Nexus Digital Audio Codec PDF User Manual

The Comrex Nexus is a device designed to send and receive either 7.5 kHz or 15 kHz audio on ISDN digital telephone circuits. Its functions and pieces are best broken down and described individually: Codec is an acronym for Coder/Decoder. We define this as the part of the Nexus which takes in an analog audio signal, converts it to a digital bit stream, and performs operations on this bit stream to remove redundant information. This operation is known as Digital Audio Compression. The codec section also takes an incoming, previously compressed bit stream and converts it to analog audio. In the Nexus codec, the analog audio inputs and outputs are available to the user. The input and output compressed bit streams are connected internally to the terminal adapter section. There are different types of compression, and the type is often defined by the name of the algorithm employed. The Nexus uses the international standard G.722 algorithm, and it is also capable of a higher speed version of G.722 which provides 15 kHz bidirectional audio at 112/ 128 kb/s. For more information on this subject, see the ?G.722 Algorithm? section on page 31.

We call this the ?TA? for short. This section can be thought of as a modem, but one that only works on a special type of telephone line. This line is called an Integrated Services Digital Network line, or ISDN. A description of this service is included on page 27. The TA takes the encoded audio from the codec and feeds it to the telephone line. It also feeds audio data from the phone line to the codec. It allows you to dial and answer ISDN phone calls and provides the functions required to ?handshake? with the ISDN line as well as to troubleshoot problems should they occur. The TA in the Nexus includes a section called the NT1 which is a conditioning circuit that interfaces with the phone line. This eliminates the requirement for another external box on your circuit. The Nexus TA also can combine the two ISDN ?B? channels using a standard called BONDING. This allows for a transmission rate of 112/128 kb/s on ISDN. For more information on this subject, see the ?Inverse Multiplexing? section on page 34. This is the ?brain? of the Nexus. It communicates with the codec and TA, sets them to the correct modes and provides the user interface for configuring and dialing the TA. It also provides the ancillary data function and allows you to communicate to the TA via computer.

The Nexus has two audio inputs and two audio outputs. The main input and output are on female and male 3 pin XLR connectors respectively. These are designed for professional level, balanced audio signals. They can be interfaced with other kinds of ?consumer? equipment, but the system performance may suffer. The input audio may also be applied to a mini jack, designed to be compatible with portable tape recorders. This input level is fixed, and the audio received from it is mixed with the main audio input. The audio output is also available on a stereo headphone jack. See the ?Nexus Speifications? on page 39 for hookup information. There are three audio controls on the Nexus front panel, along with one
switch. The switch is designed to choose what type of audio signal is applied to the main XLR input. The user may choose between a MIC or LINE level. If you are connecting a microphone directly to the Nexus, or if you are using a mixer or a console with an output labeled ?microphone level out,? you will want this switch set to MIC. Most other users will want this switch set to LINE mode. In either mode, you may use the AUDIO INPUT level control (green) to raise or lower the input level to the Nexus.

The output audio of the Nexus is a mixture of the audio from the decoder section along with the audio being sent to the encoder. This is so you can monitor what you are sending as well as what you are receiving. If you do not wish to receive any of your own audio, simply turn the LOCAL PROGRAM control (red) all the way down, and adjust the RETURN AUDIO control (yellow) to the correct level for your system. If you wish to receive some of your own audio, adjust the relative levels of the local and return audio to suit. The Nexus headphone output features the same mix of audio, adjustable via the same output level control. When setting levels, adjust the INPUT LEVEL control first. You do not need to
place a call in order to set this level. Simply feed some audio into the Nexus at the level you will typically use. Adjust the INPUT LEVEL control until the PEAK indicator on the Nexus front panel lights just occasionally on program peaks. This indicates that the internal peak limiter is active on occasional peaks and the input level is correct. If the input level suddenly increases, the limiter will protect the audio and keep it from clipping. If the PEAK light never comes on during your program audio, you may be underdriving the audio circuitry which can create noise problems. If the
PEAK light is on most of the time, you run the risk of overloading the limiter and causing distortion. If using the tape input jack, you will need to adjust the output level of your tape machine until the peak light comes on occasionally.

You may also adjust the LOCAL PROGRAM output without placing a call. This control is primarily for those who will be monitoring only the output of the Nexus and need to hear themselves as well as the return audio (as an example, the host of a call-in talk show at a remote location whose calls are being taken at a radio station). The outgoing Nexus channel is used to send the host to the station, and the return Nexus channel is used to send the callers to the host. The host will hear a mix of himself and the callers. In this scenario, the station where the callers are being received must send a mix-minus back to the host (that is the callers and any commercial breaks, but not the host’s voice, which will be mixed in locally by the Nexus). With the input level set correctly, adjust the LOCAL PROGRAM control until the level is appropriate for the system or for your headphones. Users who do not wish to have any local audio mixed with the output will turn this control all the way down. After you have completed the rest of the setup procedures and established your first call, you can set the RETURN AUDIO control.

Download Nexus Digital Audio Codec PDF User Manual


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March 13, 2010 | Posted in Audio Video, Electronic Manual, Nexus Technology

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