FYI, BMW information regarding the radio service menu. This should be the same for CD53 (in-dash cd/radio) and BM53 (Radio for NAV).

Service Mode Functions
1. Serial Number: Display of the radio serial number.
2. Software version: Display of the radio software version. Displayed as
(calender week, year, version)
3. GAL: Speed-sensitive volume control. Can be adjusted from level 1-6 using the 6 preset audio buttons. Vehicles equipped with DSP do not use this feature.
4. Field strength and Quality (F/Q): The station currently displayed can be
assessed for field strength and quality. An “F” (i.e. F15) number is used to indicate the strength of the signal being received by the radio. This is a good test of the antenna system, station signal, and the radio itself. A “Q” (i.e. Q-00) number is used to determine the quality of the radio station including both the audio and RDS signal if applicable.
5. DSP: This function provides information about whether the vehicle is fitted with DSP. The value is displayed as a one (fitted) or zero (not fitted) and is communicated by the DSP amplifier via the I/K bus.
6. TP Volume: Provides adjustment for traffic report minimum volume. Not for US.
7. AF: Alternative Frequency tracking setting. Not used in the US.
8. Area: Used to select the appropriate market setting (USA, Canada, Europe, Japan and Oceania). Adjust using the pre-set buttons.
9. Index: Display of the revision index.