The Challenge: Upgrading an Older Car's Screen
With the development of display technology and the emergence of oversized screens with ultra-high resolution, it's often necessary to replace the original, low-resolution display in some older cars to get a higher picture quality.
For example, pre-2015 BMW, Mercedes-Benz, and Audi cars often have small, low-resolution displays. A popular upgrade is to replace the original screen with a modern unit. After replacement, the new display must still show original car information, such as tire pressure, maintenance alerts, and radio data. This new display is an independent system with its own audio output module. However, it outputs a low-level signal (typically less than 2 volts), similar to headphone audio. This signal must be fed into the car's original amplifier to drive the speakers.
How to Connect Audio to the Stock Amplifier
So how is this audio signal sent to the stock power amplifier? Here are five common methods:
Method 1: AUX (Analog Auxiliary Input)
The first and most common way is to use the original car's AUX channel, which is an analog audio input. When you select the "AUX" source on your stock system, the amplifier will play the audio from the new display. While this is the most common method, as an analog signal, it can sometimes be susceptible to electrical interference or noise.
Method 2: USB Input (Digital)
Since 2013, some Mercedes-Benz models removed the AUX channel, and BMW's EVO system followed suit after 2016. In these cars, you must use the USB port for external audio. This requires converting the display's analog audio to a digital signal using a USB audio adapter. This digital signal is then sent to the original radio's USB port. A digital connection is generally more resistant to noise than AUX, offering the potential for better sound quality.
Method 3: Bluetooth Audio
Since 2013, most premium cars include Bluetooth for music and phone calls. The aftermarket display can have a built-in Bluetooth transmitter that pairs with the car's original Bluetooth receiver. Audio is then streamed wirelessly. This is also how modern CarPlay and Android Auto systems can work, using a "separation of sound and picture" principle: the picture is on the new screen, while the sound is handled by the car's native Bluetooth. This method is very convenient and offers excellent sound quality, though this can depend on the supported Bluetooth codecs (e.g., AAC, aptX).
Method 4: Fiber Optic Interface
A common misconception is that the fiber optic harness behind the head unit is for audio; it's actually for data like time and vehicle information. True high-fidelity audio transmission over fiber optics is only possible in cars equipped with a factory fiber optic amplifier. This requires a dedicated fiber optic box, also known as a MOST (Media Oriented Systems Transport) interface. The new display's analog audio is fed into this box, which converts it into a signal the car's fiber optic system can understand, effectively simulating an original audio source. This method delivers the best possible sound quality.
Method 5: External Amplifier (Bypass)
This method bypasses the stock amplifier entirely. The audio signal from the aftermarket display is sent to a new, external amplifier, which then powers the original car speakers. This gives you more control over power and tuning but is a more involved and costly installation.
Comparison and Recommendation
All five methods can solve the audio problem for an aftermarket display. Comparatively, the fiber optic box offers the best sound quality, but it is also the most expensive. A close second, and the approach I would recommend for its balance of quality and convenience, is using the original car's Bluetooth. The native Bluetooth systems in these cars are typically high-quality, providing clear, noise-free audio for both music and calls.
The Future of Car Audio Integration
Whether on a long trip or a daily commute, audio quality has a big impact on our driving experience. Improving it is a worthwhile effort. I believe the ultimate solution for the future will be aftermarket displays that can output a direct digital or fiber optic signal. This would allow for a pure, lossless connection to the car's digital amplifier system, providing the absolute best sound quality possible.
What do you think?
