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

Which type of connection sounds the best?


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.

Tell us what kind of vehicle you drive, and we'll let you know if we have a USB adapter for your factory stereo.

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.

[Read more about adding a Bluetooth transmitter to your car.]

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 transmitters and 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 for best sound quality

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 (find out about the improved sound of aptX).

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. You can start the process by entering your vehicle info in our OutfitMyCar tool. If you need help figuring out the best option for you and your car, give us a call at 888-955-6000.

Last updated November 18, 2015
  • Dale

    Posted on 5/19/2015 6:27:50 PM

    Other than a CD, what is the best way to listen to music in my 2012 Infiniti G37sport? Stored music on the 9.3GB hard drive is stored at 132kbps. I believe the USB connection simply engages Bluetooth. Not sure if there is a way to connect directly thru the USB. I would like to stream music thru my Iphone5 and Bluetooth if the quality is close to CD. Thank you!

  • Alexander Hrabe from Crutchfield

    Posted on 5/20/2015 10:23:15 AM

    Dale, while streaming music via Bluetooth is convenient and sounds satisfying enough, it does not match CD-quality sound. You could try to connect the hard drive you mentioned to your stereo via USB, but there's no guarantee the two will be compatible. If you have high-res music files, the best way to listen to them is by plugging a loaded flash drive into the stereo's USB port. If you're looking for a new aftermarket stereo for your Infiniti, give us a call. An advisor can help your find the right stereo for your needs.

  • Niklas from United States

    Posted on 5/20/2015 7:12:20 PM

    @Dale Any source would be fine... Alexander stated that Blutooth wont be CD quality well CD qulaity is 1,411Kps.... and high end MP3 files are 320Kps, 132Kps will work perfectly fine on blutooth.

  • Lucille Brandner from United States

    Posted on 6/2/2015 10:10:49 PM

    I have a car with the capability to connect devices to its audio system through a usb port. I have many cds that I would like to take on trips and since my car has no cd player, I am looking for a portable cd player that would be capable of outputing sound through my car audio. Can you recommend a good portable cd player that would do this?

  • Alexander Hrabe from Crutchfield

    Posted on 6/3/2015 11:04:58 AM

    Lucille, I think you'll find a portable CD player with a USB output to be a bit of a white whale. We no longer carry portable CD players, and most that you'll find elsewhere will probably only have a 3.5mm plug audio output. If you have a laptop, you could spend a bit of time converting your CDs to digital media files. If you load a flash drive or media player with those music files, you may be able to plug that into the factory USB input you describe and play your music that way. However, given your love of CDs, the best solution may be to upgrade your car's system with a new in-dash CD player. You can use our vehicle selector to confirm which stereos fit your car or give us a call and we can help you choose the right fit.

  • John Purcell from United States

    Posted on 7/5/2015 6:31:47 PM

    Hello. I have a very basic 2006 Ford Escape, fitted with the standard factory installed am/fm stereo CD player. This factory unit has no auxiliary inputs of any kind, no external inputs, at all. I almost exclusively listen to books on CD or MP-3, so sound quality is not much of a factor. The radio receiver works quite well, but, sadly, the CD player function is, I think, dying. Despite cleaning the unit, upon inserting a disc, the unit scans ahead, track after track, never stopping to play any track. I have tried everything that I can think of to rectify the problem, but nothing succeeds in getting it to simply play the disc! The car is old, but in great shape and runs well. I don't plan on a new car anytime soon but, with old cars, one never knows! I do not want to spend much money, as I am disabled and on a fixed income, plus, since sound quality is not a huge issue, I don't want to spend much on an expensive new deck or system. Other than buying an inexpensive new deck and installation kit (I am capable of installing such equipment, to a degree) are there any easier, less expensive options, such as MP3 players with FM transmitting ability that I can try? I mostly drive on highways or in fairly rural areas, so conflicting FM signals shouldn't be much of a problem. Would such a set up work for me? Can you suggest any such equipment? My need is rather urgent, as I have a long trip coming up, and I'll need my books on MP3 to accompany me! Please help, if you can. Thank

  • Alexander Hrabe from Crutchfield

    Posted on 7/6/2015 9:54:09 AM

    John, I've passed your information along to a Crutchfield advisor who will be contacting you via email soon. If you'd like to speak with us right away, give us a call at 1.888.955.6000 for a recommendation.

