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AoIP Evolution With GatesAir at Washington’s WAMU

Intraplex IP Link codecs and Dynamic Stream Splicing technology solidify signal chain

WASHINGTON — The American University-owned WAMU(FM) has been the primary NPR member station for the Washington area since the birth of NPR, broadcasting a mix of a national content, hourly NPR news and locally-produced public affairs programs.

In recent years, WAMU has transitioned almost exclusively to IP for audio production, contribution, and distribution. This includes our AoIP studio plant, studio-to-transmitter links, and backup links to NPR’s Network Operations Centers. We rely heavily on GatesAir’s Intraplex IP Link codecs to ensure uninterrupted audio delivery.

Intraplex IP Link codecs provide audio connectivity to all of our transmitter facilities. Our IP Link codecs are the primary STL paths to our auxiliary transmitter site, collocated with WETA(FM) in Arlington, Va., as well as to our satellite station WRAU(FM) located in Ocean City, Md. Both sites are fed via diverse public internet links. Dropped packets are inevitable even with the most stable internet connections, but Intraplex IP Link codecs have a feature called Dynamic Stream Splicing, which virtually eliminates audio dropouts.

Dynamic Stream Splicing works by combining packets from multiple streams to reconstruct any missed audio frames. This can be done over separate internet connections, or even a single internet connection by delaying one stream by 50-100 ms and adding forward error correction (another built-in feature of the Intraplex IP Link codecs). We also use Dynamic Stream Splicing on the return path from WRAU so we know that what we hear at the station is what listeners hear over the air 150 miles away. We’ve been blown away at how well this feature works, and it’s extremely rare that the Intraplex IP Link cannot deliver flawless audio.

Another great feature of the Intraplex IP Link is the ability to set up multiple failover audio sources. These can be additional RTP streams (from a different source Intraplex IP Link device), AAC or MP3 web streams from a CDN, or even a USB Drive preloaded with audio. This flexibility enables us to deliver continuous audio to listeners even in the event of audio or total primary stream loss.

Audio from our Intraplex IP codecs is delivered to AES switching networks and ultimately the audio processors located at each transmitter site. For our main site, which maintains a native Livewire path as its primary connection, the Intraplex IP provides a redundant backup via diverse links in case of interruption to the leased fiber link that conveys our Livewire connection. It’s a very robust setup.

We also convey our nationally distributed program, “1A,” to NPR’s primary NOC in Washington and backup NOC in Saint Paul, Minn. While we still rely on legacy T1 technology to get us across town to NPR, we are working to transition to a new standard that would utilize the Intraplex IP Link over the public internet at 256 kbps AAC to reach both the primary and backup NPR NOCs, which makes the Dynamic Stream Splicing application that much more valuable. At each NOC, IP Link codecs receive the signal and provide the same stream reconstruction service as we use at our transmitter sites.

This is a fairly straightforward operation, though once again the reliability and stability of the IP Link has proven exceptionally valuable. If the main NOC in Washington were to go down, the backup NOC in Minnesota picks up the stream to ensure an uninterrupted national broadcast. Once decoded, the signals are then re-encoded in IP Link devices for national satellite delivery.

Our most recent use of the IP Link codecs involves reimagining our main program channel streaming service. We’ve begun experimenting with the IP Link hardware to eventually replace Microsoft Windows-based stream encoders to deliver the audio to StreamGuys, our CDN and streaming media service provider. Like many stations, we were often faced with the typical performance glitches that come with computer misbehavior.

We believe that moving to dedicated hardware for audio encoding with the IP Link 100 will eliminate the shortcomings of a computer-based encoder. We plan to utilize the IP Links in a true redundant configuration so that our web stream remains on the air if either of our wide-area network connections is lost, or if we need to perform updates on the Intraplex IP Link hardware.

A common benefit across all of these applications is ease of setup and use. GatesAir has made the process very simple to understand, and easy to replicate across any application. It is consistently the same process regardless of the network characteristics or type of connection.

As T1 and ISDN become more difficult to maintain and IP solutions continue to evolve, there is no question that broadcasters should aggressively complete the transition to IP. Having redundant connections will provide excellent protection, while advanced error correction and mitigation techniques in products like IP Link will ensure the best possible quality and performance for any audio contribution and distribution needs.

For information, contact Keith Adams at GatesAir in Ohio at 1-513-459-3447 or visit

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