Audio quality degradation over FM transmission

I would like to know why a CDDA/MP3 FM radio transmission at 88-108 MHz results in degradation of sound quality. As I understand it, standard CDDA/MP3 audio is sampled at 44.1 KHz. 88-108 MHz is well above this threshold, and I don't see how the transmitted audio "data" can't travel along this invisible pipe at an adequate enough speed to result in perfect end-to-end transmission (assuming no interference/loss). I have heard that USA public FM radio transmissions could be compared to

64 Kbps MP3 audio. I guess that I am just interested in the physics of how this works, so don't be afraid to hit me with some math :-)

Kristian Hermansen

Reply to
kristian.hermansen
Loading thread data ...

Kristian, Refer to: h**p://

formatting link
Figure 3. The allocated modulating bandwidth for the L+R (Mono) signal is 15KHz. Any modulating signal above 19KHz would interfere with the 19KHz pilot signal, which is necessary for the receiver to reconstruct the 38KHz stereo carrier. Sorry about the earlier "Blank Post". I inadvertently hit the "Post message" button. Regards, Jon

Reply to
Jon

Will I lose the low or high frequency bands? I mean, will I get less bass or less treble? What is the frequency window?

Why can it not be CDDA quality? For instance, why can't a CD be streamed over the radio without loss in quality? The frequency of the music is 44.1 KHz and the FM frequency is between 88-108 Mhz -- which is a magnitude of 10^3 apart. I'm confused :-(

Yes, I am. Got me :-)

What if I had a high-quality transmitter?

Kristian Hermansen

Reply to
kristian.hermansen

Long.........

Kristian Hermansen

Reply to
kristian.hermansen

Actually, these things won't be very relevant. I am using this in my car! The device will site less than 15 feet from the receiving antenna in the same vehicle. Can we make some assumptions that way?

No, it's an analog signal propagating from my MP3 player's attached transmitter to my car's antenna on the back -- which hooks into my FM stereo :-) Easier now?

Anyways, I think someone answered that since it is encapsulated in a 75 Khz band, I am not going to lose much quality at all -- except for the

15 Khz band instead of 20 Khz. I don't think I can hear below 30 Hz or above 18.5 KHz anyways...so no problem losing the highs!!!

Kristian Hermansen

Reply to
kristian.hermansen

Nope, the MP3 player doesn't have that quality. The reason there are MP3s is because of compression, they eliminate some of the 'unneeded' data that is stored on the CD so that it will take up less space. This compression means that there is more noise and less dynamic range in an MP3 than the original. You also loose some of the highs.

Next, you have your MP3 player. When he said designed down to a price, he was talking about the MP3 player, not the transmitter. They often do not have the 'best' audio stages, being just good enough to not sound too crappy, usually. Now, you take this already distorted, noisy source data, the MP3, put it out from the so-so audio out of an MP3 player, couple it (usually badly, with impedance mismatches and other signal degraders) into your FM transmitter with a MAXIMUM frequency range or probably 15K, and you will hear a lot poorer music than the original CD. Such is engineering.

Charlie

Reply to
Charlie Edmondson

Well! In "this recent hot weather", 108°F/42°C, my car radio has been working just fine ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
 Click to see the full signature
Reply to
Jim Thompson

I know that MP3 is lossy. All I am asking is if the audio input can

*physically* (due to transmission) match the audio heard over an FM receiver exactly. Just assume that you have the highest quality CD source media, CD player, FM transmitter, Antenna, FM receiver, and Speakers -- and all that is left to degrade the signal is the modulation of the input over the FM channel -- will the input match the output??? This is what I want to know. I mean, the act of taking a 100% clean CD audio input -- piping it over FM (with no constraints but the theoretical math involved in modulating the signal over say 108 MHz) -- will it end up 100% clean on the other side? I don't care about any other factors. Mathematically, does the transmission of CD audio input via FM have the effective output of about 256 Kbps MP3 lossy audio on the receiving end? Is it a degrading function (forget the very low and very high frequencies)? Wow, I must have reworded this about 50 times now...lol Forgive my naivety!

Kristian Hermansen

Reply to
kristian.hermansen

I know that MP3 is lossy. All I am asking is if the audio input can

*physically* (due to transmission) match the audio heard over an FM receiver exactly. Just assume that you have the highest quality CD source media, CD player, FM transmitter, Antenna, FM receiver, and Speakers -- and all that is left to degrade the signal is the modulation of the input over the FM channel -- will the input match the output??? This is what I want to know. I mean, the act of taking a 100% clean CD audio input -- piping it over FM (with no constraints but the theoretical math involved in modulating the signal over say 108 MHz) -- will it end up 100% clean on the other side? I don't care about any other factors. Mathematically, does the transmission of CD audio input via FM have the effective output of about 256 Kbps MP3 lossy audio on the receiving end? Is it a degrading function (forget the very low and very high frequencies)?

