Sssshhh . . . the sound of public radio is about to get a little quieter. But if all goes according to a plan unveiled last month by the Public Radio Satellite Service, listeners won’t notice the change in audio levels for programs distributed to stations around the country.
On Feb. 26, PRSS plans to remove a 6 decibel (dB) boost that was added to its uplinked audio levels in 2013. The higher levels had been adopted to adjust for differences between PRSS technical standards and those of the European Broadcast Union (EBU), which were built into IDC SFX 4104 Pro satellite receivers adopted by PRSS during its last technical upgrade.
“The boost was done when the new receivers were rolled out,” said Steve Densmore, PRSS director of broadcast operations. The combination of the Canadian-built receivers and NPR’s move into new headquarters created problems for some stations on the receiving end of PRSS’s services.
The SFX 4104 Pro receivers have three outputs: traditional analog, an Audio Engineering Society–standard digital and a separate digital output that can connect to the Livewire digital audio routing standard pioneered by equipment manufacturer Axia. Because the SFX 4104 Pro receivers were built to the EBU standards, their analog outputs operate at a different level from the SMPTE (Society of Motion Picture and Television Engineers) standard used at PRSS headquarters.
“If you put a –20 dB tone [into the PRSS system] and measure it coming through the satellite link and out the receiver, that –20 will be –20 out of the Axia Livewire port and also –20 coming out of the AES port,” Densmore said. But for those stations using the analog port, the output was –2 (dBM). The -2 level was below the standard “house” audio level used at many stations, so the incoming PRSS audio had to be amplified before going to live control rooms and automation systems.
“Stations using the analog port wanted something done about why that level was so low, so the solution was to boost the level 6 dB [at the transmission end],” he said.
That solution took the analog output from –2 dBM to +4 dBM, a more comfortable level for stations still depending on an analog audio source. Because the Livewire port on the SFX 4104 Pro receivers has an adjustable output level, users were able to change a setting so it continued to output at –20 decibels below full scale (dBFS).
But the solution created problems for other PRSS clients, the majority of whom use the AES digital output, which doesn’t have an adjustable output level. By boosting the input level 6 dB, “what we’ve done is take away 6 dB of headroom on the AES port,” Densmore said.
Unlike analog signals, which “clip” above their maximum levels by distorting relatively gracefully, digital signals become unlistenable if pushed above “full scale.” The extra 6 dB of headroom gives stations and producers more leeway to produce content with wide dynamic range without clipping, which is why PRSS is now eager to remove the boost.
“What we want to do is get things back to where we were,” Densmore said.
“Ultimately what we want is minimal disruption to stations,” said Erich Shea, PRSS manager of planning and communications. That’s of particular concern because the level change comes right in the midst of a larger overall PRSS push to implement new loudness standards for program producers.
Those standards, announced to stations by email Dec. 19 and outlined in additional detail on the PRSS website, will bring PRSS in line with other distributors, including PBS.
To help public radio producers prepare for the change, PRSS is producing a webinar Jan. 28, Shea said. Now that the standard has been announced, “the next step is that we’re asking people to start producing to that standard, and then we’ll start monitoring and advising content producers about how they’re meeting that standard.”
“In the long run,” he said, “it’s going to lead to a better listener experience.”
In the immediate term, however, PRSS must first help hundreds of station engineering teams implement the level change in February. It will be a simple process for many: Stations using the Livewire port can simply adjust its output level within the PRSS receiver. But those using the AES digital or analog outputs have to tweak levels elsewhere in their distribution systems.
The change requires extra legwork for engineers at some smaller stations and statewide networks, such as the CoastAlaska group, which includes seven stations at five locations scattered across hundreds of miles of rural Alaska. Engineering chief Rich Parker said he can’t get to every station right away to make the adjustment.
“I have three stations here [in Juneau] that will be fairly easy, but the other four are a couple hundred miles away, and there’s airfare and scheduled overnights to consider,” he said.
PRSS is assisting Parker and other engineers to help smooth the transition. The process may include advance testing by sending tones at both the old and new levels, and possibly dual feeds of key NPR newsmagazines for several days after the level change.
“At the end of the day,” Parker said of some of his far-flung stations, “they’ll just have to push the faders up as hot as they can, and we’ll have to manually tweak the distribution amplifiers later, either on site or remotely.”
“It’s not going to be the end of the world. We’ll do as much as we can ahead of time.”
Very interesting, indeed.