Reliable Connections

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Reliable Connections

Jan 1, 2005 12:00 PM, By Chriss Scherer, editor

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Using telephone codecs is so easy, it's hard to imagine station operations without them. They are flexible and provide reliable communications and data paths from almost anywhere. With the increase in clutter in the RPU spectrum, and greater challenges in securing tower space for a wide coverage area, the humble codec proves its value many times over every day.

While remotes and live events are the primary use, many stations are using POTS and ISDN codecs for back-up STLs and other itinerant applications. This is facilitated by the availability POTS lines as well as the proliferation of ISDN service. In many cases it's cheap insurance to maintain an additional POTS or ISDN line at a transmitter or off-site studio for use as a back-up STL when needed.

When first introduced, codecs were single-purpose units designed for either ISDN or POTS use. This is still true for some models, while newer designs have become modular in their approach, providing a variety of options depending on the added modules. This evolution has also affected their connection capabilities. Most ISDN codecs offer connectivity to various data interfaces and IP connections. Many can serve as hardware encoders for streaming applications. Both ISDN and POTS are available with enhanced audio features as well.

Getting more with less

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The Mobile Konnector from Konexx provides a tip/ring interface through a telephone handset connection.

While ISDN service provides the highest data connectivity rates, ISDN lines are not always available without advance planning. In some areas, ISDN is still not offered or it may already be discontinued. Once touted as the next major step in telephony, ISDN never saw the widespread use that was so heavily anticipated.

POTS lines, on the other hand, are everywhere. On short notice, you can probably borrow a POTS line for the last-minute remote or unexpected news event. When direct connection to a POTS line is not possible all is not lost. If the only choice is a line from an office PBX, the phone can be adapted for POTS codec use. Devices such as the Mobile Konnector from Konexx can be inserted between the telephone handset and the phone base to emulate a tip/ring POTS connection. The data rate will likely be less than directly accessing the POTS line, but may suffice if there are no other options.

POTS codecs are also evolving to uses with wireless telephony. The low bit-rate challenges of POTS lines have provided valuable lessons for GSM use. Right now GSM provides the most reliable wireless communications path, but it is limited to 9.6kb/s. Significant data reduction is required to transmit audio through this path, but it is possible with satisfactory results. High-speed circuit-switched data (HSCSD) offers a path that is up to four times as wide as GSM, but it is not yet available in North America. TDMA service provides up to 14.4kb/s, but its data-burst nature makes real-time audio unstable.

Another way to obtain a portable wireless connection is a satellite phone. With a 64kb/s service, most ISDN codecs can be configured to tie into a satellite phone system. The airtime is not cheap, costing several dollars per minute, but it is another way to create a wireless connection.

Decisions, decisions

Deciding between a POTS or ISDN codec can be determined by the needs of the user and the availability of service. As mentioned earlier, if ISDN service is available, it will provide the highest quality connection. The availability of POTS lines makes them attractive if short lead times are the normal routine. In most cases and with a good connection, a POTS codec will provide satisfactory results with a voice transmission.

Compatibility between various codecs is another concern. Most ISDN codecs share some common coding algorithms. G.722, Layer II and usually Layer III are the most common. These algorithms allow codecs from different manufacturers to communicate. Some ISDN codecs provide AAC or Apt-x as well, but because they are not common to all codecs, their use may be limited to a reduced set of codecs and possibly only those from the same manufacturer.

POTS codecs do not share the same widespread compatibility. Except for the claim from Tieline that it will communicate with Comrex units, each manufacturers' POTS codecs will only work with units from that same manufacturer.

The delays in an encoding system must be considered when making a connection. Some encoders offer shorter delay times than others. When separate algorithms can be used for each side of the bidirectional path, a high-quality algorithm is commonly used to send audio to the station and a lower-quality algorithm to receive IFB and cueing information back at the remote site. This tradeoff in quality takes advantage of the coding delays used. Layer III and Layer II have longer delays than G.722, Apt-x and AAC-LD.

Whatever algorithm is chosen, keep multiple encoding passes to a minimum. This topic has been greatly debated since audio encoders were first introduced. To reduce the data-rate needs, perceptual encoders discard inaudible audio data. After a few passes, the multiple encoding passes may become audible.

ISDN basics

In North America, the U interface is the most common, but several other interfaces exist. Among these, S, T, U, V.35 and X.21 are more commonly seen, although there are others. Most codec manufacturers will support more than one interface.

