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BlueBus provides several audio configuration options to integrate with your BMW’s audio system and achieve the best sound quality.

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DAC audio gain

The DAC (Digital-to-Analog Converter) gain setting controls the output volume level from the BlueBus PCM5122 DAC. EEPROM address: 0x41 (CONFIG_SETTING_DAC_AUDIO_VOL_ADDRESS)

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Gain values

The gain is stored as a single byte value with the following range:
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Minimum
0x18 (24)
+12dB gain - Maximum output level
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Unity
0x30 (48)
0dB gain - No amplification or attenuation
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Maximum
0xFF (255)
Approximately -101dB - Minimum output level
The gain value is calculated as:
  • Values below 0x30: Positive gain of +(0x30 - value) / 2 dB
  • Value of 0x30: 0dB (unity gain)
  • Values above 0x30: Negative gain of -(value - 0x30) / 2 dB

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Default gain

The default DAC gain is 0x44 (68 decimal), which equals -10dB. This setting is applied during firmware upgrades at config.h:48.

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Adjusting DAC gain

  1. Navigate to Settings → Audio
  2. Select DAC Gain
  3. Rotate the knob to adjust the gain value
  4. The display shows the gain in dB format (e.g., “DAC Volume: -10dB”)
  5. Press the knob to save
The setting is applied immediately to the PCM51XX DAC via PCM51XXSetVolume() at menu_singleline.c:374.
  1. Scroll to “DAC Volume” setting
  2. Press edit/save to enter value scroll mode
  3. Scroll to adjust the gain value
  4. Press edit/save to save and apply
The display shows values like “DAC Volume: -10dB” or “DAC Volume: +6dB”.
The minimum gain value is enforced at 24 (+12dB) to prevent excessive output levels that could damage speakers or amplifiers. Any value below 24 is automatically corrected to 24 before saving.

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DSP input source

If your BMW is equipped with a DSP (Digital Signal Processor) audio system, you can configure the input source. EEPROM address: 0x42 (CONFIG_SETTING_DSP_INPUT_SRC_ADDRESS)

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Available options

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Off (Default)
0x00
DSP uses default input source - BlueBus does not control DSP input
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Digital (S/PDIF)
0x01
default:true
DSP receives digital audio via S/PDIF optical connection from BlueBusSends I-Bus command IBUS_DSP_CONFIG_SET_INPUT_SPDIF when enabled.
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Analog
0x02
DSP receives analog audio from the vehicle’s radioSends I-Bus command IBUS_DSP_CONFIG_SET_INPUT_RADIO when enabled.

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DSP detection

BlueBus automatically detects if a DSP is present on the I-Bus by monitoring module status. The DSP setting only appears in menus if ibus->moduleStatus.DSP is non-zero. If no DSP is detected, the menu displays “DSP: Not Equipped” and the setting cannot be changed.

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Switching input source

When you change the DSP input source, BlueBus immediately sends the corresponding I-Bus command to reconfigure the DSP. This allows seamless switching between digital BlueBus audio and analog radio audio.

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Autoplay

Control whether BlueBus automatically starts playback when a device connects. EEPROM address: 0x40 (CONFIG_SETTING_AUTOPLAY_ADDRESS)
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Off
0x00
Bluetooth devices do not automatically start playback when connected
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On
0x01
Bluetooth devices automatically start playback when connected

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Lower volume on reverse

Automatically reduce audio volume when the vehicle is in reverse gear. EEPROM address: 0x44 (CONFIG_SETTING_VOLUME_LOWER_ON_REV_ADDRESS)
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Off
0x00
Volume remains unchanged in reverse
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On
0x01
Volume is automatically reduced when reverse gear is engaged
This feature monitors the I-Bus for reverse gear status and temporarily lowers the volume to help you hear parking sensors and surrounding sounds.

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Reading DAC configuration

The current DAC volume setting is read from EEPROM and applied during PCM51XX initialization: 
// From pcm51xx.c:95
unsigned char volume = ConfigGetSetting(CONFIG_SETTING_DAC_AUDIO_VOL);
PCM51XXSetVolume(volume);
All audio settings take effect immediately when saved. DAC gain changes are applied directly to the PCM5122 chip, and DSP source changes send I-Bus commands to reconfigure the DSP module in real-time.

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