Tieline Technologies

Tieline G3 Offers Remote Audio Over the Internet

Mike RabeyGetting remote broadcast audio from the remote site to the studio usually employs one of 3 well-known routes. RPU transmitters can provide good quality audio ... or wretched audio, depending on the terrain and the level of RF pollution in your market. POTS codecs are handy, if somewhat fidelity-challenged. ISDN links sound great, if you can afford the line charges and have two weeks lead time.

So I was intrigued by the opportunity to test drive the new Tieline Audio Over IP option for its G3 codec. Now that high-speed internet is available at most businesses, the concept of achieving a high-quality stereo audio path from a remote site, over the internet, has become feasible. 128 kBps stereo audio, for free, is an attractive idea. The Tieline G3 already has the RG45 connector and associated circuitry in place, so only the software needs to be updated to implement this new capability.

Obviously, this type of service requires high-speed internet service - dial-up installations need not apply. Also, at least one of the ends needs to have a public IP address. (This end will almost always be at the studio.) This will require a dedicated static IP address, not the dynamic address used by most broadband providers. This is where your IT specialist or ISP provider will have to get involved. Here at Entercom Indianapolis, IT manager Gill Rudolph performed a NAT (network address translation) on an address on our local network, creating an address the Tieline could call home.

After the trial units were dropped off by Tieline GM Kevin Webb, the first order of business was to set them up on the bench for testing. The two codecs were connected back-to-back with a crossover cable, and stereo audio was fed into one unit. The output from the other unit was monitored with a high-quality amp and speakers. After first connecting to each other, the units started off at a 9.6 kBps data rate, which sounded marginal. However, as I manually stepped the data rate upward, audio quality quickly improved. At 24 kBps, the Tielines switched into stereo mode, and I continued to increment the data rate up until it reached 128 kBps. At this point, the audio quality was truly remarkable. Careful listening was required to differentiate the output from the source material. Tieline offers the choice of proprietary algorithms: "voice" for use at low data rates, and "music" for better quality audio at higher rates.

Now I tried connecting over the office network, with one unit in my office and the other in a production studio. The units connected instantly, and audio quality remained excellent. Next, I tried the NAT settings by connecting from a Comcast cable modem in the building, to the static address that Gill had set up on the office network. Umm .. it wouldn't connect. Changed some settings ... still didn't work. At this point I gladly left the situation in the hands of the IT guy, and after about an hour of experimenting, Gill had found a combination of IP address and port setting which worked.

Tieline offers the choice of TCP (Transfer Control Protocol) or UDP (Universal Datagram Protocol). Everybody is familiar with TCP/IP from transferring files over the web. TCP/IP provides robust data transfer via a system of handshaking and double-checking of each data packet. This is great for data integrity, but the overhead and bandwidth requirements slow down audio transfer substantially. UDP/IP is more like a "mass mailing" protocol: there's no handshaking, no data checking, the sending end just spews out the bits, and hopes the other end gets them all. It may sound slap-dash, but for rapid audio transfer it's actually quite reliable, especially with the FEC (forward error correction) which Tieline builds into the software. Unfortunately, our firewall wouldn't pass UDP, so we used TCP for all testing. It worked quite well, notwithstanding a fairly noticeable delay from end to end.

It was time to put the G3 system to the test ... a live remote broadcast of WZPL's "Smiley Morning Show" from a local comedy club. Dave Smiley and his zoo chose to air a live reading of "Rudolph the Red-Nosed Reindeer" (the play, not the song) on the Friday morning before Christmas day. The manager of the club informed me that they had SBC DSL internet service, and invited me to test the connection 2 days before the broadcast date. I went to the site, plugged the Tieline into their ethernet switch, and it promptly connected to the studio unit. I listened to the backfeed in flawless stereo. Good deal, I thought, now the worst part will be getting up at 4 am Friday morning.

Friday morning, I arrived at the site at 5 am. The morning show producer was busy setting up six mics into the Mackie mixer, and feeding the mixer output into the house wiring to the sound booth. In the booth, I fired up the Tieline and connected to the studio. I connected the return audio to the house PA, so everybody could hear the audio coming from the studio. Then I called the studio operator, and had him listen in cue while the producer tested the mics. "It sounds weird, man. It's all phasy and low level." Crap! The house wiring to the booth must be out of phase! Luckily I had my tool kit with soldering iron. In a flash, I reversed the wires in one of the XLR's going to the Tieline. Problem solved!

Finally, it was showtime. I had set the pan-pots on the mixer to spread the mics across the stereo soundstage. This gave the live reading an "open" sound that was quite pleasant. The producer went to his car to listen to the air signal, and came back to the sound booth grinning from ear to ear. "That sounds great! It's the best sounding remote I've ever heard!" Surprisingly, a large crowd had assembled at the comedy club at 6 am to watch the show, and their laughter and applause at the antics of the morning zoo made the broadcast even more enjoyable to listen to. The Tieline dumped the connection once ... I think it may have been when the office manager arrived at work and connected her computer to the internet. Luckily, it happened during a commercial break, and I was able to reconnect before the remote segment resumed. All in all, the remote was a spectacular success, in no small part thanks to the flawless sound which the Tieline G3 provided over the public network.

Mike Rabey
Chief Engineer
Entercom Indianapolis
mrabey@entercom.com