Kenwood CD-Changer Emulator

Many Kenwood-radios do have a connector for an external CD changer. The cable between the headunit and the changer usually has 13 pins and combines the wires for the power supply, the audio and the data.

Audio-Players (MP3-player, soundcard) with line-level can be attached to the headunit this way. But the changer-input can't be selected as audio-source as long as the headunit hasn't detected a supported device. This can be a AUX-cable and of course a CD changer. Sometimes receivers for DAB or Sirius Radio are also attachable.

AUX-in

Kenwood intentionally allows the changer input as auxillary input. There is an official cable available: The CA-C1AX. A version for DIY is simple and only needs a 13-pin DIN-plug, a diode and a resistor. The three audio wires (left, right, ground) have to be connected to a Audio/RCA-jack.

 

CDC-Emulator

The data-lines of the changer-cable are used to send control commands from the headunit to the changer and to receive status information from the changer.

So it is possible to control the media player running on a CarPC for example using the buttons of the headunit. The ID3-Tgas of the currently played song can be shown on the display of the headunit.

Kenwood uses a SPI-interface with additional signaling-lines for both transmission directions. This hardware interface didn't change for a long time, but there are some different variants of the used protocols.

Kenwood Protocol B

My Kenwood headunit is using the protocol B. Which protocol is used for which headunit can be seen here: http://www.kenwood-electronics.fr/WebFiles/File/fr/download/Dossiers%20techniques/Compatibilit%C3%A9s%20Sources_Changeurs_2006.pdf

The internet has some informations of the protocols and their command set (http://www.adriangame.co.uk/kenw-dc-pro.html or http://www.mictronics.de/projects/cdc-protocols/#Kenwood). It should be straight forward to use a microcontroller in a way, that it behaves like a Kenwood CD changer to the Kenwood headunit. Unfortunately the informations I found were not complete. The configuration of the SPI was missing. Though SPI is a standard interface, there are some things that can be freely defined. Who is the master of the SPI-bus who generates the clock-signal. What bitrate is used? What is the polarity of the clock-signal? At which time is a bit valid on the bus?

So I tried many different configurations, but wasn't able to get a working connection from the microcontroller to the headunit. Fortunately I got access to a Digital-Osziloscope for a few hours. So I could examine the communication between a real changer and the headunit.

First finding: The headunit is the SPI-master, the clockrate is 250 kHz, the clock-line is high at idle, the bits have to be valid on the bus at the rising edge of the clock-line.

Second finding: Before data is sent via SPI, there is something like a handshake.

 

Connecting a CarPC

Understanding the Kenwood protocol and being aware of the used timing, it is possible to program a microcontroller as link between the headunit and the PC. The used Atmel controller is programmed in a way that it behaves like a CD changer to the headunit. Additionally there is a connection to the PC using the serial interface. To use the buttons of the headunit to control for example the media player Centrafuse, I developed a Centrafuse-plugin.

 

 

The PCB

 

Downloads

Layout (Eagle)

Schematic (Eagle)

The software for the Atmel is available on request.