For DTV audio, all stations should be implementing an AES digital audio infrastructure.
As stations put together their DTV facilities, many key questions need to be answered about the topology and capabilities of the system. These are often called “core” decisions, because they define the basic design strategies of the DTV station.
Consumers are being exposed to high quality audio via CD, DVD and now DTV and digital cable. Increasingly, these programs are multichannel (beyond stereo) particularly in the case of feature films, which have been almost routinely mixed in 5.1 over the past decade. Now, episodic shows for television are being released with 5.1-channel audio.
At the initial design stage, the choices between designing for minimal two-channel audio and designing for full six-channel audio (5.1 audio) are not that different. Tools exist (described below) to carry multichannel and/or multi-program audio over a traditional AES “pair.” These tools can also allow the DTV station to seamlessly integrate 5.1-channel audio into a nominally two-channel program stream.
For DTV audio, all stations should be implementing an AES digital audio infrastructure. A single “level” of AES digital routing is the minimum baseline capability that was assumed by the ATSC “Top Down” Committee in its examination of DTV station requirements. Often, stations will implement a two-level (four-channel) infrastructure because it matches the number of audio channels on broadcast VTRs. AES audio distribution allows for transparent and accurate audio distribution throughout the facility. It also future-proofs the station for further developments in audio and data transmission.
Dolby AC-3 is the audio standard in the ATSC television system. It is designed to carry from one to six (5.1) audio channels and can support multiple audio programs. This system is now called Dolby Digital, and consumers will often see references to this name on their DTV receivers as well as on their DVD players and cable set-top boxes. The station's audio programs are encoded into Dolby Digital at (or near) the DTV video compression and transmission equipment. If the station is using the built-in encoder that is sometimes part of the video encoding system, these encoders are currently only two-channel encoders (i.e., mono or stereo.) If full 5.1-channel audio is desired, a separate 5.1-channel encoder (a Dolby DP569) will be required. It can encode any channel format from mono to 5.1 channels. The built-in encoder can still be used for encoding a second-language version of the program (similar to the SAP capabilities of the NTSC system) or any of the new auxiliary services such as descriptive video (visually impaired), hearing impaired, or emergency audio channels.
Program acquisition: If programs are delivered on videotape for local playback, this could exist as a separate multichannel audio tape (DA-88 or similar format) that must be synchronized to the video source (often a digital VTR). In this case, you must be able to lock the two transports to the same reference, and synchronize them using timecode or other means.
Multichannel audio programs could also be delivered on the same videotape as an encoded Dolby E signal. Dolby E allows up to eight channels of audio to be delivered via satellite or VTR, and can be carried on a single AES pair throughout your system. Broadcast networks and cable programmers are using the Dolby E format to store multichannel audio programs along with the video on the same videotape or video server. This simplifies routing, timing and synchronizing the combined elements throughout the system.
Monitoring: Monitoring the audio signal is important in any operation. Programs delivered to DTV stations by networks and others will almost certainly be delivered using digital sources (satellite, digital video server, etc.) Stations need to be able to monitor digital audio signals at many points in their facility, just as they monitor their programs today. Analog routing and distribution will not go away in the new DTV stations, it will just be relegated to “utility” routing and confidence monitoring, rather than actual “on line” audio signals. Products are available from many manufacturers that handle monitoring of digital audio signals in ways that make the audio format transparent to the operator. Often it is as simple as ordering the digital version of the monitoring product. Stations that have made the transition to AES routing on their NTSC stations will already have experience with this. Products also exist to monitor and troubleshoot Dolby E, Dolby Digital and PCM signals carried on an AES link.
Local production and MCR: If the station wishes to do local 5.1-channel audio production, then traditional multi-channel mixing and recording equipment could be used, along with a “widening” of the audio infrastructure within those operational areas.
The audio signals, whether they are 5.1 or stereo, only get encoded into Dolby Digital immediately before transmission to the viewer/listener. A full description of the Dolby Digital encoding process is beyond the scope of the question, but it is important that the programs be encoded with a “channel configuration” that accurately describes the program type. Stereo programs should be encoded as “2/0” and 5.1-channel programs should be encoded as “3/2L” (the “L” signifies the “0.1” LFE channel.) This, and other encoder settings, aids the operation of the consumer's Dolby Digital decoder, routing the number of audio channels in the program to the (possibly fewer) number of speakers the consumer may have installed in their home. This could vary anywhere from mono or stereo speakers built into the DTV set to a full-blown Home Theatre installation.
If the station chooses to implement Dolby E, there are some unique capabilities of the DP572 Dolby E decoder that have special significance in Master Control operations. The “Voice Over” feature of the DP572 allows local voiceovers to be inserted over the 5.1 audio signal, or the audio source can be switched completely to a separate two-channel source. This is usually the station's normal stereo audio, and this can be used for local inserts, news or commercials. When the decoder has been told to switch to an external two-channel source, the Dolby Digital metadata will be changed to the proper mode for stereo operation. The voiceover feature can allow the Dolby E decoder to function as the audio section of the MCR switcher, delaying the requirement for full multichannel audio production facilities, at least in Master Control.
Audio monitoring in Master Control should be set up with a professional Dolby Digital decoder to properly monitor the signal in any format the listener may have. This includes mono, through stereo and surround, all the way up to 5.1, with a subwoofer. There are also graphic displays available to visually monitor the content and relative phase of the multichannel audio signal. If you feel that your master control area is not suitable for critical listening, consider outfitting a conference room (in addition to your control room) with a 5.1 monitoring system. If this is the same room where you entertain clients and screen their programs, it gives you a chance to view their programs and to demonstrate your audio capabilities in their best light.
The average station does not need to alter their planning all that much to include 5.1-channel audio programming as part of their schedule. As viewers are exposed to more and more 5.1-channel programs via DVD and digital cable, any lack of such programming on the DTV station will become apparent. As the infrastructure requirements are very similar, it is not that difficult to add 5.1-channel audio capability to your DTV station. By specifying the proper equipment now, you future-proof your facility by supporting 5.1-channel programs today.
More information about implementing the Dolby Digital DTV audio standard can be found in the Broadcast Implementation Guidelines, available from Dolby at www.dolby.com/tech/ddbigpr.pdf
Kenneth Hunold is a broadcast applications engineer for Dolby Laboratories, Inc. in New York.