  • Naveen from San Diego, CA

    Posted on 7/14/2015 7:11:32 PM

    Thank You

  • alex from sydney

    Posted on 7/25/2015 11:31:22 PM

    Am I right that any external bluetooth adaptors are essentially FM transmitters?

  • Melina from Memphis

    Posted on 7/26/2015 6:18:37 PM

    I have a 2011 toyota highlander. How can I listen to music from my microsoft zune using the usb port? Thanks

  • Alexander Hrabe from Crutchfield

    Posted on 7/27/2015 10:03:03 AM

    Alex, actually we have several Bluetooth adapters that are wired and don't use FM modulation. Some use your factory AUX input. Some use a wiring harness that links into your factory system. Check out all our options here.

  • Alexander Hrabe from Crutchfield

    Posted on 7/27/2015 12:14:36 PM

    Melina, if your Highlander isn't already equipped with a USB input, we don't have a way of equipping your factory radio to read data from your Zune via USB. The only way to gain that feature is to install a new aftermarket stereo compatible with the Zune (which might be a tall order since Microsoft no longer makes them). However, the easiest, cheapest way to listen to music on your Zune through your car stereo is to use a mini-to-mini stereo cable to plug your Zune into your stereo's AUX input.

  • Derek Schaller from Madison, WI

    Posted on 8/5/2015 10:55:10 AM

    Thank you for the well written tutorial that included very objective information (like SNR and FR) that was well organized (progression and chart) with practical context (other factors to consider).

  • Johnny from Albany

    Posted on 8/17/2015 7:08:20 PM

    I have a new 2015 Nissan Versa with four speakers. Using a factory CD/Radio/AUX Input stereo system. My problem is I'd like a crisp high end sound quality. I don't want to blow my speakers because the volume knob is really high to actually listen but it seems that the quality is better from my laptop than my android phone using the AUX input. I'm not sure if I should get a DAC converter or a power inverter to use so I won't drain the car battery. I know the best quality is Flac than mp3, WAV files are at best too but takes too much space for music too. I'm not sure if I should stick with my laptop method as my music player or what. At most listen to music without turning the volume so high even though the quality for most of my music is mp3 192 or 320Hz. What should I do?

  • Alexander Hrabe from Crutchfield

    Posted on 8/18/2015 10:59:26 AM

    Johnny, having a laptop set up in your car doesn't sound like the most convenient way to listen to you music, or the safest. As a first step, I'd double-check to make sure the volume is all the way up on your Android device, and then you may be able to move your stereo volume to a more reasonable setting. If that doesn't work, and you're in the market for a new stereo, you could install one with a front USB port for a thumb drive you could load up with your mp3s. An aftermarket stereo will also drive your speakers with more power than your factory stereo, and you'll hear a nice boost in sound quality there as well. You can use our vehicle selector to confirm which stereos fit your car.

  • Jeffrey E.

    Posted on 9/9/2015 5:48:19 PM

    Just here to say this article was exceptional. Not too technical but just enough specificty to provide both a solid base of information and perspective on the differences. First time on the site and will now be browsing around.

  • Bill Kelsey from Copperopolis Ca.

    Posted on 10/4/2015 10:38:27 PM

    Im using a iPhone 5c connected via Bluetooth to a Pioneer AVIC-D3 and some songs sound as if theyre playing at about half of the volume. Im still using version 8.1 IIRC on the phone. Thanks.

