Fibre optic amplifier in your car: when you need a decoder

There is one situation that comes up again and again in premium cars: someone replaces the original radio with an Android head unit with CarPlay, fits everything properly, starts the engine… and there is no sound at all. Not a single speaker. The screen lights up, navigation works, the phone pairs, but the car is silent. The first thought is that the unit is faulty, and it almost never is: what is happening is that the car has a fibre optic amplifier, and the audio does not travel to the speakers over copper wire but as light.

In this guide we explain what that system is, how to tell whether your car has it and what you need so that an Android head unit works with the factory amplifier and speakers.

What is a fibre optic amplifier?

In a car with a conventional radio, the radio itself amplifies the signal and sends it along wires to each speaker. In cars with a high-end sound system that changes: the radio is only a source, and the amplification is done by a separate box, usually hidden under a seat or in the boot.

To carry the audio from the head unit to that amplifier, many European manufacturers used an optical bus (known as MOST): a fibre cable along which the digital signal travels as pulses of light. It is an excellent system — immune to electrical interference and able to carry several signals down a single cable — but it has one practical consequence: if you remove the original unit, the optical ring is left open and the amplifier stops receiving a signal. And a conventional Android head unit, however good, only has electrical outputs: it cannot speak fibre.

How do I know if my car has it?

There is no single rule, but there are several fairly reliable clues:

• It has a branded sound system: Harman Kardon, Logic7, Bose, Burmester, Bang & Olufsen or similar. It does not always mean fibre, but it is the most common sign.
• There is an amplifier box under a front seat, behind a boot panel or in the spare wheel well.
• On the original radio connector you can see two small dots, side by side, looking like tiny lenses or coloured plugs (often one lighter and one darker). Those are the fibre transmitter and receiver.
• When you pull the unit out, a thin, stiff, transparent or whitish cable appears, very different from the normal cables in the harness.
• Models where it is common: Mercedes ML and GL, R-Class, S-Class and E-Class of certain generations; Porsche Cayenne; BMW with HiFi/Logic7 systems; some Audi and Volvo models with premium sound.

If you are not sure, the easiest thing is to photograph the rear connector of the original radio and send it to us along with the model and year: it is visible at a glance.

The solution: a fibre optic decoder

A fibre decoder sits between the car and the new screen. It picks up the signal from the optical bus, converts it into analogue audio and provides an RCA output that the Android head unit does understand, in both directions. Put simply: it closes the optical ring you have opened and acts as a translator.

With the decoder fitted:

• The original speakers sound again through the factory amplifier, without losing quality or channels.
• The microphone is kept for calls: our decoder includes an external microphone that goes on the sun visor or the pillar.
• Nothing in the car has to be cut: it connects in line with the existing connectors.
• The rest of the car stays the same: climate control, steering wheel controls and dashboard warnings do not depend on the audio.

We have two key parts for this: the fibre optic decoder for Mercedes ML, GL, R-Class and Porsche Cayenne, with RCA output and external microphone, and the 6-metre extension cable for fibre optic amplifiers in BMW, designed for installations where the amplifier is a long way from the radio slot.

How is it installed?

1. Disconnect the negative battery terminal before you start.
2. Remove the original unit and locate the fibre connector: the two optical terminals in the harness, usually held in a separate tab.
3. Release the fibre carefully. Do not bend it sharply: it is the one cable in the car that can be ruined simply by forcing it, because the light stops passing through.
4. Connect the fibre to the decoder and run the RCA cable from the decoder to the audio inputs/outputs on the new screen.
5. If the amplifier is far away (boot, under a seat), use the extension cable so nothing is under tension.
6. Place the external microphone on the sun visor or the pillar and run its cable along the edge of the roof lining.
7. Reconnect the battery, start the car and check sound on all four speakers, volume and calls.

If you reach the fibre connector step and are not confident, that is the moment to hand it over to an installer: the rest of the job is simple, but fibre is best handled once and properly.

How does the cost compare with the alternative?

The alternative many workshops offer when they do not want to touch the fibre is to replace the whole system: new amplifier, new speakers and new wiring. That is a full audio rebuild and the quote shows it. A decoder is a small part that solves the problem while keeping the amplifier and the speakers you already paid for with the car. You can see the current price on the product page; all our items include a warranty, after-sales support and free shipping across Europe.

Frequently asked questions

Do I lose sound quality with the decoder?
You do not lose the amplifier or the speakers, which is where the quality of the system lies. The decoder performs the conversion needed for the screen and the amplifier to understand each other; the result is very close to the original sound, with the advantage that you can now use the equaliser on the screen.

Does any decoder work in any car?
No. Each family of vehicles uses its own connector layout and its own protocol. That is why the product page lists the models covered and why we ask for the exact year if you have any doubt: it is better to confirm it than to return a part.

And if I fit the screen without a decoder?
Normally there will be no sound at all, or sound from only one channel or at very low volume. Nothing breaks, but the car stays silent until the optical ring is closed correctly.

Do hands-free and the reversing chime still work?
Yes. The decoder includes an external microphone for calls, and the car's acoustic warnings continue to come through their usual path.

What if my car does not have fibre?
Then you need none of this: the model-specific screen simply plugs in and sounds. If you are still unsure what kind of system you have, also read our guide on the CAN Bus and decoder boxes, which is a different component and sometimes gets confused with this one.

See fibre optic decoders

You can see the fibre optic decoder for Mercedes ML / GL / R-Class and Porsche Cayenne and the 6-metre extension cable for BMW on their product pages. If you do not know whether your car has fibre, send us the model, the year and a photo of the original radio connector: we will tell you exactly what you need before you buy anything.