Surround monitoring
Dec 1, 2009 12:00 PM, By Howard Mullinack
Basic changes can improve surround-sound monitoring in the broadcast control room.
“Late Night with Jimmy Fallon” uses nearfield monitors with sound absorption at the bottom of each speaker to minimize reflections off the console.
When preparing to originate programming in surround sound, there are some fundamental guidelines for loudspeaker selection and placement, equipment for monitoring loudness and mixing techniques.
For the local TV station, it's been a long tortuous transition from analog video and BTSC stereo to transmitting ATSC digital HD video and surround sound. HD video is far more revealing of details and imperfections. The same holds true for digital surround-sound audio — the 3in speaker is no longer the norm in the home TV set. According to the CEA, more than 20 percent of viewers are listening to the audio on high-quality surround systems, with this percentage growing each year.
In the analog world of BTSC, we monitored and mixed for the limited dynamic range required for the vast majority of viewers and for analog audio carrier's technical limitations of 25kHz deviation and 75µs pre-emphasis. BSTC digital audio offers a dynamic range of more than 100dB and with it the opportunity for creating great sounding mixes and the challenge of loudness deviations that exceed the comfort range of the viewers. Loudness complaints are on the rise; legislation is pending in Congress to mandate controls and penalties (H.R. 1084, The Commercial Advertisement Loudness Mitigation Act).
These opportunities and challenges have been under study by the ATSC. Last month, the ATSC released the “ATSC Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital Television (A/85).” The document covers the ITU-R BS.1770 loudness measurement, target loudness and dynamic range management, metadata management, audio monitoring setup, and two essential “Quick References Guides,” one for station engineers and management, and the other for audio mixers and editors. An upcoming issue of Broadcast Engineering will feature a detailed report on A/85.
Speakers and speaker placement
The typical broadcast audio mixing environment is often far from ideal. There are, however, a few basics than can easily be followed to assure that what you hear is representative of what your viewing audience will hear on the typical high-quality system.
This “Saturday Night Live” mix studio uses sound absorption on the rear wall to minimize reflections.
Speaker selection is critical. In a panel discussion at the Audio Engineering Society convention in New York in October, Sam Berkow, principal consultant for SIA Acoustics, offered valuable advice: “For effective monitoring, the LCR speakers should be identical, with the surrounds tonally similar to the LCR. We found that monitors that worked for stereo mixes are not always acceptable in surround. This is because the off-axis energy of the L and R speakers is absorbed by acoustical treatment, but for the C speaker, treatment is less effective because of the glass window. Finding monitors that are well behaved in their off-axis response can really affect the way that they are heard in a real working environment. We also found through testing that better off-axis response results in better subjective preference and less listener fatigue.”
The front LCR speakers should be positioned equidistant from the sound mixer's central position. (See Figure 1.) A time delay resulting from different path lengths produces a comb filter effect at the mixer's ear. If necessary, delay can be used to compensate for less-than-ideal placement. If the surround speakers are closer to the mixer than the front speakers, they too should be delayed.
The LFE subwoofer adds its own challenges. Berkow found that “rooms that worked well for standard stereo mixing may have all sorts of problems when you introduce subwoofers. The position of the subwoofer can be very important in how it interacts with the acoustics of the room and the LCR loudspeakers. Some basic acoustical measurement can be very helpful when positioning subs, and there really are very cost-effective ways to resolve low-end problems in many rooms.”
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