And if not, what is the theoretical maximum source input before you start incurring loss on output? Obviously one could not send audio sampled at 20 Ghz (insane) and get all that information over to the receiver -- since 20 Ghz of information can not fit into that 108 Mhz frequency band. I synnonomize this with mapping a much larger data set onto a smaller data set (like an MP3 encoding). With two CD recordings in a studio done at the same time using slightly different audo equipment, it is mathematically possible for these two differing audio samples to end up as an identical MP3 file in the end -- even though there are subtleties in the original source audio, right? Would the same be true of FM transmission of two similar sources? If not, then you have a widening transformation of the original vector space. Otherwise, it is a narrowing transformation resulting in loss of input during mappings. Wow, I must have reworded this about 50 times now...lol Forgive my naivety! I know I must be confusing some of you, because I am confusing myself now...

Kristian Hermansen

Reply to
kristian.hermansen

I'm in somewhat mountainous terrain here. But about the only multi-path stuff I ever see is in the downtown "canyons"... probably all that steel.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
 Click to see the full signature
Reply to
Jim Thompson

Err, no. Brimming over with various sorts of wrongness...

For a good signal, pretty much the only thing that you could notice is that the bandwidth is a bit lower, as the FM signals bandwidth is about 15Khz, compared to the CD bandwidth of about 20Khz.

This is due to the way the signal is fitted into one FM channel, of 75Khz (0.075Mhz) width, so you can have more than one channel on the FM band. FM radio is much, much higher quality than 64K, if you're in decent signal area, I'd go so far as to say that a good quality signal is usually better quality than 128K.

As to "degradation of sound quality", you're not using a FM transmitter plugged into a MP3 player are you? These are generally designed down to a price, not up to best audio quality.

Reply to
Ian Stirling

Do you want the long answer or the short answer ?

Graham

Reply to
Pooh Bear

Treble. There is no real reason why you should lose bass, though it may decay under

10Hz.

Irrelevant. It's transmitted in a 75Khz bandwidth, about 32Khz above and below the nominal frequency, and would have exactly the same information content if it was centered on 88,108, or even 1Mhz.

Reply to
Ian Stirling

Actually, it gets worse than that. In that 75 KHz channel, you have 15 KHz of LEFT+RIGHT, another 15 KHz of LEFT-RIGHT (or is it RIGHT-LEFT), plus MUSAC, or whatever. The LEFT-RIGHT channel is transmitted as a double sideband suppressed carrier AM signal; so, it actually takes up 30 KHz. I don't know this for a fact, but I suspect that if an FM station is transmitting digital music, it is in the slot formerly used for MUSAC.

To transmit a 75 KHz bandwidth of signal, the FM transmitter actually uses about 200 KHz of spectrum. That is just the way FM works. Defined by Bessel Functions.

The 88 to 108 is immaterial, as has been pointed out. In the FM receiver, the first thing that happens is that whatever station the radio is tuned to gets converted to a narrow slot at 10.7 MHz. You can't make the slot wider, or you would get interference from the adjacent channel.

Tam

Reply to
Tam/WB2TT

Are they transmitting digital signals in theFM band? This is confusing me. If so I have no information

Or are they transmitting mp3 in analogue on FM (Zenith GE)stereo. If this is the case they are processing the sound at the radio station with compression and dynamic equalisation, to give the station sound " a personality" This is a nasty habit that stations do to make them sound louder than the next station in the FM band.

They use mp3 for storage of the music to save disc space. The conversion from linear encoding (uncompressed to mp3 is not perfect

88/108 MHz has nothing to do with the encoding, it is just the carrier. It is generally transparent to what is conveyed on it.

sorry, no maths involved

martin

Reply to
martin griffith

Long + incomprehensible maths please

martin

Reply to
martin griffith

Now look here Martin. Do we really want to go into frequency deviation and error correction protocols vs S/N ratio - line of sight signal strength - loss and reflection via obstacles - multi-path distortion and all that other shit ? Not to mention weather-related signal transmission / reception obstacles.

Graham

and that's just the beginning......

Oh god and it's digital *too* - ok add some more crap !

Reply to
Pooh Bear

I sense this immeasurable force pulling tight on my leg......

Graham ;-)

Reply to
Pooh Bear

That's standard *analogue* transmission. With all its attendant faults.

Now pls try doing the same with a bigger bandwidth digital signal with error correction !

Don't bother ringing me back !

Graham

Reply to
Pooh Bear

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.