Hello,

Today, I'm talking to you about the sound system in the 2014 Mercedes Benz GLK and how to improve it. When upgrading to an Android or Linux screen, will the sound quality be reduced? Through this post, you will better understand how the sound system in your car works and be able to choose a screen upgrade that suits your needs and purposes.
The default audio system on the standard GLK model features a CD head unit and a 5.8-inch color screen.
The front doors include 2 tweeters, 2 midrange speakers, and 2 woofers, while the rear doors include 2 coaxial speakers.
For the higher-end version, it is equipped with a Harman Kardon 7 speaker system with higher-quality speakers, more speakers, and an additional subwoofer located in the spare tire well.

Look at the first diagram, which shows the typical audio source connection architecture of a GLK with a screen and DAC upgrade.


With the default screen, the sound quality is decent, but this screen does not have Carplay. Upgrading the screen with Carplay functionality may affect the original sound quality.
In the diagram, we have four ways to feed the audio source into the head unit's amplifier system.
Suppose we listen to music on a phone with the best music source (lossless formats, Apple Music Hires, etc.). Let's see how the audio flows when connecting this phone to the car's screen and amplifier.
Number 3 and number 4: When installing a new screen, the screen will have a separate audio processing module – used to process audio sources into the head unit and music files plugged into the head unit's memory (including USB and internal memory). Path number 3, your phone will connect to the screen via Bluetooth; Path 4 – via USB cable or Wi-Fi for wireless CarPlay connection. Thus, the audio is processed once by the screen's audio processor and converted into an analog signal sent to the amplifier via the AUX port. If your screen is a high-end model, this may be acceptable, but with low-quality Chinese-made screens, this process can degrade the source audio quality.

If your screen allows the digital signal to be sent for external processing, we have a better solution (marked SPDIF in the diagram). In this case, the screen acts as a display, and the audio pre-processor then sends the digital signal for external processing. Audio quality will no longer depend on the screen.
Number 2 and number 1: Fortunately, some Chinese monitor models on the market allow the monitor to function solely as a display, with the phone's audio transmitted to the amplifier via Bluetooth or a cable to the AUX port. The audio signal remains unchanged from the source and goes directly to the car's amplifier.

If your monitor has this mode, we have an additional option: adding an external decoder for the GLK (Option 2)

I'll delve into this case a bit more because it's the most effective solution among the four options.

The system connection diagram will be simplified as shown in the figure.


Audio from the phone will be transmitted to the DAC device via the Bluetooth LDAC protocol, or via a USB cable connected to the DAC device. Some devices support both communication standards.
If you don't mind the complexity of cables, using a cable will save you money compared to buying a device with LDAC protocol.
Through the DAC device, you will have an analog output signal and feed it into the AUX input of the amplifier (I use an MMI cable to a 3.5mm jack for my GLK). With a DAC device featuring a high-quality decoding chip like the ES9018, ES9038, ES 9039, etc., you'll have a high-quality audio source that's more than capable of powering your car's amplifier, and you can use it for future upgrades with higher-end amplifiers and speakers.
Thus, with a screen featuring passthrough/display-only functionality, you can fully upgrade the audio source for your GLK's sound system.
Reference costs for my system:
- 12.3-inch Linux screen from China with the ability to use the car amplifier's original audio: 230%
- DAC supporting LDAC and ES9039 decoding chip: 50$
- Audio Technica 3.5-3.5 dual male signal cable: 8$
- MMI to 3.5 cable: 5$
- Type C to Type C cable for iPhone to connect to DAC: 5$
Total upgrade cost: 298$
This is the cost for self-build and self-installation.

The video on my car: https://photos.app.*******/gbNQLvRjQZyKGdrb9

References
https://mnaganov.github.io/2020/03/i...k350-x204.html
https://mnaganov.github.io/2020/04/i...k350-x204.html