Tuesday, June 14, 2005

Car Stereo iPod input

Do you have a car stereo that can control a CD changer? Would you like to make use of this input as a way to connect your MP3 player, portable CD player, or other device to your system? If so read on...

A simple idea

In 1997 I bought a new Mazda Protege, and it came with a standard AM/FM Radio/Cassette deck, and it has controls for a external CD jukebox. At the time, the price for these players was more than I wanted to spend. I already had a portable 'discman' player, and I wanted to find a way to connect the discman to my car stereo. Sure I'm aware of the cassette adapters and the FM modulators, but I wanted to make a direct connection to the 'head unit'. I removed the head unit and took a peek at the connector, only to find that it is totally un-labeled and has 16 contacts! I purchased the Mazda service manual for the car, and looked up the wiring for this connector. The manual shows only the speaker and power connectors, but gives no indication of the wiring for this mysterious connector. I searched everywhere, but I could find no helpful information. One car stereo installer I spoke with suggested that the connections to external CD players is purposely made in such a way as to prevent you from using another manufacturer's CD changer (or discman) - forcing you to buy one of their units instead of the competitors. I gave up on my search and used one of those annoying cassette adapters.

My second attempt

Several years later got a portable MP3 player which sparked a second attempt to find information on this connector. I opened up the head unit, and found some stenciling on the bottom of the circuit board. With that information, I had a place to start. The stenciling indicated the Left and Right audio inputs and Signal Ground. But this still doesn't solve the problem. The head unit has to know that there is a CD player attached - or it refuses to accept the audio inputs. When you press the AUX button on the front panel the display flashes "no CD".

I assumed that there must be some sort of jumper needed that would fool the head unit into thinking that there was a CD changer attached. Using some the information that I got from the stenciling -- and some trial and error -- I found a solution!

It wasn't exactly what I expected. As I said, I was hoping to fool the head unit into thinking that there was a CD player attached. I never achieved that, but I stumbled in on a hidden feature - which I'll call the LINE-INPUT mode. When I jumpered two pins on the connector, the display showed the word 'LINE' and I could hear my MP3 player through the car speakers!

Doing it yourself

Ok - if you've read this far - you probably want to know how to do this yourself. But first a few notes. If there was one standard interface to CD changers - then this would work for everyone - unfortunately chaos reigns even with units made by the same manufacturer. This means that my wiring info most likely won't work if your head unit is different than mine. But you may be able to learn from this example and apply it to your situation. Of course the standard disclaimer applies. If you attempt this and fry your equipment - I can't be held responsible.

Determine if this will work for you

I would recommend that you start by comparing the physical size and shape of the connector on the back of the head unit to the picture below.

Rear view of head unit

Close up view of mysterious CD changer input connector

The label from the bottom of my unit for reference:

If your connector matches the above photos you may be in luck, I would also recommend checking voltages on the pin labeled ACC (should see about 12V) and and 0 volts on the pin labeled P-GND when you have the car key in the ignition and turned to to Accessory mode.

The Detailed info
There were no pin numbers associated with the connector, so I arbitrarily assigned numbers to the pins as shown below:

Here's the signal names that I found stenciled on the bottom of the circuit board, and the
functions that I determined by trial and error.

The Mystery Connector (for CD Changer)
PIN Signal name Function
1 NC Not used
2 TNS Unknown (something to do with dimming lamps?)
3 BATT Constant 12V power.
4 ACC 12V DC when Key is in accessory mode
5 D-GND Data Ground (for use with BUS pin #13)
6 AUX-O Unknown (Let me know if you know about this)
7 L-IN Left audio input
8 R-IN Right audio input
9 NC Not used
10 -ILL Controls panel illumination. No connection for bright, increased voltage dims the panel light.
11 MUTE Input - Connect this to Pin#3 and the speakers are muted.
12 P-GND Power ground
13 BUS Data bus for communicating with CD Changer
14 AUX-I Input - When connected to Pin#3 the line inputs are selected
15 SET-C Unknown (Let me know if you know about this)
16 S-GND Signal ground for audio inputs

The only pins that we are concerned with are 4,7,8,14 and 16 and the rest can be left alone.

Finding a mating connector

I was in a hurry, so I didn't attempt to find the exact correct connector, I turned to my junkbox and found two connectors that (after cutting to size) could be placed in the connector side by side. It wasn't pretty, but It looks like it will make a good electrical connection. I glanced through an AMP connector catalog and found a connector that looks more appropriate, but even it may need some creative cutting to make it fit properly. The eight pins are spaced at 1/10 inch, and the two rows are separated by 2/10 inch. So there are a total of 16 pins. I put in wires for all of the pins for purpose of my tests, but most pins are not needed, and can be omitted.

Wiring it up

I recommend using shielded cable for the left and right inputs to keep noise down. It is connected to a mini stereo jack (or you could use RCA jacks). As for wiring the AUX-I -- this deserves some explanation. I had hoped to be able to plug in my MP3 player press the AUX button on the front panel of the head unit, and listen. Unfortunately this didn't work. The only way I could get the aux inputs to work is if I have a normally closed button connected from pin 4 (ACC) to pin 14 (AUX-I). For safety sake I put a resistor in series with the button to limit current in the event that the ACC (+12v) accidentally shorts to ground.

