Playing Music From a Phone or Music Player in Your Car

The Nulaxy KM29 is a low-cost and easy way to get other devices to play through your car’s sound system

If you’re driving somewhere, especially if traveling alone, you’ll crave something to alleviate the boredom of watching the brake lights of the car in front and the mile markers pass every minute or so.

Just about every car has an AM/FM radio.  A radio is one of the longest-standing pieces of car electronics, with AM radios dating back to 1930 and the FM band being added in 1952 (here’s a great history of the evolution of car radios).  Other types of car entertainment sources have come and gone over the years; things that are now seldom seen such as cassette tapes, 8-track cartridges, CDs, video players, even lp records for a while.  More recently, new types of audio sources have appeared in some vehicles including satellite radio, digital audio, internet streaming, and accepting music feeds either through an “Aux In” port, a USB port, or via a Bluetooth connection.  Whatever else your vehicle might have, it almost certainly still has an AM/FM radio.

But, as you probably know from personal experience, once you’re out of your home area, finding appealing stations to listen to is a problem, and a repeating problem every hour or so as you travel in and out of each radio station’s coverage area.

Radio, while wonderful, has some downsides – in particular, the annoying ad breaks every few minutes.  These days there are many other sources of music and entertainment, often without such interruptions, but not all of them are accessible through all car entertainment systems.

A common problem these days is when you have a type of music/entertainment device that can’t readily be connected to your car’s audio system.  The three most common ways of connecting a separate device to your car system are either through a Bluetooth connection, an “Auxiliary In” cable, or by USB, and it is far from uncommon to be unable to match the output options from a device to the input options of your vehicle.

In older days, there were other interesting solutions to how to connect things, for example I still have a “cassette adapter”.  It is in the form of a regular cassette but instead of tape inside, it has a wire that you can connect to other devices via their Auxiliary output, and inside the cassette, there is a unit that sends out the electrical/magnetic signal to be read by the tape heads in the car’s cassette player.  The sound quality is not always excellent, and there can be the noise of the car cassette player’s transport mechanism turning and turning, because it expects to be turning a cassette.

Of course, nowadays very few if any cars have cassette tapes – they were replaced by CD players, and now CD players in turn are also disappearing.  But if you’ve a cassette player, this can be a low cost and simple solution for providing an auxiliary input into your car’s sound system.  Amazon sells them for as little as $10.

If your car accepts Bluetooth input, and your music/audio source broadcasts a compatible Bluetooth signal, that might be a satisfactory solution.  Alternatively, if they both accept compatible auxiliary audio outputs and inputs and you can simply run a wire between the two, that can be a better (ie simpler) solution.

But, what if neither your car nor your other audio device have a common method of connecting?  What do you do?

These days, there are good answers to that question.  There are many units that act as a “bridge” or “interface” between various types of audio device outputs and various types of car inputs.

The units can accept audio inputs from up to four different sources :

  • Bluetooth
  • Auxiliary input cable
  • USB port
  • Micro-SD port

In turn, they send their received signal on to a vehicle’s FM radio, in the expectation that no matter what else a vehicle might or might not have, it is almost certain to have an FM radio.  Happily, an FM radio signal is also a reasonably good quality way of sending an audio signal to your vehicle’s sound system.

Some units can also feed their received signal on to a car through an auxiliary cable.  This is not a commonly found feature, however.

Let’s talk about the issues to consider with each of these four connection methods.


Most of the units we looked at (in April 2021) have a reasonably current version 5.x of Bluetooth.  A few had earlier 4.x versions of Bluetooth, and at the time of writing, the most up to date version of Bluetooth is 5.2 – it started to appear in devices from early in 2020.  For an in-car type application, BT 5.2 doesn’t have much practical benefit over 5.1 or 5.0.

Devices that use BT 4.2 or earlier are becoming quite out of date – BT 5.0 appeared in 2016, so our preference is to have a unit that supports 5.0 or above Bluetooth.  Note that we only say this because we feel that a more “modern” device is probably better engineered in all respects.  A BT 4.2 device is likely to work with all versions of BT, both older and newer, so you can still probably connect your audio player to a BT 4.2 unit, and it is unlikely you’ll notice any difference in sound quality.

