CHICAGO — The AudioTX STL-IP Plus is a novel twist on IP program delivery. As engineers, we all recognize that the connection from studio to transmitter is probably where our programming is most vulnerable. Usually it involves one or more outside connectivity providers. Failures often have us waiting on hold, opening trouble tickets and other time-sucking activities that we can ill afford. The answer up to now has been redundant delivery schemes — perhaps keeping that old equalized telco circuit as backup for the T1.
But with T1 service facing sunset, what to do? We all know that IP delivery schemes suffer from dropouts. The promise of 0.99999 (five nines) reliability should mean arithmetically less than a second of outage every week. But when the data passes through a hundred devices along the way, that percentage is degraded further. Sometimes I think that ISPs just make up their statistics anyway. As evidence, where I’ve used simple codecs with low latency like the Barix and equivalents, short delivery interruptions occur pretty regularly. This just isn’t acceptable where I work.
So I started looking around for something better. Intuitively, I knew I was looking for a product that supported multiple IP streams and that could switch between them seamlessly. The odds of both streams being interrupted simultaneously seemed low. Any such solution would have to time-align the received packets, since data packets traveling different IP routes might get there at different times. This dictated that a buffer be employed on the receive end, allowing time for the receive codec to recognize a dropout and act to fix it. There are now several products that do just that, supporting two identical streams that the user can direct over separate paths. These devices provide separate RJ-45 jacks on the backplane, further increasing redundancy.
The problem with this “Version 1” architecture is that it fails to acknowledge that different IP paths have differing throughput capabilities. Maybe you have a DSL and a cable modem circuit at the transmitter. Or maybe it’s a 54 Mbps Nanobeam and a DSL. Whatever you have, it’s reasonable to assume the throughput and reliability will be different for different methods. Being locked into one coding algorithm and one data rate for both routes means you must choose the worst performer for all.
The Audio TX STL-IP Plus recognizes this weakness. Lime Broadcast, the manufacturer, started over in how audio delivery is handled. They designed what they call SureFlow 5. Instead of two identical streams, it allows as many as five — but the truly revolutionary feature is that users can choose the coding algorithm and bitrate for each stream independently and assign the order of preference for use in the audio output. In addition, the IP stack in the device allows the user to route outbound data to separate upstream gateways for each stream. The devices are, of course, full-duplex.
In Chicago I have a 100 Mbps duplex 11 GHz link between studio and transmitter and a Comcast cable modem at the transmitter site, along with 6 Mbps office-type MPLS at the studio. I am running four simultaneous, fully-redundant streams from studio to transmitter.
First up is a 48 kHz linear PCM at about 1.5 Mbps, just like the old Intraplex this system replaced. Next is a 320 kbps MP2, also on the 11 GHz link. Yep, two streams on the same medium. Why? Because the statistical probability of both streams suffering a dropped packet at the same time is pretty low; in fact, this adds more “timing-diversity” than you might imagine as the different streams have differing packet sizes and encoding latencies too. Streams three and four exit the studio via our ISP. One is a 320 kbps MP2 and the other is a 64 kbps MP2, again sharing the same outbound route and arriving via the Comcast cable modem. This plays the odds of packet drop in the Comcast world.
All this is accomplished by embedding metadata in the streams with time sync marks that the receive AudioTX STL-IP Plus uses to time-align the received audio. All four streams are decoded to AES and any required substitution happens only then, seamlessly, in the order of preference established during setup.
The device has a bunch of other features, like setting packet size for each stream, logging, email alerts, contact closures and all the other things you’d expect. But this strategy of allowing simultaneous delivery across multiple paths with data rate and packet size chosen to match the path characteristics has made the performance bulletproof.
I was a little surprised that this design wasn’t protected by a patent. From what I know, it’s probably too late now. So look for other makers to copy the idea eventually, though the DSP-based designs now popular lack the computing power needed, or this function would probably be offered as a firmware upgrade. So for the foreseeable future, if you are looking for a solution that addresses the real world statistical probabilities of IP delivery, this is your answer.
For information, contact Mo Dutta at Lime Broadcast Group in England at 011-44-121-256-0200 or visit www.stl-ip.com.