The U interface is the connection between the raw ISDN network and the NT1 (Network Termination, type 1). It is almost always a two-wire interface, but this is not an international standard. Outside the United States and Canada, it is completely the domain of the telephone company, and because the telephone company provides the NT1 the lack of a standard does not matter to the user.

In the United States and Canada, the U interface is standardized by the American National Standards Institute (ANSI) and uses 2B1Q line coding. It has a range of 18,000 on copper lines from the telephone company connection. Telephone companies sometimes use other transport technologies, either to provide service over T-1 or fiber or to extend the range beyond 18,000 feet. However, it must always be converted back to the 2B1Q coding. 2B1Q specifies two-bits per quaternary (quat). A quat is a signal that can take one of four states: 00, 01, 11 or 10.

The raw bit-rate on the 2B1Q U interface is 160kb/s. This path comprises two 64kb/s channels, referred to as B channels and a single 16kb/s channel, referred to as a D channel. The B stands for bearer and D stands for data. The D channel carries the call setup and teardown information. The remaining 16kb/s of data are used for data framing and synchronization, and an embedded operations channel to convey information on block errors to the far end.

Only the center two pins nector are used on the RJ-11 or RJ-45 jack with the U interface.

The V interface is the two-wire interface at the telephone company's central office. This is rarely encountered because it is used exclusively by the phone company.

The S interface uses a modified form of alternate mark inversion (AMI) framing similar to a T-1. It is a four-wire interface defined by international standards, with a raw bit-rate of 192kb/s. In addition to the two B channels (at 64kb/s each) and the D channel (at 16kb/s), there are bits for framing, synchronization and two embedded operations channels. In some parts of the world this is referred to as the S0 (S zero) interface.

The S interface is a bus architecture that supports as many as eight terminals and a single NT1. An S interface is provided with an RJ-45 jack, and it uses the four center pins. The remaining pins are sometimes used to power the terminal or NT1. When power is provided, it is usually placed on pins seven and eight.

The T interface is a subset of the S interface. Unlike the S interface, only a single terminal can be used on the T interface.

X.21 and V.35 are standards for serial data ports, not unlike RS-232. RS-232 can be used for synchronous or asynchronous data, although it is typically used for asynchronous data. V.35 and X.21 are used solely for synchronous data. The usual application is to connect data terminal equipment (DTE), such as a computer terminal or codec, to data communications equipment (DCE), such as a CSU/DSU on a T-1 or dedicated digital line. In addition to the transmit-data and receive-data and handshake pins, these interfaces include pins for clocking, as required by synchronous data applications. V.35 is most common in the United States and Canada, while X.21 is commonly used in other parts of the world.

Hit the road

One of the most important features to consider is the operation of the codec. A dedicated remote engineer may not always be on hand to establish the link and troubleshoot problems. The ability to store presets that can be easily recalled by logical names will make this easier. Having a non-technical user select a preset called �City Hall� or �Riverfront Stadium� is much simpler than specifying a sampling rate, data rate, coding algorithm and configuration settings.

Information on the various ISDN interface standards was provided by Rolf Taylor of Telos. Wireless network info was provided by Tom Harnett of Comrex.

Resource Guide

A sample of available POTS and ISDN codecs

Audio Processing Technology (APT)

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Worldnet Tokyo

The Worldnet Milano is designed for fixed-link and ISDN connections. It incorporates Standard Apt-x and Enhanced Apt-x. It communicates via fixed links from 56kb/s to 576kb/s or from one to four ISDN links from 128kb/s to 512kb/s. An S interface is standard and a U interface is optional.

The Worldnet Tokyo supports Standard Apt-x, Enhanced Apt-x, MPEG 1 and 2 Layer II/Layer III, G.711, G.722 and MPEG 2 AAC. It also provides MUCAS and J.52 inverse multiplexing bonding algorithms. Simultaneous connections can be made to different destinations. The unit can connect to four ISDN lines. An RS-32 ancillary data path is provided, as are four TTL inputs and outputs for remote control. It can be controlled via RS-232 or TCP/IP.

ATA Audio

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Scoop EZ

The Scoop EZ can be used with POTS, ISDN, wireless and Inmarsat connections. It includes a two-channel audio mixer with phantom power, a selectable compressor/limiter, and auto answer and configuration of incoming call type (ISDN/POTS). At 9" � 6" � 3" it weighs less than 4lbs. The unit is now available for use with GSM networks by inserting a SIM card into the unit. Audio bandwidth via GSM is 300Hz to 3.4kHz.

