[SI-LIST] Re: what's de-emphasis
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sun, 1 Nov 2009 23:14:14 -0500
Joe wrote:
> The Pre-emphasis definition is acceptable. The De-emphasis definition is
> clearly wrong, in my opinion. ...
Those definitions (in the original post from Adiu) are correct for analog
transmission, such as FM radio where the added noise is the problem and it
increases linearly with the modulation signal's frequency. It is common to
emphasize the higher frequencies at the sender (transmitter) and reduce the
higher frequencies by an equal amount at the receiver, to improve the
signal-to-noise ratio. The medium, over which the signal is transmitted,
doesn't distort the signal; it just adds noise.
This is in contrast to digital signals on copper for example, where
pre-emphasis may be applied to help overcome the waveform distortion of the
medium between the sender and receiver. In that case, de-emphasis has a
totally different meaning.
As Michael wrote, you need to dig harder to find the right definitions that
apply for the kinds of signals and media you are looking at ... which sort
of means you need to know what they mean before you can find the right
definitions.
The definitions quoted are the traditional meanings of pre- and de-emphasis,
throughout most of the 20th century. Their meanings have changed now that
they have been applied differently to handle other problems.
Andy
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