We have never found Bluetooth to be an appealing solution to anything.  It invariably ends up as more complicated than it should be, because device designers “over-engineer” and try to make it “too clever”, but end up just making it too complicated instead.  Our ideal BT application would be totally “unintelligent” and do nothing more than wirelessly emulate a simple piece of wire, with nothing to configure or adjust or worry about.

Even with our most modern BT devices, we continue to be plagued with problems of devices “forgetting” connections, or not auto-connecting, or, if not the device forgetting things, us forgetting how to control the device through the obscure series of layered menu options and flashing colored light codes.  We don’t like BT and try and avoid it whenever possible as a result.

Auxiliary Input Cable

This is our preferred method of connecting devices, because it is simple and straightforward, and generally works well.  The key factor to be aware of is the “volume level” that is being transmitted out of the auxiliary output or headphone jack of your device and what the matching “volume level” is that the receiving device is expecting.

On occasion, these may be mismatched such that either the sound ends up ridiculously quiet, or way too loud and distorted.  These problems can sometimes be solved – perhaps your audio source has both a “line out” and a “headphone jack” – maybe one gives a better volume level than the other.  Perhaps varying the volume level on your device varies the output that is sent to the receiving device, and that might solve the problem too.

You can also get separate amplifiers and attenuators, so you feed the output first from the audio source player to an amplifier/attenuator to adjust the volume, and then from that to the receiving device that will send the signal on to the car’s system.

USB Port and/or Micro-SD Card

Some devices might offer one of these options, some might offer the other, and some might offer both, so we’re combining them in this discussion, because the same types of issues apply to both.

Furthermore, if your device will read audio from a USB device but not from a Micro-SD card, you can get an adapter that accepts a Micro-SD card (and perhaps other types of memory card too) and converts it to a USB plug.  Here’s a bunch to choose from, courtesy of Amazon (of course).

The biggest problem we consistently see is that the device you’re plugging the card into will only read devices with a limited storage capacity.  This is often stated as 32 GB, which probably means they will read the SDHC type cards that can have up to 32GB on them, but not the newer cards – the SDXC cards – that can go as high as 2TB.  It also might imply that they are only compatible with FAT32 formatted drives (which have a sort-of limit of 32GB per drive/partition).

FAT32 is the default format for such devices, because it is the most widely supported format by most operating systems and equipment, so this might not be a problem.  exFAT is another format that you’ll find with many larger than 32GB external storage items, and NTFS, originally a Microsoft proprietary protocol, can also used, but not so likely to be supported in these bridge units.

We don’t know, but we think some units which claim to be able to read up to 64GB cards might actually be capable of reading larger capacity cards too.

The limit, at least as stated, never seems to exceed 64GB, and that’s a surprisingly very low number.  I’ve 400GB and 512GB Micro-SD cards full of music and would hate to have to either split that over seven or eight smaller Micro-SD cards, or decide which tracks to put on a card and which to leave off.

There are two other things to consider as well.

The first is what music and audio formats the unit will read and be compatible with.  Ideally your music is stored in FLAC format, or failing that, in MP3, either VBR or 320kbps CBR.  Most of the units will play FLAC and MP3 files, but if you have your music stored in some other format, be sure that the unit will be able to play that file format.

The second thing is how the unit will navigate its way through your music files and allow you to conveniently select not just single tracks to play, but complete “albums” or directories or playlists.

My sense is that most units are expecting to see a bunch of music files in the root directory of the USB thumb drive or micro-SD card.  Some will also allow you to move between folders and sub-folders to reach the files you want.  You might need to experiment with how you load up your music files to get them playing the way you want.

Most units will have a play/pause button, and a skip forward/backward pair of buttons, but it is unusual to see much more, although sometimes there will be a multi-function button or two which can give the other buttons other sets of commands as well as do multiple things themselves.  Some units might have tone adjusting controls, but they are almost certainly unnecessary – you probably have them in your car audio too.

USB Charging Ports

It is common for most of these units to provide one or two, sometimes even three USB charging ports as an extra “bonus” feature.  One of the ports can often work as an input port for audio recorded on a thumb drive.

In addition to “normal” USB charging, many units also provide some type of “special” charging too.  Allow me to (very briefly) explain.

Over the last ten years or so, portable electronic devices have become much more power hungry.  They have larger capacity batteries in them, and draw larger currents when operating.