The Scoop Studio is a 1RU version of the Scoop EZ without the battery backup and mixer functions.

The Hifi Scoop 3 ISDN offers Layer II, Layer III, G.722 and ADPCM. It also incorporates the ITU-T J52-compliant algorithm to automatically negotiate different algorithms. The unit is available in two, four or six B channel configurations. It occupies 2RU.

Harris Intraplex

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The Harris Intraplex Intralink ISDN monitor can control as many as six BRI ISDN lines in a 3RU shelf. It handles any combination of program audio, voice and data using standard Intraplex channel modules. Five shelves can be linked for a maximum capacity of 30 BRI, and it is compatible with most industry codecs using Layer II, Layer III or G.722. The unit delivers MPEG program audio up to 128kb/s and delivers LAN data at 128kb/s. It is Telex and Clearcom voice- and data-compatible for remote intercom use and is field-configurable via a Windows-based user interface.

Tieline Technology

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I-mix G3

The Commander G3 two expansion slots accept 15kHz mono POTS, dual mono POTS, stereo POTS, mono/stereo ISDN, GSM wireless to landline and telephone coupler modules. Stereo POTS connections can send audio to two destinations or be used for an IFB. The optional Digital Matrix Router can route audio input to any audio output. The three-input mixer features two mic/line inputs, an RCA auxiliary in/out, two headphones, independent channel on/off cue-talkback and send-return audio mix functionality. Two POTS lines can be bonded for phase-matched stereo operation. The ISDN module supports G.711, G.722 and Layer II. An additional 15kHz mono POTS module and optional failover software can be installed for automatic backup. The GSM Module provides 7.5kHz wireless connections.

The I-mix G3 has the same features as the Commander G3, but adds a six-input mixer with five XLR connections and one RCA connection. Remote audio levels can be controlled from the receive end. Both models can be controlled via USB, Ethernet or RS-232.


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The Matrix is a POTS and ISDN codec. It delivers 15kHz full-duplex audio on a POTS line via an integrated V.34 modem, and it is compatible with the Bluebox, Hotline and Vector. With the ISDN module, it provides Layer III or G.722 encoding. It can also be used with the GSM Module to provide a wireless 7kHz audio path. The software can be flash upgraded through the multi-purpose dataport. The Telcell option allows connection to a telephone line or to the hands-free port on most mobile phones. A battery kit is available including a rechargeable NiMH battery that will run the unit for seven hours in POTS/ISDN, or four hours with GSM Module. It is also available in a 1RU version.

The Nexus ISDN codec is available in a portable package or a 2RU studio unit. It includes a built-in terminal adapter. It provides a full-duplex, 15kHz audio path of two B channels using Turbo G.722 encoding. It can store multiple ISDN line configurations and provides an ancillary data channel.

The Envoy ISDN codec builds on the features of the Nexus portable unit by providing a four-channel mixer (two mic inputs and two mic/line inputs) and three headphone outputs.

The Bluebox delivers 15kHz full-duplex audio on a POTS line. It provides two contact closures at each end of the Bluebox. One is enabled on connection, the other is a momentary contact closure that is initiated by a push button on the keypad. Calls can be answered manually or automatically. As many as 44 digits can be entered into the units' dialer. The unit's software can be flash upgraded through a computer port. It includes a cell phone jack, which connects to the hands-free port on most mobile phones to act as a phone coupler.


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AudioTX Communicator

The AudioTX Communicator runs on Windows 98 and above and uses any standard sound card. The unit can be used with a laptop and connects to all major ISDN audio codecs. The software will automatically detect the codec on the other end and reconfigure itself. The system can communicate via IP or over private leased lines. The software supports a single BRI and communicates via Layer II, Layer III, G.722 and G.711.

The AudioTX POTS is a software-based POTS codec. Operating on Windows 98 and above, the software works with the computer's soundcard and modem. The software automatically detects and adjusts for line quality to provide up to 7.5kHz of audio bandwidth. The software also works with high-speed GSM/data-enabled mobile phones.


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The Eagle is an ISDN codec that features AES/EBU digital inputs and outputs, dual display and multiplexing capability between the two ISDN B channels in 1RU. It transmits via G.711, G.722, Layer II, Layer III or AEQ-LD-2, a low-delay algorithm. The auxiliary data channel can be configured separately for transmitting and receiving. All AEQ products can be controlled via RS-232, RS-422 or through the AEQ Easy control system.

