(click thumbnail)A frame of RDL silence sensors used to monitor the actual off-air signal. Photo by Cris Alexander. When a station moves into the HD Radio world, its engineers have to change their thinking about a lot of things. One important consideration is the 8.4-second audio diversity delay that will be added to the analog audio path.
What listeners hear on their radios — and what will be heard on the studio off-air monitor unless remedial steps are taken — will be 8.4 seconds behind real time. That makes real-time off-air monitoring impossible.
Some stations, even big-market FM facilities, simply leave the diversity delay off, figuring that until a certain percentage of listeners are listening to the HD signal, it doesn’t matter. That momentarily solves the monitoring problem, but it also creates a big problem for the listeners who do have HD Radio receivers.
Fringe-area HD Radio listeners — or core listening area listeners if there are multipath and other signal anomalies within the coverage area — will find the station unlistenable as the “blend to analog” feature of the radio works back and forth between the delayed digital and undelayed analog signals. That’s a big tune-out factor and will not help sell IBOC receivers, so it’s advisable to get the diversity delay set right from day one, and that means finding a way to monitor in the studio.
Stations that operate profanity delays full-time have likely addressed the issue already and developed ways to handle the studio monitor issue. Others will have to get creative.
There are two important considerations: providing on-air talent with a processed “air monitor” feed and providing some sort of confidence monitoring.
Some engineers have taken an old audio processor, run a program DA output through it and connected it to the studio headphone feed, switching between a “live” (i.e. “delayed”) air monitor feed and the pseudo-air monitor feed from the processor with the monitor mute relay. That works, but it can be confusing to talent when 8.4 seconds “disappear” when they turn the studio mic on.
Perhaps a better option is to provide those two feeds — the live, delayed off-air feed and the pseudo-air monitor feed from the processor on the EXT1 and EXT2 monitor inputs, respectively. The normal monitor mode would be the pseudo-air monitor feed.
A silence sense unit, available inexpensively from RDL or others, can then be used to monitor the real off-air signal, operating a highly-visible warning light in the studio when more than five seconds of silence is detected in the live, off-air monitor. When the warning light illuminates, the operator can then punch over to EXT1 and check the live monitor and take remedial action if necessary.
The live monitor should also be used to feed office speakers in the lobby, restrooms and common areas so the station staff can keep an ear on what’s really going out over the air.
Some provision for monitoring the HD Radio signal should also be provided, either by means of another external studio monitor input, if available, or a muted HD Radio receiver in the studio. It’s a good idea to connect a true HD Radio off-air feed to a separate silence sense detector and warning light to alert operators and engineers that there is a problem with the IBOC audio.