Model 42A Interface

The Model 42A is designed to create broadcast-standard IFB circuits from line-level audio sources. The unit’s primary application is to interface analog outputs associated with digital matrix intercom systems with broadcast IFB user devices. The Model 42A provides four independent IFB circuits. For convenience, the four circuits can be connected by way of eight 3-pin male XLR-type connectors; four of which are located on the back panel and four on the front. Each IFB circuit provides DC power and two analog audio signals to support the connected IFB user devices. The Model 42A’s audio quality is excellent; little hiss, hum, or other artifacts are present. To ensure optimal operation, the unit provides resources for visually and audibly monitoring the audio output signals. In addition, the DC output voltage is monitored for over-current and short-circuit conditions.

Installation of the Model 42A is very simple. Audio input connections are made using a 25-pin D-subminiature connector. The IFB output circuits interface using standard 3-pin XLR-type connectors. The compact, one-rack-space package is constructed using heavy-gauge steel components. The unit’s mains power input can range from 100 to 230 volts, 50/60 hertz. This "universal input" ensures correct operation virtually anywhere in the world.

There may be persons not familiar with the term IFB. That’s not unreasonable as it’s a somewhat obscure acronym for interruptible foldback. On its own, the term foldback is an alternate way of describing a cue or monitor function. Adding "interruptible" before it means that the cue source can be temporarily replaced with an audio signal originating from a producer, director, or other production personnel. IFB circuits are often used in the broadcast industry for talent cueing applications, both in studio and field settings. Both "dry" and "wet" IFB circuits can be deployed and their characteristics are worth reviewing. The term "dry" IFB typically refers to a transformer-balanced line-level audio circuit with a nominal level in the range of 0 to +8 dBu. This is essentially a standard audio circuit that is commonly used to interconnect audio equipment. The term "wet" IFB refers to a circuit that combines DC power and one or two channels of analog audio. The nominal level of the DC power source is typically in the range of 30 to 32 volts. The audio is unbalanced with a typical nominal level of –10 dBu. The Model 42A implements wet IFB circuits. As such, in this user guide the term IFB will always represent this type of circuit.

IFB circuits provide an effective means of delivering power and two channels of audio to user devices by means of standard audio cables. These cables, ubiquitous to the audio industry, interface using 3-pin male and female XLR-type connectors. Using IFB circuits and standard audio cables it’s a simple matter to support user devices such as listen-only belt packs and announcer’s consoles with no external power source required. Whether the IFB source and user devices are 100 or 1000 feet apart, reliable operation can almost always be provided.

The Model 42A supplies four independent IFB circuits. Each circuit consists of two audio inputs and a 2-channel "wet" IFB output. The audio inputs are transformer coupled, have a nominal level of +4 dBu, and are compatible with balanced or unbalanced sources. In on-air television broadcast applications the audio sources will often be analog outputs from matrix intercom systems. Two sources are typically designated to serve as user cue signals. Generally one source is configured in the matrix intercom system as "interrupt" while the other is configured as "program." An alternate term often used for the "interrupt" channel is "program-with-interrupt." This may be more descriptive as the function is actually a program source that gets interrupted with talkback audio. The "program" channel is typically a continuous source of program audio. An alternate term is "program-only." For other applications, the Model 42A’s audio inputs can be connected to a 2-channel or stereo audio source. This configuration may prove useful in radio broadcasting, audio-with-picture, or recording studio applications. The specific application will dictate whether both cue signals will be utilized by the end user. In on-air sports broadcast situations a "double-muff" (stereo) headset will provide a user with both audio channels, one for each ear. In interview or news-gathering applications it’s common to use a single "earpiece" to provide a user with program-with-interrupt audio.

Maintaining excellent audio performance was a major Model 42A design goal—the hiss, hum, and noise associated with typical IFB circuits was simply not acceptable. The Model 42A meets those requirements with audio that is "on-air" quality: low distortion, high signal-to-noise ratio, and ample headroom. On-air talent and guests, production personnel, and technicians will all appreciate the clean, quiet cue signal.

Each Model 42A IFB circuit provides DC power and two channels of unbalanced audio by way of two 3-pin male XLR-type connectors. The DC output is nominally 30 volts with a maximum rated current of 200 milliamperes. A major strength of the Model 42A is the IFB circuit’s ability to effectively deliver DC power over a variety of conditions. Unlike other interface devices that use a common but less-than-ideal circuit topology, a unique IFB circuit was developed by Studio Technologies to achieve the desired performance goals. The result is a major improvement in effectively supporting IFB user devices over a wide range of conditions. Connected devices can draw up to the full rated 200 milliamperes of current with little drop in DC voltage. This output voltage stability is the key—whether drawing 50, 100, or 200 milliamperes, the output will remain close to 30 volts. In practical terms this means that reliable IFB-based cue systems can now be deployed in more stadiums, concert halls, or motor racing facilities than was previously possible; longer cable runs, more user devices, excellent performance.

To help confirm proper installation and ensure correct operation, the Model 42A includes a full-featured audio monitor section. Two 5-segment LED meters provide an indication of the audio levels as they appear directly on the IFB output connectors. This capability makes it simple to adjust and maintain correct audio levels so that optimal IFB performance can be maintained. A headphone output is also provided, allowing "real-world" checking of IFB audio quality. In addition to being able to manually select the IFB circuit to be monitored by the meters and headphone output, an "auto scan" mode is also included. This allows each of the four IFB circuits to be monitored in a continuously repeating sequence.