The Swing is a portable audio codec with a built-in ISDN terminal adapter and a double-port interface compatible with U.S. and Euro-ISDN. An internal digital POTS hybrid can be used simultaneously, and it can connect to a cell phone. The three-input mixer has two mic inputs and a selectable mic/line input. ISDN coding is available via G.711, G.722, Layer II and AEQ-LD-2.

The Course is a 4RU frame that can hold 10 one- or two-channel multiformat audio ISDN codec boards. Coding is via G.711, G.722, Layer II and AEQ-LD2.

Telos Systems

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Zephyr Xstream

The Zephyr Xstream is a 2RU ISDN codec. It encodes audio via G.722, G.711 Layer III, Layer II, MPEG-2 AAC and MPEG-4 AAC-LD. The ISDN terminal adapter is built in. A 10Base-T Ethernet port allows remote control and streaming of MP3-coded audio over a LAN, WAN or the Internet. TCP/IP network connectivity also allows upgrade of system software via FTP. It includes a front-panel headphone jack with level control.

The Zephyr Xstream MXP is a portable version that adds a four-channel stereo mixer in a portable chassis or in a rack-mount version such as the Zephyr Xstream MX. The four inputs are switchable as mic or line, with phantom power on the first two. The selectable AGC/limiter processing has presets designed by Omnia. Two separate local headphone mixes can monitor send audio, receive audio or both.

The Zephyr Xport can transmit via POTS or ISDN to a Zephyr Xstream ISDN codec. It transmits audio via AAC Plus over POTS, or AAC-LD or G.722 over ISDN. It has a built-in mixer with mic and line inputs, and selectable dynamics processing by Omnia. The controls and menus were designed for non-technical users. The unit can be controlled via Ethernet and RS-232.


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The Opticodec 7400 connects via ISDN or IP and encodes audio via Layer II, Layer III, G.722 and G.711. Up to 94 speed-dial entries can be stored in the directory. It can connect at data rates from 56kb/s to 384kb/s. Eight remote control inputs and outputs are transmitted, as well as a 9.6kb/s ancillary data channel. Remote control and administration is possible with the included software for Windows 98 and above. The Opticodec 7000 is the portable version that adds an audio recorder in a compact case.

Musicam USA

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The Netstar is available in two models, which are identical in features. The model 500 has full controls on the front panel, while the model 300 has no front panel controls and is designed for unattended use. Both can be controlled via the built-in Web server or RS-232. The unit can send and receive real-time stereo audio, contact closures and ancillary data via ISDN, dedicated data lines and IP. Audio can be encoded using G.711, G.722, Layer II, Layer III, MPEG 2 AAC or MPEG 4 AAC-Low Delay. Stereo audio is available through an Ethernet port. ISDN is standard, and V.35, X.21 and RS422 interfaces are available. The Roadstar is the portable version and is housed in a rugged case with a four-channel mixer, phantom power and a headphone monitoring system.

Available in five models, the CDQ Prima ISDN codecs transmit audio via Musicam Layer II, Layer III and G.722 with bit-rates from 24kb/s to 384kb/s. It can be set to dial on audio and hang up on silence, and automatically configure itself to connect to the appropriate encoding algorithm, sampling frequency and connection rate. A plug-in module provides connection directly to an ISDN line or to a V.35, X.21 or RS-422A interface. An intelligent headphone circuit can monitor local and return audio on models 120, 220 and 230.

The Prima LT ISDN codec offers the same audio specifications as the CDQ Prima line of codecs, with functions designed for easier setup and use. An optional Windows remote control program is available for easy point-and-click operation. The Prima LT Plus adds enhanced audio metering, digital audio I/O standard, the ability to bond to three ISDN lines for a maximum connectivity rate of 384kb/s and two ancillary data channels.

The Liberty POTS codec provides a bidirectional 15kHz audio path at rates as low as 24kb/s. It includes one mic/line XLR input, input leveler, a line input, ?" mini jack for most cell phones and a headphone monitor jack. Audio levels from the far end can be controlled via the POTS link.

The Roadrunner is a portable ISDN codec and three-channel mixer. It transmits mono audio over a single ISDN line. Two independent headphone circuits provide individually adjustable send and receive levels. It includes an internal ISDN terminal adapter and integrated NT-1 where required. It transmits audio via G.722, Musciam Layer II and Layer III.