Integrating modern cell phone technology with classic cars presents a unique challenge, especially when it comes to seamless Bluetooth connectivity. Many vintage vehicles lack built-in Bluetooth systems, requiring enthusiasts to explore innovative methods for bridging this technological gap. This guide delves into the initial steps of reverse engineering a car phone system to enable Bluetooth audio, focusing on understanding the existing audio signals and preparing for integration.
Early observations reveal intriguing insights into the audio transmission within older car phone handsets. Analyzing the signals from the transceiver to the handset’s pins 1 and 2 uncovers a dual-wire system for analog mono audio. When a button is pressed, generating a loud beep, the waveform on both pins displays the same audio signal, yet one is negated relative to the other.
This approach, sending the same signal over two wires with one negated, is a common technique to enhance signal integrity. The true audio signal is derived from the voltage difference between the two wires. This method effectively minimizes interference, as any noise affecting both wires equally will be cancelled out in the differential signal. The observed signal is centered around 0V, with peaks reaching approximately +/- 0.3V at maximum volume.
However, the complexity deepens when considering the handset’s dual speaker system – earpiece and loudspeaker. Both speakers draw their audio from these same two wires. This implies a mechanism within the handset to discern which speaker should reproduce the sound, a process more nuanced than simply switching based on whether the handset is on or off hook. Even when off hook, certain sounds like call failure tones are directed to the earpiece, while button press sounds still route to the loudspeaker. Notably, button press sounds can interrupt earpiece tones, indicating a system capable of prioritizing audio output to a single speaker at a time.
The key to speaker selection might lie in a periodic signal observed on the lines when no audio is playing and the handset is on hook. This “noise” appears consistent across both wires, unlike the negated audio signals.
Hypothetically, this could be a digital signal embedded within the lines, signaling the “type” of incoming sound and instructing the handset to direct it to the appropriate speaker. This signal could operate in conjunction with the audio, allowing for interruptions like button beeps overriding earpiece audio.
Therefore, the two wires likely carry a combination of signals:
- Analog Audio: Represented by the difference between pin 1 and pin 2 signals ([pin 1] – [pin 2]).
- Digital Signal for Audio Destination: Potentially encoded in the sum of pin 1 and pin 2 signals ([pin 1] + [pin 2]), where the analog audio component cancels out, leaving the digital message.
Further investigation revealed that these “digital signal” spikes are actually noise originating from pin 5, which carries serial data from the transceiver to the handset. With this understanding, the next crucial steps involve capturing and decoding the digital communication to fully grasp the system’s control mechanisms. Planning a test device to generate these digital messages is essential for further experimentation and control. Audio integration with a Bluetooth chip, while a primary goal, will follow the successful decoding and emulation of these control signals. The initial milestone is to dial a number on the handset and trigger the corresponding Bluetooth command on a modern cell phone, establishing a fundamental link between the old car phone system and contemporary mobile technology.
Any insights or advice from those with expertise in automotive audio systems or reverse engineering are highly welcomed to refine this Bluetooth integration project.
