Clarifications on Frequency Modulators
Well, I just got done wading thru all of the posts about FM Modulators. It appears there are some misconceptions flying around.
First off, FM stands for Frequency Modulation. It always bothers me that everyone (not just u guys) uses the term "FM Modulator". This term is rather redundant, but just so everyone knows what I'm talking about I will continue to call them that. =)
First off, although an MP3 is sampled at a certain rate just like a CD, it has *nothing* to do with how it will sound on a tape adapter vs. a FM modulator vs. a direct AUX input. Understand that, just like a CD, the 'digital' format is converted to analog. I saw one comment that a tape's 'sampling rate' is something like 2.1k. Umm, there is no 'sampling rate' for a tape! Its analog!
Now, what you should be concerned pertaining to sound quality with an analog signal is this: FREQUENCY RANGE. Your hearing (depending on how many concerts you've been to) works from about 20 hz to about 20,000 hz (20 Khz). 20hz to around 200hz is considered bass, 200hz to around 2 Khz is considered midrange, and 2 Khz to 20 Khz is considered treble (approximate break points). A CD will cover the entire spectrum of what you can hear (20 hz - 20 Khz). A tape will get close to that; a *good* tape deck will get down around 30hz and up around 18 Khz. Your factory head unit won't get quite that much range. Transmitted FM signals start out with pretty good frequency range, but by the time they get to your car and go through your tuner, the range will be diminished. Most good FM tuners will have a frequency range of around 30hz-16khz.
Now, keeping all of this in mind, realize that MP3s are CUT OFF above 16 Khz!!! Whether you use a tape adapter or frequency modulator, it will not make much difference in the range of frequency you will hear.
Now there are other issues that might persuade you to chose one option over the other. I would not purchase either a frequency modulator or a tape adapter unless it was a good one! Crappy ones will have terrible frequency response because they are made with crappy parts. I have a good frequency modulator, and it works great. I would never use a tape adapter because there are moving, mechanical parts involved, and there would be an ugly wire hanging out of my dash, and, after hearing a good frequency modulator, I don't like the quality of the tape adapter near as much (mainly because of the hiss). In addition, its a lot cheaper to make a decent tuner than it is to make a decent tape deck, so your factory head units will generally be more suited toward a frequency modulator. And for those with CD players, well, your option has already been decided. =)
Another option which I am going to use for my boat, is the FM Transmitter option. I have found quite a few web sites that sell *good* stereo FM Transmitters for under $100 that have, shall we say, slightly illegal transmitting power. I need the transmitter for the lake. With a bunch of boats tied up partying, nothing would rule more than having everyone tuned to your own personal radio station.
The other option, the AUX input is hard to find. Reverse engineering the required signals to fake your CD changer controlling head unit into thinking its plugged into a CD changer instead of your MP3 player is rather time consuming, and definitely not for everyone. You can find some head units (usually not the best quality head units) that have AUX ins. I still have an old SoundStream TC-308 (best head unit I ever owned, $900 new back in '89 for a tuner/tape deck!) that has AUX ins and am using that currently in the boat. It really is the best sounding option I must say.
Anyway, there is the summary. If anyone has any questions or wants to refute anything I said here, please do!
PS I have one extra frequency modulator (a good one) I bought new for $55. Its still in the wrapper, hasn't been used. Would sell for $35 if someone is interested.