Designing Control Rooms for ATSC Audio

When we last looked at ATSC A/85 we were primarily concerned with the key issue addressed by the document: loudness management.
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When we last looked at ATSC A/85 we were primarily concerned with the key issue addressed by the document: loudness management. A/85’s official title, “ATSC Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital Television,” is a partial red herring because, while it obviously details processes for managing loudness, it also includes recommendations aimed at standardizing the control room environment, and creating consistency in how audio for television is created in the first place. Since the document is relatively long—72 pages for the 2013 revision—many people may not realize that it covers other factors that need to be considered when producing audio content for television.

Anyone familiar with cinema audio production will feel right at home with the control room recommendations in A/85 because it specifies that all mix rooms should be as similar as possible in their monitor setup, sound pressure levels and frequency response across the audible audio spectrum. Whereas cinema mix rooms are typically set to monitor at 85 dB, A/85 specifies different target sound pressure levels (SPL) for television mix rooms depending on room type and volume.

Because television audio is often mixed in some less than ideal environments, rooms are categorized and adjustments made to ensure that consistency is maintained despite the situation the mix engineer finds himself stuck in. Audio control rooms are split into five categories, from high-end control rooms all the way down to environments where the mix is being done through headphones.

The higher-end, higher-volume rooms have a recommended SPL at the mix position of 85 dB, while the lower-end rooms are calibrated to 76 dB, and headphone mixes are to be monitored at 74 dB SPL.

Higher SPL is necessary in larger rooms because there is more space to fill, and allowable because problematic boundaries are farther away from the sound source. SPL must go down in smaller rooms since there is less space to fill and because boundaries are closer to the sound source so room modes are easier to excite.

The one thing truly new for television mixers in this recommendation is that, just as in cinema, the level control should be calibrated, then left alone for the duration of the mix. Most television mixers, including me, are accustomed to grabbing the monitor level control whenever we need to. How this plays out in real life will depend greatly on the mix engineer and whether they are comfortable leaving that level control alone. It may be helpful to remember that the goal of all this is to create consistency when producing content in any given room, regardless of size.

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ATSC A/85 room categories and SPL recommendations (*see ATSC A/85:2013 for additional clarification) Achieving consistency in mixing and monitoring rooms requires proper speaker placement, as well as handling modes and other acoustic problems inside the room. A/85 addresses these issues in Annexes B and C, covering low-frequency modes, high-frequency absorption, minimizing reflections, monitor placement and monitor choice. It recommends a practically flat response curve for all rooms and suggests equalizing the system if necessary to achieve it.

However, I have found that it is better to deal with acoustic anomalies in the room before equalizing the system because modes and cancellations will continue to cause problems in the room until they are dealt with even if the system is equalized flat at the mix position.

As for which loudspeakers should be used for monitoring, the only real recommendation is that they should be “perfectly flat,” something that may prove difficult to achieve, but is certainly worth attempting. Even a perfectly flat monitor will not reproduce sound with a perfectly flat frequency response in a deficient room. A more realistic goal would be to install the best professional-level monitor with the flattest response available within the budget, then install these monitors throughout the facility.

Placement of monitors is critical and the document mentions that care should be taken when positioning to minimize reflections from console surfaces, side and rear walls, and any other reflective surface. Annex B closes with references to two documents that may be helpful to anyone requiring additional assistance with monitor placement, and additional information regarding acoustics in audio control rooms can be found in my earlier three-part series on that topic in this column.

Finally, I’d like to relay an update on my experience using loudness meters in combination with legacy meters. Even before the CALM Act became law my team and I were installing loudness meters in our live and audio post rooms, but we experienced low adoption of the devices because some mix engineers didn’t quite understand why they should change how they measured despite our instructions about the upcoming changes and imminent legislation. It appeared that it might be necessary to remove all meters that could not measure in LKFS from the mix rooms to force mixers to start learning how to use the LKFS meters. However, after spending some time behind the desk it has become apparent that having legacy meters in the control room gives the mix engineer a quick reference they’re familiar with that helps them get the initial mix off the ground, from which point they are able to mix by ear with only periodic glances at the loudness meter or any of the others.

It does seem a bit odd that this recommended practice, which is at the heart of the CALM Act, has given us a new meter and two new numbers (LKFS and True Peak) that we must pay attention to, yet also instructs us that the best method to make all of this work properly is to ignore the meter and those new numbers the majority of the time and just trust our ears. Fortunately this works very well in practice, but can take some time to grow accustomed to. Having a controlled, consistent environment to mix television content in is an idea that is long overdue and this, along with proper monitoring tools, is the linchpin that makes this entire process work.

Jay Yeary is an audio engineer and consultant. He can be reached through TV Technology or via Twitter at @audiojay.