Get the best sound out of your portable music player in the car


Robert Ferency-Viars

Robert Ferency-Viars is the managing editor for the Crutchfield car A/V learning content, and has been with the company since 1999. A Virginia native from the heart of the Blue Ridge Mountains, he loves spending time with his wonderful wife and sons, listening to music, writing, and playing games with friends. Robert's love for car audio began at 16 when he installed his first car stereo.

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In the old days, it was easy: CD sounds better than cassette. And that's all there was to it. To get the best sounding music, you'd listen to the CD on your car stereo. Today, we can choose from a wide variety of music sources (iPod®, smartphone, portable audio players, and more), and also from an array of ways for piping the music from your favorite source into your car stereo.

The question is, how do you determine the best way to connect? Some ways are easier than others and some ways will yield better sound than others. Here we'll take a look at the different ways to connect your music player to your car stereo and see which ones will provide the best sound.

The most common methods are:

  • USB input
  • auxiliary input
  • Bluetooth® transmitter
  • wireless FM transmitter and hard-wired FM modulator
  • cassette adapter

Which method sounds better?

Answering this question relies upon two technical specifications: signal-to-noise ratio (SNR) and frequency response (FR). SNR is a measure of signal strength compared to background noise in the signal or equipment. A higher number, in decibels, is better. FR is a measure of how much of the audio spectrum, from bass to treble, gets reproduced. It's measured as a range in Hertz, and the wider the range, the better.

The FM radio, cassette player, and CD player all have different measurements for these specs. A CD player can have an SNR of 90 or 100 dB, while a cassette player offers an SNR of 50-70 dB. The frequency response of a CD player tends to be better too, often in the neighborhood of 10-20k Hz. Cassette players don't deliver as much detail on the extreme ends of the spectrum and tend to have an FR around 30-18k Hz. Even so, that difference isn't nearly as significant as the disparity in signal-to-noise ratios between the two. That's why it is a cut-and-dry situation that the CD player sounds better than the tape player. We'll use these numbers to evaluate some of our connection options below.

usb input

USB inputs are common on newer stereos.

USB inputs — the preferred connection

The USB input is now a common feature on aftermarket car stereos, as well as in many factory systems. And that's a very handy thing because the USB input provides the best sound quality possible from an external music source.

The music signal remains in a digital form until it gets converted into an analog signal by your stereo, so no unnecessary manipulations are made to it. USB doesn't limit the frequency response or signal-to-noise ratio of the music, so what you hear is exactly what you have stored on your music device.

Besides being the best-sounding form of input, the USB input will often serve as a power source for your device too. But if your stereo does not already have a USB input, then we need to proceed further down the list of connectivity options.

Auxiliary Input Adapter

There are adapters available for adding an auxiliary input to many factory stereos, like this one for select Chrysler factory stereos.

Auxiliary inputs

Feeding the sound through an auxiliary input will usually yield a strong signal because it's a direct audio connection with full-bandwidth (20-20k Hz) frequency response. An auxiliary input does not have a signal-to-noise ratio of its own. The SNR depends on the signal's source, i.e., the audio player. That means the signal isn't getting degraded by the process of sending it to your stereo. The good thing about an auxiliary connection is that it is a direct delivery of the signal from the source to the car stereo, without any unnecessary electronic manipulations.

Each time the audio signal is manipulated, for example, by digital-to-analog conversions, or FM conversion or transmission, there is some signal degradation. This degrading is usually insignificant and inaudible, but it adds up with each manipulation and can result in a minor loss of sound quality. Using an auxiliary input eliminates several conversions so the signal that your stereo receives is exactly the same signal that your music source is creating. That means optimum sound quality for you.

It would seem that auxiliary and USB inputs should deliver the same quality of sound, but frequently, they don't. One reason is the difference between the digital-to-analog converter (DAC) in your stereo compared to the converter in your music player. Unless you're using a high-resolution portable audio player, the DAC in the stereo is going to be superior to the one in the music player because car stereos have the power and speakers required for bigger, better sound, so the DAC has to be able to reproduce a high-quality signal. Older portable music players and smartphones on the other hand, like the iPod®, were originally designed to make small earphones sound pretty good. When you use your music player's headphone jack to connect to your stereo, that means you're using the music player's DAC.

Bluetooth adapter

A Bluetooth adapter transmits music wirelessly with full-bandwidth frequency response.

Bluetooth® streaming

Bluetooth wireless technology has garnered tremendous popularity, because of its ability to stream music to your stereo without a cable. The music signal is broadcast wirelessly from the Bluetooth transmitter to a Bluetooth receiver. The Bluetooth functionality is either built into your music player and car stereo, or added via external devices (like the adapter pictured here).

