# 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. Or set to 2 for Polar Stereo

- **`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).

#### 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

```ini
# 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

1. **RDS Streams**: Maximum of 4 RDS streams are supported. Exceeding this limit will cause an error.

2. **String Length**: Device names are limited to 63 characters maximum.

3. **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.

4. **Audio Levels**: Keep audio levels reasonable to prevent distortion. Start with conservative values and adjust as needed.

5. **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