On-air Telephone Interfaces

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VoIP is the future

The final option for telephone interface isn't delivered by a phone line at all. The concept of VoIP (Voice over IP) is gaining popularity quickly, and in some instances broadcasters aren't having much choice in the conversion.

The generic term VoIP can have two meanings. First, it can refer to traditional analog lines brought to a station's demarcation point, then converted to IP telephony for use over a PBX system. In this instance there usually isn't much benefit for the broadcaster to make this conversion ‗ it's better to grab the broadcast lines at the demarc and route them to the studio interface equipment in the traditional way. This isn't always possible in an environment like a college campus, where an entire Centrex-style system is likely to be replaced by a VoIP network, and access to legacy lines is unreasonable.

Telos 2101

The Telos 2101 can access BRI-ISDN and PRI-ISDN lines for on-air use.

The second meaning of VoIP is direct access to a VoIP service provider over an Internet trunk. Services such as Vonage and Broad Voice can offer significant savings over traditional telcos, offering full-featured phone service at less than $10/month in some cases, especially if the lines are used primarily for incoming calls.

The good news is that after years of incompatible VoIP standards, both types of VoIP circuits are converging to follow the SIP protocol, which removes most incompatibilities between hardware and providers. This means that whether you're programming your SIP-compatible device as a PBX extension or a direct interface to your provider, setup is the same.

Challenges with VoIP

While it has cost advantages, VoIP interface is not always without peril. If you are subscribing to a low-cost provider via DSL or cable modem Internet connection, that link will be provided without QoS (quality of service), meaning that other Internet traffic on the local LAN or within the Internet at large can cause the transmission to drop out. This risk is usually removed within a PBX environment with traffic shaping, as well as when subscribing to the VoIP service provided by your DSL or cable provider (often at higher prices).

VoIP phone lines usually offer a choice of audio coding. For on-air lines, it's best to stick with G.711, which is the same mildly-compressed audio codec used with traditional telephony. But that algorithm requires 64kb/s plus IP overhead, and multiple channels can fill an Internet access trunk pretty quickly. The other most popular choice for audio coding is the G.729 family of algorithms, which reduces the data stream by a factor of eight. While you may notice a slight quality reduction with G.729, this codec usually isn't the limiting factor on a poor phone call, given the horrible quality of most cell phones. And the significant bandwidth savings can provide for higher stability on contended Internet connections.

VoIP integration bridging the gap

VoIP lines can be converted to traditional analog POTS lines using inexpensive ATA (analog terminal adapter) hardware bridged into traditional broadcast hybrids. But as is the case with T1 and ISDN, you are adding analog-digital and four-two wire conversions that must be reversed in the hybrid. An additional risk present in these conversions ‗ one that isn't as relevant in traditional telephony ‗ is that the delay associated with VoIP can allow a significant and irritating echo to return to the caller if the ATA is not doing a good job of echo cancellation.

Fig. 4 shows a block diagram of the Comrex DH42, to be introduced at the 2008 NAB Show. The product allows interworking between up to two POTS lines and two VoIP lines in a studio environment. The system maintains send-receive separation on its VoIP lines, and acts as a digital hybrid on its POTS lines. Other than that, it treats all these lines equally, providing AGC and filtering to maintain call quality. All four lines may selectively be put on-air, conferenced or routed to a downstream device like a PBX. Using such a system, broadcasters can reap the benefits of VoIP technology while keeping a foothold in familiar POTS territory.

Preparing for the future

Given the onslaught of changes in technology and the need to do more, cheaper and faster, it's likely that POTS and ISDN lines will cease to become cost effective or easy to get from the telcos. Long-term, VoIP technology is poised to become the only game in town for voice telecommunications. In fact, we have had a significant increase in requests for a professional digital hybrid audio interface for SIP-based VoIP connections, which would seem to validate this observation. But being able to get your callers on the air is still the bottom line ‗ regardless of the circuit. Giving broadcasters the tools to make that happen ensures that technology won't hinder that ultimate goal.

Hartnett is technical director of Comrex, Devens, MA.

Resource Guide

Manufacturers of hybrids, call screeners and other on-air telephone products


AVT Audio Video Technologies
+49 91152710

Broadcast Bionics
+44 1444 473999

Broadcast Tools



Condron Broadcast Engineering

JK Audio

KLZ Innovations

Radio Systems


Telos Systems

+49 2173-967 30

+31 15 262 5955

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