  • Dave Delamere from Crutchfield

    Posted on 10/5/2015 12:31:57 PM

    Bill, it looks like the Pioneer AVIC-D3 required a separate Pioneer Bluetooth adapter, and its older version of Bluetooth may be causing some issues. If you bought the receiver at Crutchfield, you could call Tech Support for free help troubleshooting your system. Their toll-free number is on your invoice. If you purchased your equipment elsewhere, you can still get expert Crutchfield Tech Support - 90 days-worth for only $30. Click on this link for details.

  • Patsy Callaghan from Yakima, WA

    Posted on 10/16/2015 4:50:41 PM

    Hi folks, We just bought a new Honda CRV which has two USB ports and an HDMI port but no AUX port. We had ALSO invested in two PONO music players, which are not Bluetooth equipped. We tried a universal FM transmitter, but maybe because of where we live, there's just too much static. Any ideas?

  • Alexander Hrabe from Crutchfield

    Posted on 10/19/2015 10:32:11 AM

    Patsy, aside from replacing your radio, a wired FM modulator like this will probably be the only way to play music from your PONO(s). With a wired connection, you'll get a clearer signal than with the universal transmitter you've already tried. If you need any recommendations, just give us a call.

  • Melvin Low from Penang

    Posted on 10/28/2015 2:38:18 AM

    I had an AVF car portable media player and it sound really bad although the songs I put is in high quality MP3 320kbps I wonder is the problem related to format problem or the car portable media player is just bad quality sound output I got good quality media player but nowadays I hardly find a good quality cd to burn. So would I change to bluetooth media player or media player with a USB plug or the problem are not those I listed above?

  • Alexander Hrabe from Crutchfield

    Posted on 10/28/2015 8:27:06 AM

    Melvin, there could be a number of things that could be making your music sound "bad." Old factory speakers, not enough power going to those old factory speakers, a bad connection somewhere, etc. It might be worth borrowing someone else's media player and testing it in your vehicle to see if the sound improves. There's also the possibility that you're not satisfied with the resolution of your media files. To find out more about high-resolution audio, check out this article. If you're sure it's the music player that's at fault and you want to replace it, a wired connection will give you better quality sound than a Bluetooth connection, but you'll need to make sure that your device is compatible with your stereo via USB. Alternatively, an AUX connection will suffice.

  • Alan from Rock Island, IL

    Posted on 11/6/2015 5:22:21 PM

    When I listen to music via a Bluetooth speaker, it seems to me to compress the dynamic range of the music I listen to in a rather noticeable way. I have a Bose Bluetooth speaker (don't judge, good sound and well-made for a small portable). I have both plugged audio in directly and used the Bluetooth with my phone utilizing the highest quality on Spotify. With an album high on dynamic range (Oceansize-Frames) I get quite perturbed at the loss of dramatic effect from loud to soft when using Bluetooth, but dont detect such an extreme alteration on the wired connection. I assume that part of the effect stems from Spotify, but as I stated, it doesn't seem too noticeable unless I use Bluetooth. Is this just in my head or is there a noticeable difference? PS, in my vehicle I utilize a cassette adapter and find it to be far superior to an fm transmitter. Years ago, I went through probably 5 different transmitters trying to find a good one, but unless you are in the middle of nowhere, I always get interference from radio no matter how many stations I try. For my factory speakers (decent quality in a 2001 Toyota avalon. Can't think of the brand, but it's not the standard audio package. Junk CD player, sadly) it performs very well. I have no complaints about the audio quality. I personally find that it is far superior to a typical cassette, likely due to wear and tear they endure. Also keep in mind that a CD is better, unless it skips. Spotify NEVER skips ;)

  • Alexander Hrabe from Crutchfield

    Posted on 11/9/2015 9:08:55 AM

    Alan, your ears are not playing tricks on you. You'll get a fuller, more dynamic sound with a wired connection. Bluetooth requires signal compression to function and so some musical information is lost in the transfer. If you're interested in better-sounding Bluetooth, check out this article on aptX.

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