Motorola DigiCipher II broadcast encoder, where it is married and timed to the HD video. This DigiCipher II encoded feed is the one that is distributed to the YES Network HD affiliates. Next time we’ll see how YES handles the SAP channel and metadata.
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