Reverse engineering classic Car Phone systems presents a fascinating challenge, especially when aiming to integrate modern Bluetooth technology. Initial investigations into the audio signals of a car phone handset, specifically pins 1 and 2 connecting the transceiver to the handset, revealed some intriguing complexities. This analysis focuses on understanding these signals to pave the way for seamless Bluetooth integration in vintage car phones.
The first observation involved examining the waveform on pins 1 and 2 while a button was pressed, producing a loud beep. The resulting waveform, as depicted below, showed an identical analog mono audio signal on both wires, yet with one being the negation of the other.
This differential signal approach is a well-known technique for transmitting audio over two wires. By sending the signal and its inverse, the true audio signal is represented by the voltage difference between the two wires. This method is particularly effective in noisy environments, like a car, as any interference picked up along the wires is likely to affect both wires equally, thus cancelling out when the difference is calculated. The signal was found to be centered around 0V, with peaks reaching approximately +/- 0.3V at the highest volume.
However, the system’s sophistication became apparent when considering the handset’s ability to direct sound to either the loudspeaker or the earpiece. Both speakers draw their audio from these same two wires, implying a mechanism for speaker selection beyond simply on-hook or off-hook status. For instance, even when off-hook, certain sounds like a “call failed” tone are routed to the earpiece, while button press beeps are still played through the loudspeaker. Intriguingly, button press sounds can interrupt earpiece tones, suggesting a priority system and single-speaker output at any given moment.
The key to this speaker selection might lie in another signal observed on the lines when no audio was playing and the handset was on-hook. This signal appeared as periodic noise, identical on both wires and not negated, unlike the audio signal. This is illustrated in the image below:
Initially hypothesized as a digital signal dictating the “type” of sound and its destination speaker, further investigation, detailed in a linked forum post, revealed this “digital signal” to be noise originating from pin 5, carrying serial data from the transceiver to the handset. This noise, superimposed on the audio lines, initially misled the analysis.
Therefore, the two wires primarily carry a differential analog audio signal ([pin 1] – [pin 2]). The speaker selection mechanism is likely controlled through a different communication channel, possibly via the serial data line initially mistaken as a digital signal riding on the audio lines.
Moving forward, the focus shifts to capturing and decoding the digital communication occurring, and devising a testing apparatus to generate digital messages for deeper system exploration. Audio integration with Bluetooth will be addressed subsequently. A significant milestone will be achieved upon successfully dialing a number on the handset and triggering a Bluetooth command on a modern cell phone to initiate the call. This breakthrough will provide strong motivation to tackle the audio aspects of the project.
Expert insights and advice remain highly valuable at this stage. Any knowledge or experience relevant to confirming or refuting these assumptions and plans for integrating Bluetooth into this classic car phone system would be greatly appreciated.