The original USB standard allowed for up to 0.5 amps of 5V power to be drawn through a USB port, to power or charge a device.  That was fine for early phones, but when tablets came out, and phones got ever-larger screens, we soon ended up with devices that were drawing more than 0.5A of current, meaning that a USB charger would never be able to charge up a unit while it was turned on.

Distressingly, that often still happens on airlines where they many times limit the current on their seatback ports to a very low amperage.

So, over the last few years, there have been a number of revisions to the USB specification, allowing it to provide more power to connected devices.  That’s the good news.

The bad news is that there are a number of ways that devices now “signal” to USB power supplies to advise what rate of power they can accept, and even now, what voltage they would like, too.

Not all devices and power supplies understand each other’s communications.  That’s not necessarily a problem – typically a power supply will degrade down to a low amperage if it isn’t told to deliver more.  But it does mean there are now a complexity of different ways that this information can be exchanged.

As well as just directly, there are two commonly used protocols – QC and PD (Quick Charge and Power Delivery), and these appear in different versions.

I could write a lengthy article on this topic, suffice it to say that the higher the version number (3.0 or 4.0) the better, and if a charger supports both protocols, that is better than a charger that only supports one (or none).  And the more amps that a “normal” USB charging port can provide, the better, too.

Other Features

Some of the units will also show your battery’s voltage on their display.  That can be useful information and give you a valuable advance warning of your battery starting to age and die – look for lower than normal voltages when the car engine is stopped, and larger than normal voltage drops when starting the vehicle.

Many of the units will also act as a hands-free unit for your phone allowing you to take and make phone calls.  That can be a helpful feature, too.

Design/Screen Size & Color/Pivot/Stalk/Controls

Another more minimalist style of bridge unit

One of the key considerations is to have a unit that has easy to understand intuitive controls, so you don’t need to read a manual each time you want to do something.

Another key consideration is that you want to be able to control the unit as much as possible without needing to take your eyes off the road.  Plus, any necessary information that is displayed should be displayed in as large a size as possible, so if your eyes are no longer brilliant at reading things nearby, you can still make out what the display is showing you, without needing to juggle glasses while driving.

But having said that, how much information is needed to be visible, and how much control do you actually need to do on an ongoing basis?  Probably you don’t need to do much at all after setting up a connection between your player, the bridge device, and your vehicle.  In that case, you don’t really need much in the way of displayed data, and assuming you can control the audio from your player device, there’s no benefit to having another set of controls on the bridge device.

This is definitely a case of “less is more”.  Don’t fall prey to “creeping featuritis”.  Don’t lose sight of your simple requirement to merely connect a sound source to your car’s sound system.

These comments need to be rethought if you might want to play audio directly from USB and Micro-SD cards.  Then you’ll need some basic information to help guide you through your selection of tracks and albums.

Most of the units plug directly into the vehicle’s cigarette lighter/accessory power socket.  A few have built in rechargeable batteries.  We prefer to plug in to the vehicle’s power socket, so we don’t have to worry about keeping the battery charged.

Some of the units have displays built in to the piece that plugs in to the power socket, others have a flexible stalk and a display at the end of the stalk.  Depending on how important it is for you to be able to see the data on the display, and the layout of where your power socket is and how visible a fixed display might be if plugged directly into the power socket, the display at the end of a stalk concept might be more convenient in some cases.

One thing about choosing a screen.  If you decide you need a screen, get the biggest screen possible, and be sure to get a color screen – it is so much easier to see your way around a screen if it has color-coded elements in it, and of course, the bigger the screen, the easier to read.

Finding a Free FM Frequency

It can potentially take quite a while, stepping through all the hundreds of different FM frequencies, to find one that has no radio signal on it, and, of course, as you drive, you’ll occasionally move into a region where the formerly unused frequency now has a radio station broadcasting, and so you’ll need to repeat the process from time to time.

Some of the units have a useful feature that will automatically seek to find an empty unused frequency and stop on that frequency, once found.  That can save you time, especially when you’re driving and trying to concentrate primarily on the road, not on the gadgets in your vehicle.

Some units will transmit on frequencies down to 87.5 MHz (the traditional FM frequencies start at 88 MHz).  If your car radio will receive down to that frequency, you are more likely to find a free frequency there because there are no regular US radio stations that broadcast at frequencies below 88.1 MHz.

