Quote:
Originally posted by verdeyw
But higher frequency means higher bandwidth, in most practical cases. DSL doesn't just use some high frequency 'f' it uses the range of freqencies from 0 to 'f' (or from some other lower frequency to f). If DSL could only use one frequency (a very narrow band signal), it would only be able to broadcast a continuous sine wave, which, by itself, could carry no information. Once you start turning the sine wave on and off, in order to transmit some data, the band needed starts spreading out, and if you want to have high-speed data, then you have a broadband signal.
Likewise, an FM radio station has more frequency space available, not due to nature or physics, but because the FCC licences FM stations 200kHz apart (which is why some digital radio tuners only up and down by 0.2MHz), while AM stations are licenced only 10kHz apart. That means that audio frequencies above ~5kHz have to be filtered out of the AM station's broadcast signal before it's transmitted, while FM stations can keep frequencies up to 15kHz (I think).
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There are many different ways to transmit data with electromagnetic transmissions. FM Radio uses Frequency Modulation. AM Radio uses Amplitude Modulation. The reason FM needs a larger frequency is because FM modulates the frequencies up and down. AM Radio modulates the amplitude of the wave instead. AM doesn't need a large frequency band because the frequency doesn't change.
The reason having a larger range of frequencies available for transmitting increases the bandwidth is primarily because it increases the number of channels for transmission. The spectrum of available frequencies is divided into multiple channels and each channel can sustain a certain transfer rate; adding more channels increases bandwidth.
It's important to remember that just because something's broadband doesn't mean that it will have a higher transfer rate than something that is narrowband. The word broadband has begun (incorrectly) to refer to high bandwidth which is unfortunate because they mean different things.
Matt