The technology used by Bluetooth streaming is superior to most FM transmitters (see below), which were the previous form of wireless connection. First of all, Bluetooth signals are digital transmissions, that are capable of carrying more information, and thus rendering better sound quality, than FM transmissions, which are usually analog signals. Secondly, Bluetooth technology uses "spread spectrum" transmissions to broadcast parts of the signal over a constantly changing range of coded frequencies. This serves to block out most unwanted signals (i.e., interference) since the Bluetooth receiver is only accepting "packets" of data that are specifically addressed to it. As a bonus, this frequency-hopping also prevents your signal from being picked up on another piece of gear.

As for the numbers, Bluetooth transmissions are full-bandwidth, having a frequency response of 20-20k Hz. The signal-to-noise ratio is dependent on the source player, so that number will vary. As a high-quality, short-range, digital transmission, the Bluetooth process itself should have little or no impact on the sound quality. And many devices and car stereos are now including aptX® for near CD-quality sound for your streaming audio.

The downside to Bluetooth streaming is that, while it is resistant to interference, it does sometimes happen and you could get some form of unwanted noise in the signal. The other knock has been its range of 30', but in a vehicle, that's generally not a problem.

FM transmitters

A wireless FM transmitter, like this one from Scosche, is one of the easiest ways to get your music into a factory radio with no auxiliary or USB input.

FM adapters

First, the numbers. FM radio is restricted (by FCC regulation) to a frequency response of 30-15k Hz. Pretty similar to your typical cassette player. Likewise, the SNR of the FM tuner in a typical aftermarket stereo is similar to a good tape player: around 70 dB. When you use an FM adapter to pipe in your tunes, the music is fed into your radio over an FM frequency — the radio thinks it's just another radio station. So theoretically, we can expect the music from our portable to sound about as good as a typical FM station.

There are two types of FM adapters to choose from. A wired FM modulator is connected to your radio via the FM antenna connection. It usually requires removing the stereo to get to the antenna connector on the rear, but it's much less prone to outside interference than wireless transmitters. On the other hand, a wireless FM transmitter broadcasts your music over the air to the stereo's FM tuner, just like the real incoming radio signals. It's much easier to use, but the wireless transmitter is directly competing with all of those radio signals and other FM interference bouncing around inside and outside of your car. So, we can expect that of the two FM options, a wired transmitter will be our best bet for sound quality. Wireless FM adapters are quick and easy, but for long drives, especially through populated areas, you'll probably be better served by a wired option.

cassette adapter

Put that cassette player to good use with a cassette adapter for your portable.

Cassette adapters

If we pipe in our music through a cassette adapter, we can expect it to sound like, well, a cassette. As mentioned above, cassette SNR is 50-70 dB and FR is about 30-18k Hz. Overall, it'll yield results similar to an FM adapter, but without any static or interference. Plus, cassette adapter kits are inexpensive, easily the cheapest way to go. The downside is the wires involved, running from your portable to your tape deck. Not very pretty.

Running the numbers

For comparison purposes, here are the SNR and FR numbers for each option.

Type Signal-to-Noise Ratio Frequency Response
BEST USB Input Same as source 20Hz - 20,000Hz
BETTER Auxiliary Input Same as source 20Hz - 20,000Hz
Bluetooth Adapter Same as source 20Hz - 20,000Hz
GOOD FM Adapter 70dB 30Hz - 15,000Hz
BASIC Cassette Adapter 50 - 70 dB 30Hz - 18,000Hz

The bottom line

A direct connection via USB will yield the best sound quality because it eliminates extra manipulations of the audio signal. Bluetooth streaming runs a close second, because it does manipulate the signal by broadcasting it over the air. It uses techniques that ensure high-quality delivery of the signal to your car stereo, such as aptX® technology.

An auxiliary input is the next best option. It's also one of the most common and easiest methods to use.

FM adapters offer lower sound quality and are open to outside interference. Cassette adapters tend toward the lowest sound quality, with better cassette performances on the upper end of that 50-70 dB signal-to-noise ratio.

The quality of the source does matter

In this article, we examined ways to get the best ways to get your music to your car stereo in terms of sound quality. However, something to think about is how you encode your music to digital files. Music with less compression (CDs, high-res digital music, etc.) will sound better than music that's heavily compressed (MP3's, Internet radio stations, etc.). If you're ripping a CD to a digital file, you can set the encoding for less compression on many programs, like Windows Media Player® and iTunes®. Likewise, Internet radio stations often let you set a higher quality for your music in the settings menu.

Personal experience

The numbers presented here are very general. It's certainly possible that your equipment could have a superb FM tuner, or a portable audio player with high-res music loaded on it, for example. Besides sound quality, you should also consider the ease of hook up and elegance of the connection. And given how noisy the car environment is, you might not hear much of a difference between any of the connection options.

In the end, it's up to you. Try at least two different options and see what sounds best in your situation. And the next time you install a new stereo, spend the extra couple of dollars to buy the extra cable to connect to the stereo's USB or auxiliary input. It's worth it in the long run.

What are the options for your vehicle?

Your options for connecting your music player to your car stereo depend on a combination of three things: The kind of vehicle, stereo, and music player that you have. If you need help figuring out the best option for you and your car, give us a call at 888-955-6000.

Find what fits your car