The Gates FM Top-Level Limiter, introduced in the 1960s and featured in the 1965 Gates catalogue, was a fully transistorised audio processor designed specifically for FM broadcasters. This unit addressed the increasing need for precise control of audio signals in the FM frequency range, particularly in preventing overmodulation caused by high-frequency content, often amplified during the pre-emphasis stage of FM broadcasting. Designed for use between a station’s limiting amplifier and FM transmitter, the device was suitable for both stereo and monaural operation.
This limiter incorporated a solid-state design and featured a frequency-sensitive audio processor with a built-in 75-microsecond pre-emphasis curve. Its advanced circuitry allowed the amplification of high-frequency signals in the 15,000 Hz range by up to 17 dB above the nominal 1,000 Hz signal. This capability reduced the risk of overmodulation at high frequencies, maintaining cleaner, distortion-free transmission even during challenging audio peaks. The limiter utilised a precise clamping mechanism to instantly manage high-amplitude signals, effectively preventing distortion and intermodulation.
The Gates FM Top-Level Limiter was lauded for its ability to enhance the clarity and fidelity of FM broadcasts. By reducing overmodulation and intermodulation, the device allowed FM receivers to produce a louder, cleaner, and more consistent signal, particularly when paired with receiver de-emphasis. Broadcasters using SCA (Subsidiary Communications Authorization) for background music also benefitted, as the limiter minimised crosstalk and signal degradation caused by pre-emphasis splatter between the main channel and SCA subchannels.
Capable of dual-channel operation, the Gates FM Top-Level Limiter supported stereophonic and monophonic configurations, making it versatile for a variety of broadcasting needs. It complemented existing station limiters or could function independently to provide precise, economical control against overmodulation. With instantaneous attack and release times, and a maximum gain of 28 dB, it provided excellent handling of input levels ranging from -10 to +24 dBm. The output was adjustable up to +18 dBm and supported 600-ohm balanced or unbalanced connections.
Physically, the Gates FM Top-Level Limiter was built to last, with a sturdy design housed in a medium grey steel chassis. Measuring 19 inches wide, 5 ¼ inches high, and 12 inches deep, it weighed 24 pounds and required 117 volts of power at 30 watts. Its combination of reliable construction, precise audio processing, and flexible functionality made it a valuable tool for FM broadcasters looking to achieve superior audio quality and signal integrity.
