As part of our ongoing projects at obd-de.com, focusing on automotive technology and repair, we’ve recently delved into the fascinating world of vintage car phone systems. Our goal is ambitious but exciting: to develop a Bluetooth Converter For Car setups, breathing new life into these classic devices and bridging them with modern technology. This journey began with an in-depth look at the audio signals running through these systems, specifically pins 1 and 2, which carry analog audio from the transceiver to the handset.
Our initial observations revealed that both pin 1 and pin 2 transmit what appears to be the same analog mono audio signal. However, a closer inspection unveiled a crucial detail: one signal is the negation of the other. When we visualized the waveform of these pins during a button press, which generated a loud beep, it became evident that the signals oscillate around 0V, reaching approximately +/- 0.3V at peak volume.
This dual-wire approach, where one signal is negated, is a known technique for transmitting audio. The true audio signal is encoded in the voltage difference between the two wires. This method offers a significant advantage: common-mode noise rejection. Any interference affecting the wires is likely to impact both in a similar manner, thus canceling out when the difference is calculated and preserving the integrity of the audio signal. This is particularly beneficial in the electrically noisy environment of a car.
However, the complexity doesn’t end there. Car phone handsets intelligently switch audio output between a loudspeaker and an earpiece. Intriguingly, pins 1 and 2 serve as the audio source for both speakers. This implies that the handset possesses a mechanism to determine the intended speaker for each sound. This selection isn’t solely based on the handset being on or off hook. For instance, even when off hook, some sounds like call failed tones are directed to the earpiece, while button press sounds are routed to the loudspeaker. Furthermore, button press sounds can interrupt earpiece tones, suggesting a priority system where only one speaker can be active at any given time.
To understand this speaker selection process, we investigated further and discovered a peculiar periodic “noise” present on the lines when no audio is playing and the handset is on hook.
This noise appears deliberately structured, resembling a digital message. Crucially, unlike the audio signal, this noise is identical on both wires, not negated. We hypothesize that this digital signal is used to communicate the “type” of sound being transmitted, enabling the handset to direct it to the appropriate speaker. This signaling could even occur simultaneously with audio transmission, allowing for interruptions like momentarily playing a button beep through the loudspeaker while a call tone is active in the earpiece.
Based on these observations, we propose that pins 1 and 2 carry two distinct signals:
- Analog Audio: Represented by the difference between the signals on pin 1 and pin 2 ([pin 1] – [pin 2]).
- Digital Signal for Audio Destination: Encoded in the sum of the signals on pin 1 and pin 2 ([pin 1] + [pin 2]).
By adding the signals from both wires, the analog audio component should cancel out, leaving a baseline around 0V punctuated by occasional blips representing the digital message. We speculate that these blips might represent binary data, with positive blips as ‘1’ and negative blips as ‘0’. Decoding this digital signal presents a significant challenge in our project to create a functional bluetooth converter for car phones. Similarly, emulating this signaling when routing audio from a Bluetooth chip to the handset will be equally complex.
UPDATE: Further investigation revealed that the observed “digital signal” spikes are actually noise originating from pin 5, which carries serial data from the transceiver to the handset. This clarifies that the speaker selection mechanism might be communicated through a separate serial data channel, rather than being multiplexed on the audio lines themselves.
Moving forward, our next steps involve capturing and decoding as much of the digital communication on pin 5 as possible. This will be crucial for understanding the speaker control commands and other functionalities. We are also starting to plan the development of a test device capable of generating digital messages for experimentation and deeper analysis. While audio integration remains a key objective, we are prioritizing the digital communication aspect initially. Our immediate milestone is to successfully dial a number on the handset and trigger the corresponding Bluetooth command on a modern cell phone. Achieving this core functionality will pave the way for tackling the audio integration and completing our bluetooth converter for car project.
We are eager to hear from anyone with expertise in car phone systems, audio signal processing, or Bluetooth integration. If you have insights, advice, or relevant knowledge that could help validate or refine our assumptions and plans, please share your thoughts! Your contributions will be invaluable as we progress in this exciting endeavor to modernize classic car phone technology with a bluetooth converter for car applications.
