A car built before phone projection existed still has a working stereo, and there are three sensible ways to feed it. They are not equivalent, and the differences are not marketing — they come from how each one moves the signal.
AUX: the analogue option, still the best sounding
A 3.5 mm cable carries the analogue signal straight into the head unit’s amplifier. No compression, no radio link, no codec.
If your car has an AUX socket, this is the highest-quality route available, and it costs almost nothing. The limits are practical rather than sonic: you have a cable, the phone has to stay plugged in, and modern phones without a headphone jack need a USB-C adapter with a DAC built in — the cheap passive ones only work on phones that still output analogue over USB-C, which most no longer do.
The buzzing problem
The classic AUX complaint is a whine that rises and falls with engine speed, appearing only when the phone is charging at the same time. That is a ground loop: the charger and the AUX cable create two paths to earth at slightly different potentials.
- A ground loop isolator on the AUX line removes it for very little money.
- Charging from a different socket sometimes helps.
- A better-quality charger with proper filtering sometimes helps.
It is not a fault in the stereo, and replacing the cable rarely fixes it.
Bluetooth: the convenient option
If the head unit has Bluetooth, use it. If it does not, a Bluetooth receiver plugged into AUX adds it for the price of a takeaway.
The quality question is the codec. Almost every car head unit and cheap receiver uses SBC, the mandatory baseline codec. Some support AAC, which suits iPhones, and a few support aptX. On a car stereo, over road noise, at motorway speed, the difference between SBC and aptX is far smaller than the difference between your car’s speakers and better ones — so codec support is not worth paying much for.
What is worth checking: whether the receiver supports hands-free calling as well as audio streaming, and whether it remembers more than one phone. Two-device pairing is the feature people miss most in a shared car.
Bluetooth receivers need power. The neat ones take USB-C and can be tucked away; the ones with a built-in battery are a false economy, because you will find it flat exactly when you want it.
FM transmitters: the universal option, with a ceiling
An FM transmitter broadcasts your audio on an unused FM frequency, and the car radio receives it like any station. It works in absolutely any car with a radio, including cars with no AUX, no USB and no Bluetooth.
The trade-off is physics. FM broadcast has limited bandwidth and a fixed noise floor, so the result is softer at the top end and hisses in quiet passages. In a city, where the FM band is crowded, you will also fight interference — a frequency that was clear at home picks up a station forty kilometres later.
Practical tips that matter more than the model you buy:
- Find a genuinely empty frequency. The low end of the band is often quieter. Re-scan on long journeys.
- Keep the transmitter away from the charging cable, which radiates noise.
- Expect to change frequency between regions. That is the format, not a defect.
Most FM transmitters also include a car charger and a hands-free microphone, which is why they remain popular — one device, one socket, three functions.
Which to choose
- AUX socket present, sound matters: cable, plus a ground loop isolator if it buzzes.
- AUX socket present, convenience matters: small Bluetooth receiver into AUX.
- No AUX at all: FM transmitter, and accept the ceiling.
- Head unit with USB that supports audio: try it first — USB audio bypasses the analogue stage and often sounds better than AUX.
What none of them fix
All three deliver a signal to the same amplifier and the same speakers. If the sound is muddy, thin or distorted at volume, the source is not the problem, and no adapter will change it. That is a speaker and door problem, and it is worth reading about separately before spending on anything else.
