FM95 Configuration File

This document describes the configuration file format for the FM95 FM transmitter software.

File Format

The configuration file uses standard INI format with sections and key-value pairs. Comments can be added using the # or ; character at the beginning of a line.

Configuration Sections

[fm95] - Main Settings

This section contains the core FM95 transmission parameters.

Audio Processing

stereo (0 or 1, default: 0)
Enable stereo transmission. Set to 1 for stereo, 0 for mono.
audio_volume (float)
Audio input volume multiplier. 1.0 = unity gain, values > 1.0 may cause distortion.
master_volume (float)
Master output volume control. Affects overall transmission level.
clipper_threshold (float)
Audio clipper threshold to prevent overmodulation. Lower values = more aggressive clipping.
preemphasis (float, microseconds)
Pre-emphasis time constant in microseconds. Common values: 50 (Europe), 75 (North America).

Stereo Processing

polar_stereo (0 or 1, default: 0)
Enable polar stereo encoding.

RDS (Radio Data System)

rds_streams (integer, range: 1-4)
Number of RDS data streams to process simultaneously. More streams allow richer RDS data but require more processing power.

MPX (Multiplex) Settings

mpx_power (float)
MPX signal power level. For BS412.
mpx_deviation (float, Hz)
Maximum frequency deviation for MPX signals. Standard value is 75000 Hz for FM broadcast.

Calibration

calibration (0-2, default: 0)
Enable calibration mode for testing and adjustment. (2 enables 40 hz square wave instead of the 400 hz sine)

Frequency Deviation

deviation (float, Hz)
Maximum frequency deviation. Standard FM broadcast uses 75000 Hz. This setting affects the master_volume automatically.

[devices] - Audio Device Configuration

This section specifies the audio devices to use for input and output.

input (string, max 63 chars)
Primary audio input device
output (string, max 63 chars)
MPX input
mpx (string, max 63 chars)
MPX (multiplex) input device for subcarrier input
rds (string, max 63 chars)
RDS signal input

Example Configuration

# FM95 Configuration File
# Lines starting with # are comments

[fm95]
# Enable stereo transmission
stereo=1

# Audio levels (1.0 = unity gain)
audio_volume=0.8
master_volume=1.0

# Prevent overmodulation
clipper_threshold=0.9

# Pre-emphasis for Europe (50µs) or North America (75µs)
preemphasis=50

# Enhanced stereo processing
polar_stereo=1

# RDS configuration
rds_streams=2

# MPX settings
mpx_power=0.1
mpx_deviation=75000

# Standard FM deviation
deviation=75000

# Enable calibration mode for testing
calibration=0

Important Notes

RDS Streams: Maximum of 4 RDS streams are supported. Exceeding this limit will cause an error.
String Length: Device names are limited to 63 characters maximum.
Deviation Interaction: The deviation setting automatically adjusts master_volume using the formula: master_volume *= (deviation / 75000.0). Set deviation last if you want to override master_volume.
Audio Levels: Keep audio levels reasonable to prevent distortion. Start with conservative values and adjust as needed.
Preemphasis: Use 50µs for European broadcast standards, 75µs for North American standards.

Troubleshooting

No audio: Check device names with system audio tools
Distorted audio: Reduce audio_volume and clipper_threshold
Poor stereo separation: Enable polar_stereo and adjust mpx_power
RDS not working: Verify rds device and ensure rds_streams ≤ 4

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