Whereas with AM radio signals, two signals at the same or close frequency interfere with each other, and make neither easily heard, generally with FM the more powerful of the two signals “takes over” and you get that signal clearly and the other signal doesn’t interfere at all.

This leads to another suggestion.  In the US, only the odd numbered frequencies are used – that is, 88.1 MHz, 88.3 MHz, and so on.  There are no radio stations broadcasting on, eg, 88.2 MHz and 88.4 MHz.  If you tune your radio to a strong signal, you’ll probably notice that it “spills over” to the adjacent even numbered frequencies – for example, 95.5 MHz will probably also be heard on 95.4 MHz and 95.6 MHz.  But the signal is much weaker at these off-center frequencies.  If you can set your car radio for 95.4 MHz, you might find that the signal from your in-car unit is able to easily “obliterate” the weaker signal from the station at 95.5 MHz, and in such a case, you might therefore find that you can drive the entire country with your in-car signal at 95.4 MHz overpowering any and all outside radio stations.

Not all car tuners will tune to the even frequencies as well as odd ones though.  But it isn’t an essential consideration.  And in general, the signal from the device in your car is usually sufficiently strong to be able to easily obliterate weaker outside signals, and even reasonably strong signals.  All you might notice is a very quiet bit of “noise” in the background in such cases, maybe not even that.

Our Preferred Unit – the Nulaxy KM29

Our main requirement was for a unit that supported Bluetooth 5.x and had an auxiliary in port.  While we already have lots of USB charging ports in our vehicles from other accessory devices, if one of these bridge units also had some good/fast USB charging ports, maybe that would allow us to get rid of other accessories, so that was something we had on our “nice to have” list.  And while we didn’t really expect to use it, if a device also had USB and Micro-SD inputs, then it might be a “tie breaker” between units.

We decided we wanted as large a screen as possible and on a stalk.  And we decided we didn’t want to pay over $25, because there were an abundance of units priced in the $15 – $25 range and no clear extra benefits going to more expensive units.

Amazon showed either 677 or 833 units to choose from!  We compared dozens of them in terms of their “on paper” feature lists and appearance/controls, and drilled down to a more careful comparison of a few units, before ultimately choosing this Nulaxy KM29 unit (pictured at the top of the article).  We’ve bought other Nulaxy products and been pleased with them, and while we view the value of a Chinese brand as being very minimal in nature, it was comforting to know that it wasn’t totally a generic brand that was as likely to disappear tomorrow as not.

The Nulaxy unit was very simple to understand and use, and provides a good strong clear FM signal.  It also has reasonably sensible and understandable controls for playing from USB and Micro-SD sources.

It has QC 3.0 fast charging and also will charge at up to 2.4A from its regular USB port.  It doesn’t provide PD power, but that didn’t trouble us, because on a long road trip, there’s plenty of time to top up one’s device battery if needed, at regular charging rates.  If you did want PD, or if you don’t like a display on a stalk, and if you’re willing to forego the aux input feature, then perhaps one of the VicTsing units might be a better choice for you.

In other words, for our requirements, the Nulaxy unit does all we want of it, does it well, and has some other features too.  Great value for only $22.

Summary – The Best Connection Method

If you end up with a choice of different connection methods, it stands to reason that one is better than the other(s).  Which should you choose?

Our approach to this issue is to first recognize that a car is not an ideal environment for appreciating highest quality music to start with.  There’s a lot of road noise that colors the sound and destroys the dynamic range, and the chances are your car’s speakers are not delivering a broad range or smooth spectrum of sound either.

So we don’t give highest priority to the connection method that theoretically might bring the highest quality sound.  Instead, our preference is for the simplest, easiest, and most convenient method.  By happy coincidence, it is probably also the connection type that gives best sound quality – a simple cable running from the Aux or Line Out or Headphone jack on your audio player and going to the Aux/Line In jack on your car’s sound system.

If that’s not an option (or if you don’t think it to be easiest), you can use Bluetooth or any other option, based on what is easiest for you and your configuration of player outputs and vehicle inputs.

Using a “bridge” device such as an FM transmitter might give you more options and perhaps more convenience.

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