Audio quality degradation over FM transmission

Digital = almost perfect, when it works analogue= graceful degradation

martin

Has your car radio ( FM band ) been bad in this recent hot weather ? Mine's shocking. Thank goodness it's analogue !

Graham

Isn't it fairly flat ( topologically ) where you are ?

We ( with all our hills and valleys ) get all this atmospherically introduced multi-path trouble ( at least I think that's the cause ). Never actually got into really examining the fine detail of RF transmission and reception. I reckoned I'd leave the finer points to someone else and concentrate on what I seemed to be best at.

Graham

Yes but....

My car's stereo has an auxilliary input for such things.

Might it not be simpler just to connect it in some such way ?

I've never come across an mp3 player with an FM transmitter. Is it legal ? Does the Chinese company making it care ?

In a car.... for sure !

Graham

Errr what ?

If you're sending an analogue signal it doesn't matter how it originated. Or how good it is / isn't. The transmission medium ( unless digital ) doesn't care how the signal was encoded. Totally irrelevant.

Graham

Firstly, FM involves some distortion even at the theoretical level, because you receiver has to discard some of the ("infinite") FM sidebands. I don't know how much distortion. The distortion will be worse at high deviation. Your car radio will have narrow band filters, which increase distortion.

Secondly, most FM radios are not kind to audio. Phase shifts in the IF, and non-linearity of tuned circuit discriminators add distortion. Especially at full deviation, distortion is not negligable. With a clean music source, I would expect to easily hear the degradation caused by a car radio. FM receivers do have decent treble, but it I think it is often audibly distorted. High end MP3 should easily beat FM radio. Just what bit rate you need for "FM" levels of degradation, I don't know.

Thirdly, your transmitter will have distortion. 2% THD or more would be not surprise me.

Your whole transmitter - receiver chain at full deviation could do anything from 7% THD down to about 1% THD.

Roger

Which is usually unnoticable, except for the lowest bitrates, and the best listening conditions.

I've measured the outputs of my cheap MP3 player with single tone outputs. It vastly outperforms my ears, and shows good results on test equipment, coming up with a dynamic range of about the same as my CD player.

Many/most low-power FM transmitters are of very bad quality, and come nowhere near what FM can achieve.

I doubt many, if any, car radios have bandwidth switches anymore.

But that is not such a big concern, as they are only brief transients. Besides, the car environment is loaded with other noise.

For S/N relative to some distortion level ("big signal ref"), you can probably expect > 65 dB even in car radios.

The cheezy low powered TX'ers are the weaker part of the link, from my experience.

Example of what a decent FM RX'er can do:

I don't know about car RX'ers -- I just thought the linked one might be an easy reference.