Initial investigation into pins 1 and 2, designated for analog audio from the transceiver to the handset in a car phone system, reveals intriguing insights. Observing the waveform on these pins when a button is pressed, producing a loud beep, shows a unique signal characteristic.
Both wires transmit the same monophonic analog audio signal, yet with a crucial difference: one signal is the inverse of the other. This waveform analysis indicates the signal is centered around 0V, with peaks reaching approximately +/- 0.3V at maximum volume. This dual-wire approach, employing signal negation on one wire, is a recognized technique to transmit signals. The actual audio signal is represented by the voltage difference between these two wires. This method effectively minimizes interference, as any noise affecting the wires is likely to impact both equally, thus preserving the voltage differential and the integrity of the audio signal.
However, the system’s complexity deepens when considering audio output destinations. The handset intelligently switches between a loudspeaker and an earpiece for sound playback, utilizing these same two wires as the audio source in both scenarios. This implies a mechanism for the handset to discern where to direct the sound. The switching logic isn’t solely based on the handset’s hook status. Even when off-hook, certain sounds, like a call failure tone, are routed to the earpiece, while others, such as button presses, still utilize the loudspeaker. Notably, button press sounds can interrupt earpiece tones, suggesting a single audio output pathway at any given moment.
The key to this speaker selection might lie in an unexpected signal observed when no audio is playing and the handset is on-hook. A periodic “noise” pattern emerges on the lines.
This recurring signal appears deliberate, resembling a digital communication rather than random noise. Importantly, this signal is consistent across both wires, without the negation seen in the audio signal. Initially, it was hypothesized that this could be a digital signal conveying information about the upcoming sound type, enabling the handset to choose the appropriate speaker. This signal could potentially be superimposed on the audio signal, allowing for interruptions like button beeps during an earpiece tone.
Based on these observations, a dual-signal hypothesis was formed:
- Analog audio: Defined 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 would cancel out, leaving the digital message.
However, an update revealed that these “digital signal” spikes are actually noise originating from pin 5, which carries serial data from the transceiver to the handset. This noise was mistakenly interpreted as a deliberate digital signal for speaker selection.
The project’s next phase shifts towards capturing and decoding the actual digital communication on pin 5 to understand the control mechanisms. Planning a test device to generate digital messages for experimentation is also prioritized. Audio integration, while crucial, will be addressed subsequently. The immediate milestone is to successfully initiate a call by dialing on the handset and triggering the corresponding Bluetooth command on a modern cell phone via a Bluetooth adapter. Achieving this digital communication link is seen as the primary step before tackling the complexities of audio.
Expert advice and insights are welcome, particularly regarding strategies for capturing and decoding digital communication in this car phone system and methods for emulating and injecting signals for testing purposes.
