Re: Audio quality degradation over FM transmission


"Ian Stirling" <root@xxxxxxxxxxxxxxxxx> wrote in message
news:42bb19ba$0$30827$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> kristian.hermansen@xxxxxxxxx wrote:
>> Ian Stirling wrote:
>>> 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.
>>
>> Will I lose the low or high frequency bands? I mean, will I get less
>> bass or less treble? What is the frequency window?
>
> Treble.
> There is no real reason why you should lose bass, though it may decay
> under
> 10Hz.
>>
>>> 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.
>>
>> 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 :-(
>
> 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.

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


.

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