Subject: Re: [linux-audio-dev] Noisy LCD
From: Richard A. Smith (rsmith_AT_bitworks.com)
Date: Mon Apr 30 2001 - 03:11:00 EEST
On Fri, 23 Mar 2001 11:36:14 -0500 (EST), Richard C. Burnett wrote:
>Well I do not have any first had experience with the noisey LCD monitors,
>but I can tell you what I have discovered. Most people say when they
>unhook the monitor cable from the video card the noise goes away. My
>first thought would be to get a better shielded monitor cable. There was
>no response to that. I am not sure exactly how an LCD of the new types
>work. All I could imagine is that whatever setup they had was poorly
>made. I would think it much easier to guard against noise in an LCD where
>the voltages are so much lower and not as many field issues their.
I'm a little late on this discussion but I can shed a a few notes..
We design and make several different LCD drivers for various OEM
applications. They are all embedded apps though so none are in any
of the commercial monitors... (Yet.. We have a few items brewing)
In general LCD's are much noisier than their CRT counterparts. There
are high frequency clock signals running all over the driver boards.
The 2 main noise sources are:
1) Back light drivers. LCD backlights are 900VAC to 1.2KVAC
depending on the load of the display. The transformers for the
backlight voltage usually run around 32 - 50 KHz or so.
2) Pixel Clock(s). This is usually at 40Mhz or higher depending on
the resolution of the display. Its way out of audible range but it
is a square wave so depending on how fast the edges are and what
other noise sources it mixes with it can generate noise in all areas
of the spectrum.
If you are using a LCD that converts from RBG then there may also be
some RAM on board for the frame rate converter... the clock signals
there can generate noise.
Your cheaper LCD systems are probally going to be rideing the edge of
the FCC limit. And as already suggested its a sheilding thing...
FCC approved equipment can still generate a fair amount of noise.
I don't have any real good suggestions on shielding as that's our
partner company's problem. *grin* But you want to minimize long slots
or long gaps at the interface of 2 piecies of metal. