Studio Technologies Model 42A Line-Level Audio to IFB Circuit Interface

Studio Technologies Model 42A Line-Level Audio to IFB Circuit Interface

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 connectors. The compact, one-rack-space package is constructed using rugged steel and aluminum components. The unit's AC mains power input can range from 100 to 240 V, 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 outputs are often used in the broadcast industry for talent cueing applications, both in studio and field settings.

Both "dry" and "wet" IFB outputs 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 analog 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 DC power source is typically in the range of 30 to 32 V. The audio is unbalanced with a typical nominal level of –10 dBu. The Model 42A implements "wet" IFB outputs. As such, in this user guide the term IFB will always indicate this type of circuit.

IFB outputs 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 connectors. Using IFB outputs and standard audio cables it's a simple matter to support user devices such as listen-only beltpacks 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.

In many cases, the Model 42A Interface will be used in on-air television applications. Whether installed in a fixed location or as part of a mobile facility, the unit is capable of providing excellent performance. In addition, the Model 42A can be used for non-broadcast applications. For example, audio recording and post-production facilities can also effectively use the Model 42A. Combined with stereo or mono listen-only beltpacks, also available from Studio Technologies, a variety of headphone cue systems can easily be deployed. And since the Model 42A's audio inputs are compatible with standard line-level audio signals virtually any analog source can be connected.

Four Independent IFB Outputs

The Model 42A supplies four independent IFB outputs. 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 a 2-channel user cue signal. 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 only program-with-interrupt audio.

Maintaining excellent audio performance was a major Model 42A design goal—the hiss, hum, and noise associated with typical IFB outputs 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 signals.