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. The CD player has an SNR of 90 or 100 dB, while a cassette player has 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. Now let's see what those numbers mean for our portable music player.

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.

A wireless FM transmitter, like this one from Belkin, is one of the easiest ways to get your music into your car stereo.

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.

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

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.

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

Bluetooth wireless technology is a newer option that's gaining in populartiy. The music signal is broadcast wirelessly from the Bluetooth transmitter to a Bluetooth receiver. The Bluetooth gear is either connected externally or built-in to your music player and car stereo. Sounds like an FM transmitter, right? There are some major differences that enable Bluetooth capable equipment to function much better than most FM adapters.

First of all, Bluetooth signals are digital transmissions, which 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 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.

Feeding the sound through an auxiliary input will usually yield the cleanest 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. Just like with Bluetooth transmitters, 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.

Many stereos have options for adding an auxiliary input. USA SPEC makes auxiliary input adapters for many factory stereos.

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.

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