esp32-cdc-faker

An ESP32 based fake CD changer for various car radios.

Introduction

For now this is only an idea but will be my side project in 2017. This project will consist of a small piece of hardware and some software. It will be able to wirelessly connect a Bluetooth or perhaps WiFi enabled device to various car radios. As for my concerns I will only be able to test this project with the ~~Dance~~ Stream MP3 radio of my Skoda. But since the important interfaces (for this project) are somewhat standardized throughout similar aged radios among different VAG brands my guess is that this project will also work for bunch of other radios.

Boundaries

The main goal of the esp32-cdc-faker clearly lies in bringing wireless connectivity to the car radios. Neither USB ports nor any kind of SD card slots will be provided by this project. If you are looking for such functionality jeez just put dem mp3's on your phone. No seriously there are other products available that already offer a wide range of interfaces. Though none of these only offer Bluetooth or WiFi which makes them bulky and requires them to be placed in the glovebox or somewhere in the center console. They are also mostly all quite expensive if you only need wireless conenctivity like I do.

Rationale

There are no compareable products available that only offer wireless connectivity for a reasonable price. Instead other products feature USB ports and SD card slots an are therefore bulky and dont fit just behind the car radio.

Planned features

Transfer audio from a Bluetooth enabled device to the car radio
Let the radio control the Bluetooth enabled device's audio by relaying button presses of play, pause, next track and so on
Perhaps support WiFi audio transfer via AirPlay, DLNA or similar
Keep the hardware small so that it fits behind the radio

Not planned features

Playing audio from USB or SD cards
Adding a microphone for handsfree. I guess the phones built in microphone is the better option since those integrate noise reduction. See Qualcomm Fluence for example

Hardware

The hardware will be built around the Espressif ESP32. This neat little IC bundles Bluetooth and WiFi connectivity aswell as more than enough computing power and IOs for my purposes. The ESP32 will be accompanied by an I2S DAC for analogue audio output to the car radio. The integrated DACs of the ESP32 do not seem suitable for my purposes due to their lowish (8 bit) resolution and possibly slow sampling rate. I2S DAC are commonly used among audio related projects featuring the Raspberry Pi and are therefore widely available plus they wont set you back much. During the project I'll be using a development board like the this and some cheap breakout board DAC. For the finnishing touch I planned to design a PCB for all the components and publish that on easyeda.com so that there will be no need to buy multiple modules but only one PCB, some ICs and passive parts.

Status

Turned out 2017 was quite short and 2018 doesn't seem to be any longer so this is still heavily work in progress. But there is progress. A2DP sink to I2S is working. Implementation based on the ESP32 examples provided by Espressif was pretty straight forward. I initially used the ESP32's integrated DAC which sounded not so well. So I got me some cheap I2S DAC from China and initial testing with some old scrap laptop speakers sounded way better using the dedicated DAC. Next step is to actually get the sound out of the car radio which requires it to enable the CDC input. This will be done talking SPI to car radio. On my device the respective pins are names DATA-IN and CLK which are MOSI and CLOCK of the SPI. MISO is not existing because the CDC uses a different interface to talk to the car radio. It's probably the NEC protocol which is commonly used in infrared remote controls. Nice thing about that is that there again is an exmaple for ESP32 using the RMT peripheral to receive NEC modulated signals. Lucky me. I've been waiting for a logic level converter the past few days which has just arrived. So tinkering can go on now.

Additional ressources

Plenty information on the ESP32 can be found on esp32.net
As an external DAC one could use this although the audio quallity needs to be evaluated
The Espressif community forum for the ESP32 is found here

Software

Some work of the software development part has already been done as some smart people already figured out the protocol that the radio and CD changer are speaking. Information on that protocol can be found in the additional ressources below. What's left is:

Port the radio/CD changer protocol for use with the ESP32's FreeRTOS based OS
~~Figure out how to use the bluedroid stack to turn the ESP32 into an A2DP sink and route the incoming stream to I2S~~
~~Figure out I2S audio output on the ESP32~~
Figure out how to to turn the ESP32 into an AVRCP
Since Android uses bluedroid a look at the source code may help
...
Make everyting work nicely together
Apparently A2DP sink to I2S works already: example on github.com

Additional ressources

Information on the radio/CD changer protocol by Martin S. can be found on martinsuniverse.net (german)
Implementation of the radio/CD changer protocol on a Raspberry Pi here: dev.shyd.de
ESP32 firmware, SDK and exmaples on GitHub
Some information on how Bluedroid is used on Android CaVa on Bitbucked
Android docs on the Bluetooth implementation source.android.com
Android's JNI implementation to expose Bluetooth to Java clients android.googlesource.com
The patches to Android that enabled A2DP sink functionality:
SBC decoder that is required to actually decode incoming audio data can be found here

Requirement analysis

See the project here.

Legal issues

I think there are four different legal issues regarding this project.

1. Usage of the properitary, reverse engineered protocol used to communicate with the CD changer.

My guess is that this won't be an issue as long as no binaries of the protocol implementation are distributed and this project is only used non-commercially. But don't nail me down on this.

2. Directly related to the first issue: Figure out what open source license fits this project.

I really don't even have the slightest clue what license covers soft- and hardware designs aswell as usage of properitary protocols best. Any advice welcome.

3. Voiding the warranty of the radio or even the car when using the hardware designed during this project.

I suppose that you will at least void the warranty of your radio when you use the hardware of this project and break anything. Thus don't use it if you don't understand what you are doing. For my part I don't care voiding anything since the car is old and out of warranty anyway.

4. Usage of this unregistered hardware in public traffic.

I assume in some places of the world it is illegal to use hardware in your car that is untested by authorities and not licensed by the car manufacturer. At least while participating in public traffic. For example in Germany the TÜV would probably need to accept the product or at least issue a clearance certificate so that it can be used legally in public traffic.

nschikora / esp32-cdc-faker