I have a cd changer in my car that uses fm modulation. I noticed that some mp3z sound really nice, while others completely lack high frequencies. The volume is also low (compared to real stations), but if I turn it up too much within the cdchanger I start to listen pops and clicks, and in some other times an extreme dynamic range compression.

I've already applied mp3gain and I'm getting much better results, the dynamic range doesn't sound so squashed anymore, but I still feel there's a lack of high frequencies. Frequencies that are present when I tune a real FM station.

So I was thinking if there is a magic number to set the low pass filter while encoding to make the mp3z sound as good (or close) as a real FM station?
Any other settings worth looking for?

Thanks in advance.

The problem is your FM modulator, not the source material. FM strongly attenuates high frequencies, which is what you're hearing. Since you mention that your modulator sounds worse then regular FM stations, I'm guessing your modulator is particularly poor.

Since you're never going to get high frequencies out of an FM modulator anyway, maybe you should look into buying a deck with an AUX jack.

FM radio has a pilot signal at 19 kHz, and for this reason FM signals typically cut off at 15 kHz. Encoding anything higher than this would be a waste of bits.

There is a reason why professional FM modulators cost thousands of €€€.
It is not as trivial as amplitude modulation, and when you want stereo, things get even more complicated

It is a Sony CDX-565MXRF, it has a built in modulator and no line-out option, getting rid of it is not an option.

There are some mp3 files that sound really good, almost as good as the FM station. But there are another ones that sound "transparent" on my computer setup but on the car sound like crap.

I'm guessing the encoding process has a lot to do with it, specially the low pass filter. According to pdg, I should set the encoder to low pass below 15kHz then. Any other useful tips besides getting rid of the cd-changer?

I suppose it's possible that your modulator doesn't adequately filter out content above 15 kHz and that this is interfering with the 19 kHz pilot signal, but this is only a guess. Simply lowering the lowpass of your encoded files could possibly fix the problem.

So I tried with -b 192 --lowpass 16, the difference is negligible. I guess the only option if I want higher quality is to dump the cd-changer and the stereo. That's the only way of getting a line-in

Radio stations add large amounts of aggressive dynamic range compression and high frequency clipping/limiting to maximise both the loudness, and the amount of high frequencies, which are broadcast. They also EQ their output to sound good on "typical" (i.e. poor quality) radios and speakers.

Your playback system won't have anything like that, so isn't going to sound like a real radio station.

The quieter, duller sound might be perfectly accurate! Or it could be some crude (high frequency?) limiter is kicking in on the modulator.

You could run the audio through some dynamic range compression or EQ to try to acheive the sound you want.

Line in might help, or might not. Depends on the system, and where the problem is. I doubt it's with the mp3s themselves - but you could always put the original CD-DA into teh changer to compare.

Cheers,
David.

I did compare with copies of the same CD-A on both the front deck and the cd-changer. On the cd-changer the volume is lower and the high frequencies are also lost. Low volume I don't mind, I just increase it at the expense of higher noise floor. I wouldn't bother with dynamic range either. Is the lack of highs that bugs me.

The thought of EQ/increasing the power of high frequencies didn't cross my mind before. Is there any program (free if possible) that can do it?

Thanks in advance.

Use the EQ on your car stereo, or else change EQ the files before your encode them to MP3 (foobar2000 would be a good choice).