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I have some MP3s that are encoded at 44.1kHz with a bitrate of 160kbps. I hook these up to my car using an FM transmitter and the sound quality isn't particularly great. Low and high notes sound distorted and crackly.

I suspect that I'll need to reduce the sampling frequency, but I am not sure. Reading up on FM broadcast radio, it seems that the FM radio channels have a bandwidth of 75kHz, so I do not understand why my audio sounds crackly.

How do I get my MP3s to play without crackling over FM radio?

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MP3 doesn't really go as high as 44.1 kHz anyway. The first step in compression filters out audio above 18 kHz, so a 36 kHz sample rate would have been sufficient. But that's not really the problem anyway. FM is analog and therefore doesn't even have a sample rate. A sample rate is the rate at which digital samples are taken.

FM does have bandwitdh, as you figured out. But the 75 kHz radio bandwidth isn't directly equivalent to 75 kHz audio bandwidth. In fact, that's the whole point of FM. AM is the modulation type for which the correspondence is a lot easier, just a factor of 2. (That's because they send the same information both below and above the center frequency, for simplicity and quality - single side band radio eliminates that factor of 2 for a slightly lower quality). But this simplicity means AM has a low audio quality - best for voice. FM uses a more robust modulation (encoding) so that the audio amplitude becomes a radio frequency change. As a result, the radio signal amplitude isn't that important anymore. And it's that amplitude which is most easily disturbed.

Effectivey you get 15 kHz audio bandwidth out of FM - a lot better than the 5 or so that AM gives you. Still, you lose about 3 kHz from the MP3 signal. In your case, that's probably just one of the causes. The limited MP3 bitrate causes some distortion, too, as does the whole digital->analog->FM path. They all add up. The easiest fix would be to obtain better quality MP3's, or a digital connection to your car sound system.

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  • $\begingroup$ +1 for both giving the background and the practical answer (higher data rate MP3's and/or digital connection). $\endgroup$ – Jim Clay Oct 28 '14 at 11:57

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