There are a couple of good reasons why radio stations execute remote broadcasts. First, it's a great way to interact with the local audience; and second, it can often be a nice revenue source for the station.
As the station engineer, you may be charged with determining the best way to execute remotes for the station. For our purposes, let's assume that you're putting together a brand new system without any legacy equipment. We'll focus primarily on 21st century techniques.
Wired vs. unwired
When setting up a system for remotes decide if with the wired route or the unwired route fits the application. This distinction is clouded somewhat with the wide availability of the public Internet, so there is quite a bit of crossover with some of the equipment that is available.
Talk shows on remote are an ideal way to interact with listeners. Chef Jasper Mirabile (seated) talks with guest Chef Gary Puetz while Dennis Eversoll, CPBE CBNT, checks the setup on KCMO-AM Kansas City.
Generally speaking, the wired route is based on older technology. You could totally rely on the local telephone company to provide wire connections back to the station. In the old days you could order an 8kHz or 15kHz audio circuit. While they often worked well (and they often didn't) they always required a test visit before the day of the remote. The tariffs are usually quite high for this type of circuit, so unless you planned on originating multiple remotes from the same location, this type of wired circuit was not economical.
In the early 1990s, ISDN codecs became available and supplanted the dedicated audio circuits. While an ISDN circuit was often less expensive (not only for installation but also on a monthly basis) it had much in common with the older style of lines: the telephone company required a couple of weeks to install it, and it still needed to be tested ahead of the remote date. To a great extent, the quality of the remote broadcast audio depended on the codec itself and not the quality of the telephone company technician that installed the line. On the other hand the complexity of the ISDN codecs sometimes scuttled the remotes.
The unfortunate reality is that ISDN is now also obsolete; some telephone companies balk when the customer wants to place an ISDN order. The reason is pretty simple: There are better ways to get more data over a single copper pair, which means more money for them. Telephone service providers are inclined to spend their capital dollars on equipment that can handle these new methods, forsaking the older.
So what are you left with if you want to make use of wired circuits for remotes? POTS (though also becoming obsolete in its pure form) is ubiquitous so that is an option, and the Internet is readily available in many places because of DSL technology. Still, there are lots of options for wired connections.
Comrex has many years of experience in building equipment used to broadcast remotes over plain old telephone circuits and not surprisingly its current line includes all the features and functionality that you would expect. The Matrix codec field version has one mic-level input, one mic or line level input, a headphone output and a line-level out as well. The studio version is 1RU and does not have the mixer and headphone features. The Matrix uses an on-board POTS codec that can provide 15kHz of duplex audio response depending on the quality of the POTS connection. However, the user can also add modules to the Matrix such as the Portable ISDN module, the Matrix GSM module or the Matrix Telcell module.
Comrex also offers the Access, which takes advantage of the increasingly diverse set of connection possibilities: POTS, DSL, cable DSL (as well as 802.11x (Wi-fi), 3G data networks, high-speed cellular data networks). It uses the Comrex-developed Broadcast Reliable Internet Codec and will perform at several user-selectable quality levels. HE-AAC and AAC low-delay are also available for use over robust networks.
Tieline offers the I-mix G3, which is a complete remote broadcast package (mixer and headphone amp built-in). The basic unit is a POTS codec, but it has an expansion slot to plug in other types of codec modules. For example, a second POTS codec module can be added, giving the I-mix the capability of bonding two POTS lines together, providing mono, stereo or even dual-mono feeds of as much as 15kHz in audio bandwidth. Alternatively, the expansion slot can be loaded with Tieline's IP software module, allowing the unit to connect to wired LAN. (ISDN and GSM plug-in modules are available for the I-mix 3 as well.)
Musicam has its own codec that will work via IP: the Netstar. This device can send and receive audio, contact closures and ancillary data via TCP/IP, ISDN or dedicated data lines. It contains not only the standard algorithms such as G.711, G.722, MPEG Layer 2 and MPEG Layer 3, but also MPEG AAC and MPEG Layer 4 AAC-low delay. This unit can deliver uncompressed 20kHz audio with near-zero delay if the IP connection supports it.
Audio TX sells a software package called Communicator that allows the user to create a remote session via TCP/IP through physical connections such as a LAN, Wi-fi or DSL. The PC or laptop on which Communicator runs can also be turned in to an ISDN codec. The Windows software includes algorithms for MPEG Layer 2, Layer 3, G.722 and G.711, and can connect with other manufacturers' codecs.
The APT Tokyo is another full-featured, multiple algorithm codec that can be used over a LAN by way of Ethernet, or via its USB connector. In addition to MPEG Layers 2 and 3, G.711, G.722 and MPEG AAC, it also includes the APT proprietary algorithms such as Standard Apt-x and Enhanced 16, 20 and 24-bit Apt-x. This device features a built-in inverse multiplexer that allows it to use four separate ISDN circuits, and hence provide up to a 512kb/s data rate.
The Orban Opticodec 7600 is a duplex audio codec for use via Ethernet, X.21 or V.35 interfaces, or as many as three ISDN interfaces or mixed with POTS interfaces. Configure and operate the codec directly with the front panel keypad and high-resolution graphical display.
A relatively new player is Mayah, and it recently introduced the Centauri II 3300/3301. This is a codec with multi-channel I/O. The bit rate is determined by the application and can be as low as 160kb/s with AAC+SBR (MPEG4 AAC HE) or as high as 6Mb/s with linear audio. Connectivity via IP and ISDN is possible.
Perhaps the best-known manufacturer of ISDN and POTS codecs is Telos Systems. The Zephyr Xport is a POTS codec that, with the inclusion of the field-installable ISDN option, can be made into an ISDN codec as well. The Xport uses AAC Plus audio coding for POTS connections; MPEG AAC low delay for ISDN connections made with an Xstream on the far end; and its G.722 option allows it to communicate with other G.722 codecs as well. The unit offers a built-in mixer with mic and line-level inputs, and independent headphone outputs that can listen to received audio or monitor mixes.