Monitoring surround sound is often a confusing concept for television broadcasters. The ability to properly hear the program audio goes well beyond the basic two-speaker arrangement typically found in stereo broadcast facilities and often competes with the ability to see the picture on video monitors.
Between program monitors, automation system monitors and other necessary displays, finding room for extra speakers is often deemed to be a low priority, especially in important areas like master control centers. However, as viewers grow more adept at understanding high-definition technologies, they are demanding surround sound to complement the video.
To properly monitor program audio for surround sound, broadcasters need to be able to hear the full 5.1 channels through an adequate speaker system. Proper speaker placement and bass management will make this a relatively simple task. At the same time, broadcasters also need to have the ability to hear other common listening configurations so they can monitor how a consumer who isn't using a 5.1 system will hear the same program. There are tools available today to provide this capability.
First, a system capable of reproducing the full bandwidth of sound from the very low frequencies to the very high frequencies is necessary. There are a few ways to meet this requirement. If there is plenty of space, a matched set of full-range speakers and a subwoofer is ideal. If full-range speakers are not practical for the main channels, then smaller matched speakers and a bass-managed subwoofer to handle the low end would be an acceptable alternative.
In this type of system, the low-frequency content from the main channels is filtered and sent to the subwoofer, along with the LFE information. This allows for smaller main channel speakers where needed, yet maintains the full bandwidth reproduction of the program audio.
Second, the speakers need to be placed correctly within the room. The left and right speakers should be somewhere between 22 and 30 degrees from each side of the center of the listening position, and the center speaker should be centered in front of the operator. (See Figure 1.) The speakers should all be at the same height or as close as possible within the constraints of the other monitors vying for the same space. When working around traditional CRT video monitors, shielded speakers will be required.
The location of surround speakers allows for some variance, but they should be placed slightly behind the listening position and aimed towards the operator. Ideally, they are between 90 and 110 degrees relative to the center speaker. In some cases, due to physical room limitations, they can be located on the back wall or on the sidewalls. (See Figure 1 on page 34.)
Another important consideration for the speakers is that they are properly level balanced to each other. It is critical that all of the speakers reproduce the same sound pressure level (SPL) when fed with a given reference signal or monitoring level.
A simple noise generator allows consumers to adjust their decoder systems for uniform level reproduction from their speakers. The professional application depends on the equipment in use for how the calibration is done.
Regardless of what system is employed, the goal is to get the same SPL from any channel that is given the reference noise signal, with the exception of the LFE channel. When bass-managed systems are used, this will require an audio spectrum analyzer to set both the overall level and the bass-managed level reproduced by the subwoofer. For full-range systems, a simple SPL meter will usually suffice.
A proper 5.1 channel monitoring environment is not limited to the master control center. The need for proper monitoring also exists in the ingest room, the audio production room, and even in the lobby or green room, where off-air signals may be delivered.
Each of these areas needs to be capable of reproducing the full-range audio, and the solution for each may be slightly different. Although all of the production areas should be equipped with professional equipment, a consumer system in places like the front lobby gives you the ability to hear your programming on equipment that your viewers might have at home. It also shows visitors what you are doing and lets them know that they could have something like this in their homes without having to buy professional equipment.
Know the downmix
Once all of the 5.1 channel audio monitoring is in place and properly calibrated, subjective assessment of the audio program can proceed. However, it is a given that some of your listeners won't be listening to the full 5.1 channel discrete program as you are. Some may be listening through a Dolby Surround Pro Logic or Pro Logic II decoder, in stereo or mono audio.
The ability to hear all of these downmixes is certainly necessary at content creation and desirable in the quality control and ingest areas as well. Products such as the Dolby DP570 multichannel audio tool were designed for making the proper downmixes and metadata emulations of the Dolby Digital consumer delivery coding system in production and quality control areas.
At some point in the facility, the ability to hear an off-air signal through a consumer system is a definite plus. Hearing the actual off-air signal as a consumer is the final test to ensure that your audio signal is making it all the way home, sounding as you intended it to.
Beyond hearing, there are other monitoring considerations: 5.1 channel metering, proper dialog level, metadata parameters and errors are all things that should be monitored. Several companies have solutions for looking at this information and making sense of it.
For metering, find a meter that displays audio signal levels, such as Wohler's AMP2-E8MDA. The inputs to these metering devices are tailored to virtually every application possible. Many of these devices include confidence speakers to hear the sound and are perfect for listening in difficult areas such as transmission where a 5.1 channel system is impractical but the sound needs to be heard to verify its presence.
Dialog level can be properly measured and monitored with the Dolby LM100 loudness meter. This meter will also show metadata parameters within Dolby E and Dolby Digital streams and count errors. Several networks are now specifying this meter for all tapes submitted to them for airing.
Tektronix makes a test and measurement set in the WVR7000 series that allows for measuring items from many sources based on the options purchased, including metadata, errors, level and Dolby E or Dolby Digital decoding. This box also includes video monitoring needs, making it a nice package of tests in one display.
Videotek's VTM-450E HD/SD on-screen monitoring system is popular with satellite uplink trucks for monitoring the signals going to and returning from the network operations center during live remotes.
Regardless of the size of your broadcast or production facility, the ability to properly monitor your audio program is necessary. Without proper audio monitoring, passing on the signal from the network or server is analogous to only having a black-and-white monitor for your program output video monitor.
As more and more consumers buy DTV systems and begin watching your DTV service with 5.1 surround sound systems, viewer complaints will decrease due to fewer transmission problems. Having the ability to monitor and fix audio problems before they are transmitted to your viewing audience will lead to happier customers.
Jim Hilson is a senior broadcast audio specialist at Dolby Laboratories.