With information from the Yahoo HackTheIBus group, I have been analyzing the data which the factory head unit sends to the Harmon Kardon amplifier, to persuade it to work.
My work colleague Christian built this circuit for me, which allows a PC to monitor the communications:
I used some monitoring software (now downloadable from Sourceforge), written by Franck Touanen, to monitor the data messages sent from the head unit at turn-on, while trying different bass/treble/fade/balance/volume/mode settings.
So I can now identify what data is sent to the HK amplifier for each of the above settings. The audio control messages are curiously designed; in particular the volume control is very strange – it has a range of 63 steps, and data messages can be sent to adjust it up or down from 1 to 15 steps. So at turn-on a number of “volume up” data messages are sent (for example: if the volume is at position 33, two “up 15” messages are sent, followed by one “up 3” message).
My next task will be to make a circuit which transmits the appropriate messages, then see if it enables me to run the Harmon Kardon amplifier without the factory head unit.
ALSO:
Jayson Walker (owner of a Chili Red MCS with many performance enhancements) demonstrated his stereo system today. He has a DVD playing head unit which plays through the navigation screen (some details here).
He has a JL Audio amplifier but does not have subs in his car; yet the bass really gets the seats vibrating. Clearly the Harmon Kardon system is missing some low end … I might yet have to follow the lead of “scooterboy” and add a small sub to my car.
M&I / GBMINI 