Audio Processing for FM Broadcasting: Compressors, Limiters & Processors

Professional audio processing is the difference between an amateur-sounding station and one that commands listener attention. In the competitive FM band, properly processed audio ensures your station sounds loud, clear, and consistent across all program material while staying within regulated modulation limits.

Why Audio Processing Matters for FM Broadcasting

Raw audio from microphones, music players, and other sources has significant dynamic range: quiet passages and loud peaks can differ by 60dB or more. FM broadcast channels have a fixed maximum frequency deviation of ±75kHz. Without processing, quiet passages disappear into noise while loud peaks cause overmodulation and distortion. Audio processing reduces dynamic range in a controlled manner, making soft sounds audible and preventing loud sounds from exceeding limits. Well-processed audio also creates a signature "loudness" that makes your station stand out when listeners scan the dial. The FMT5.0 Pro series includes integrated DSP-based audio processing with multi-band compression, limiting, and stereo enhancement, providing professional-grade processing without external hardware.

Key Audio Processing Stages

• AGC (Automatic Gain Control): The first stage of processing. AGC slowly adjusts overall gain to maintain a consistent average level regardless of input source variations. Time constants are typically slow (seconds) to avoid pumping artifacts. For example, when switching from a quiet interview to a loud music track, AGC gradually increases or decreases gain to match the target level.
• Multi-band Compression: Divides the audio spectrum into multiple frequency bands (typically 2-5 bands) and applies independent compression to each. This prevents a loud bass drum from causing the compressor to reduce treble frequencies (a problem with single-band compression) while controlling overall density. Common bands are: sub-bass (20-80Hz), bass (80-250Hz), midrange (250Hz-2kHz), presence (2-6kHz), and brilliance (6-15kHz).
• Limiting / Peak Control: The final stage before the stereo encoder. A fast peak limiter catches any remaining transients that could cause overmodulation, with attack times of a few microseconds. The FMT5.0 Pro's composite clipper operates on the multiplex signal itself, providing brick-wall limiting that protects against illegal deviation while maintaining perceived loudness.
• Stereo Enhancement: Carefully increases stereo width and spatial imaging. When done subtly, it creates a more engaging listening experience. Over-processing in this stage can cause mono compatibility issues, so the FMT5.0 Pro monitors the L-R/L+R ratio to prevent excessive widening.

Pre-emphasis: Critical Final Step

After processing but before modulation, the FM pre-emphasis filter boosts high frequencies (above 3.18kHz for 50us, above 2.12kHz for 75us). This improves signal-to-noise ratio by compensating for the triangular noise characteristic of FM demodulation. The pre-emphasis curve is standardized by region, and using the wrong setting results in either dull or harsh audio with potential overmodulation at high frequencies. The FMT5.0 Pro supports selectable pre-emphasis of 0, 50, or 75 microseconds. Always set this correctly for your region before fine-tuning the audio processor.

Processing for Different Formats

• Music Formats (Pop, Rock, EDM): Aggressive multi-band compression creates a dense, consistently loud sound. Moderate stereo enhancement widens the image. Heavy peak limiting increases average modulation. The goal is competitive loudness and impact.
• Classical / Jazz: Light compression preserves natural dynamics. Minimal peak limiting. The goal is transparency and fidelity. Listeners of these formats expect dynamic range and will tune away from over-processed sound.
• Talk / News: Single-band or gentle multi-band compression focuses on voice clarity. AGC is critical for leveling different speakers. Roll off sub-bass below 80Hz. Prioritize intelligibility over loudness.
• Multi-format Community Radio: Moderate multi-band compression with slower AGC time constants provides a balanced compromise. Some stations use different processing presets for daytime talk and evening music programming.

Built-in Processing on FMT5.0 Pro

The FMT5.0 Pro transmitters include DSP-based audio processing accessible via the LCD panel and web interface, with configurable AGC attack/release, 5-band compressor thresholds and ratios, composite clipper drive, stereo enhancement width, and pre-emphasis selection. The processed audio is monitored via a modulation meter showing peak deviation and average modulation, helping you dial in the perfect processing for your format. For stations with external processing chains, the internal processor can be bypassed entirely via the audio input configuration menu.