Field Report: Audio TX STL-IP

May 1, 2005

With telephone companies less willing to support dedicated equalized broadcast lines, and in some areas reluctant to setup new ISDaN service, the challenges for getting audio from "here to there" are becoming greater. At the same time, Internet network connections seem to be springing up everywhere--and available at attractive prices. As a statewide network, Vermont Public Radio produces live news forums and msucial events in various areas of the state; getting an ISDN line setup quickly and economically is not always possible, but at colleges or business locations there is almost always some kind of broadband IP service readily available.

The STL-IP from Audio TX is a 1RU device that provides a broadcast-quality audio link over an IP network, with low delay and quality comparable to dedicated copper lines. The unit has rear-panel XLR balanced stereo audio I/O as well as AES/EBU digital I/O, an additional port for external word clock input, and a standard Ethernet port for a 10/100 Ethernet connection to a network. A built-in Web administration tool allows configuration of the unit from a standard Web browser. Front-panel LED indicators show power and transmit and receive status.

There are also two additional ports--one for ancillary RS-232 data and one for GPIO (TTL) with four I/O and four status outputs/ control signals. Serial data can be transported in sync with the audio at speeds between 1,200b/s and 57,600b/s. The manual provides clear documentation for using these various ports for communication and control.

Get connected

The unit is shipped with a default access address of The setup computer's IP address must be changed to that subnet, and a cross-over cable or a hub connects them both. Opening a Web browser to that IP address will open the initial configuration screen. It's fairly self-explanatory from there; the manual is quite good in that respect. When configuring two units, the IP address has to be switched back and forth on the setup computer several times, which is a relatively minor inconvenience. If the units are used in various locations this can become a bit time consuming. There is a handy utility that sends a broadcast message to the unit and displays its current IP number, in case the last setup is forgotten. My one complaint was that the interface worked well with Internet Explorer but was not as happy with Firefox, my preferred browser.

One aspect that might be important to broadcast engineers is that there really isn't much that is user-serviceable about the STL-IP. The manual is clear that “the unit should not be opened or serviced except by appropriately qualified personnel.” Indeed, a quick peek under the hood reveals an embedded system board with flash memory storage. This is coupled to what appears to be a high-end audio interface and the appropriate output connections. The company rep was helpful at all points, including saying that for a custom installation there were certain customizations that could be done for a particular application; including my request for a pure software client to monitor the unit from a PC. I got the impression that this was not an entirely welcome request in general as the company's focus is on the hardware box; and in fairness, Audio TX makes another product called Communicator, which is software-only and designed to be run on Windows PCs. My feeling is that the STL-IP shares that pedigree to some extent.

Performance at a glance
Low-delay (5ms) transmission
Multiple encoding algorithms
Connection rates from 24kb/s to 4.6MB/s
Automatic link restart after loss
Analog and digital audio I/O
RS-232 for ancillary data
GPI/O for four status/control circuits
Web-based configuration screen
Complete manual stored in the unit
Field-upgradeable firmware

The basic setup adjusts the network IP settings and I/O settings for the digital or analog inputs. The clock rate can be generated internally, and the digital input section provides the ability to lock to the clock on the input. It is important that the clock frequency is set to match the transmit/receive connections for best performance. When I had a mismatch between sample rate and transmit rate I observed occasional digital ticks in the received audio. These clicks disappeared when I reset the rates to match.

For UDP and UDP multicast connections, two levels of FEC (forward error correction) can be set--50 or 100--with a directly corresponding increase in occupied bandwidth for the signals. For all connections there is also a configurable receive side jitter buffer, which can be set in 1ms increments up to five seconds to smooth out a troublesome network connection if needed.

To many from one

Each unit can send one audio source, or a choice of separate audio from left stroke right inputs, to six dedicated far-end units using the designated TX-0 through TX-5 configuration fields. Each unit also has one RX setup field for receiving audio from one other TX unit. Both RX and TX modules can be setup for outgoing or incoming TCP, UDP or UDP multicast connections by assigning a different port number to each TX field. This was a bit confusing to me at first, but it soon became obvious--the audio transmit side takes the source audio and sends it to the network interface — then ports TX-0 through TX-5 can be configured to either initiate an outbound TCP or UDP connection or to accept incoming connections from another STL-IP unit; this unit can itself be set to receive audio by accepting an outside IP connection or by initiating one. Setting it up for UDP multicast allows audio to be sent to an infinite number of far-end units on a multicast-capable network.

On a dedicated LAN in our station I sent full bandwidth linear audio from a 44.1kHz digital source to a unit in a different part of the building with almost no detectable delay. Sending audio around a campus or site with a dedicated network would certainly be an acceptable way to avoid running lots of audio cable--particularly in areas where a good network infrastructure already exists--and there would be no stranded investment in cabling if either end needed to be relocated.

The rear panel is loaded with connection for audio, control, word clock and the IP connection.

Setting the various audio parameters is done from the same Web interface. Choose anything from 24-bit, 96kHz PCM linear stereo audio using 4.096Mb/s of bandwidth, down to 16-bit, 22kHz Layer 3 mono at 24kb/s bandwidth with nearly everything in between--MPEG L2, L3, J.41 and ADPCM at various combinations of sample rates in mono (L, R or L+R), dual mono, joint stereo (L2/L3) and stereo.

With the cooperation of a wireless ISP who shares our building we setup a test over a Wi-fi connection. One STL-IP was placed at a concert hall several blocks away and connected via a DSSS wireless link back to another STL-IP at the studio. We sustained a full 44.1kHz PCM linear signal at 1.504Mb/s. This was the limit of our link because of power and antenna configurations, which could be easily improved. We then setup a full-duplex link at 44.1kHz MPEG L2 Joint Stereo at 384kb/s full duplex between the sites and the audio quality was excellent--better than the ISDN link we had previously used for a concert.

Audio TX, a division of MDO UK
+44 121 256 0200
+44 121 256 5109

For one last extreme test I took one of the units home to try on a 256kb/s up, 512kb/s down DSL connection with a Linksys WRT54G wireless router. For the first test there were some nasty dropouts on the connection. I upgraded my router to the third-party HyperWRT firmware and used the added traffic shaping feature to give priority to the STL-IP connection on one of the hardware ports and installed the latest upgrade to the STL-IP software. All of the packets had to traverse at least 21 hops from my house to the studio yet I was able to establish a link back from the station for a 44.1kHz, L2, 384kb/s joint stereo signal with only rare drop-outs. Because of the number of hops I then set the receive jitter buffer to 100ms to smooth the connection and I left it running for most of a weekend with no perceptible loss of audio--a pleasant surprise because this is the same data rate we use over a dedicated T-1 STL to our transmitters now.

You will have to evaluate your particular network link to determine if it is robust enough to sustain a full-time connection when using these units for an STL, but on dedicated networks there would be no problem shipping high-quality, low-delay audio. Given the wide variety of protocols, sample rates and bandwidth settings, it should be possible to find just the right combination that will work reliably in just about any situation, particularly for emergency backup, even over the Internet. With the falling price and increasing availability of high-speed IP links, this type of system appears to be the wave of the future for getting sound from here to there — a welcome addition to the engineer's bag of tricks.

Parker is director of engineering for Vermont Public Radio.

Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.

It is the responsibility of Radio magazine to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by Radio magazine.

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