The diagram shows a mini plug stereo plug, although I actually
used a panel mount jack, and then used a short cable to connect
to my MP3 player.


With the modification above, you will now find that when your radio is first turned on, you will be in the LINE-IN mode, and ready to listen to your MP3 player, Discman or whatever. If you want to listen to the radio or tape player, press the front panel AM/FM button or TAPE button as usual. If you later decide to listen to your MP3 player again, this is when you will need to press the new button that you've installed.

Humms and Buzzes If you are using battery power for the Discman you should have no problem. If you are using the cigarette lighter for power, you might have problems with hum. This is a ground loop condition which can be fixed with an audio isolation transformer in series with the audio cable (available at Radio Shack Cat.#: 27-054 $14.99).

If you try this - please let me know what your experience is. If you have any more info about the unknown pins on the connector, or I've omitted anything, let bme know, But - please read the FAQ section first, and don't waste your time and mine by asking me about input connectors on other cars. I do not have any more information!

FAQ, Misc and links

Q: I'd like to add a aux input connector to my stereo, but it's not a Mazda. Do you have a schematic for my car stereo connector?

A: (I've received this question several times) I don't have any other information about other car stereos. I discovered the information about my stereo by using google searches and correlating that with the information that I got from opening up my unit.... and some tinkering around. I don't have any other information about other car stereos. I ordered the wiring diagram for my car. It showed the wiring for the speakers, DC power input and antenna, but it didn't have any info on the CD connector.

Q: How do I get access to the connectors in the back?

A: You have to remove the unit.

Q: Why don't you just permanently connect pins 14 and 4?

A: That would work too, but there is one significant disadvantage to that method. If you want to listen to the radio or tape player, you will not be able to change back to the LINE input mode unless you turn off the ignition. Remember that the AUX button on the front panel is totally useless.

Q: What is a "Normally Closed" Button?

A: This is a standard designation in the industry for distinguishing two different button types. A "Normally open" button is much more common, and has the property of being an open circuit normally (when you don't have your finger pressing on the button.) Pressing down on the button
causes the circuit to close. When you let go of the button, a spring pushes the button back up and returns the circuit back to the Normal state (OPEN) Thus the name "Normally open".

A "Normally Closed" button is the opposite. It is a closed circuit normally (when you don't have your finger pressing on the button.) Pressing down on the button causes the circuit to OPEN. When you let go of the button, a spring pushes the button back up and returns the circuit back to the Normal state (CLOSED). Thus the name "Normally Closed".

There are some buttons that have BOTH Normally Open and Normally closed terminals. This would work fine too.

Q: Why not use a "Normally Open" Button instead?
Then you would have to hold your finger on the button while listening to your MP3 player

Q: How do I remove the unit?
A: It's easy once you know how. My head unit has two removable plastic pieces on the front. One on the left, and one on the right. Use a small screwdriver to pop them out. The plastic pieces are merely cosmetic They cover up two holes on either side of the front panel. If you insert a special 'removal' tool, it will release the latch that holds the head unit. I didn't have the 'special removal tool', I just used a couple of long nails (use the thickest nails that will fit in the hole). With the 4 nails inserted in the holes, pull the unit out. It may be difficult to find a place to grab on to. I stuck my finger in the cassette opening for leverage. I found a picture that shows the process.

Q: What is the wiring for the other connectors - Front and rear speakers and power?
A: Here's a diagram. I'm not sure what the pin labeled "to Panel Light Control Sw" is used for.

Reverse engineering

It occurred to me that the head unit must receive a signal from the CD changer on the 'mystery connector' that lets it know that the CD changer is attached. Signals from the head unit would also be sent to the CD changer when the user presses Play / Stop / Track Forward etc. One idea that I had was to borrow a CD changer, plug it in, and monitor the signals on pin #13 (labeled 'Bus') to figure out the protocol. But I never had to do that. I was concerned that even if I did figure out the protocol, it would involve serial data that would be difficult to duplicate - perhaps using a micro-controller. No doubt that is what the folks at http://pac-audio.com/ and did. They reverse engineered the protocol, and designed a little circuit with a micro-controller that re-creates the necessary signals to fool the head unit into thinking there is a CD changer attached. If you check out the sites below, you'll see that they have lots of different models - this must be due to the many different protocols (and connectors) that the different head units have. They cost about $75.00 which is rather high when compared to the price of a new head unit with CD and an aux input connector on it costs about $120.

Can you make one of these yourself? Not likely unless you have access to an appropriate CD changer, and a signal analyzer to capture the signals that are sent by changer to the head unit, and some mirco-controller circuit design and embedded systems experience, and a lot of time.

Here's some links that I came across that you may find helpful:

Maker of car CD interface hardware: http://pac-audio.com/

Miata Wiring info: http://www.miata.net/garage/m2interface.html

I have no connection with these companies, but they looked interesting.
-Converted from Geocities web page on 6/14/2005
-Updated images 6/20/2010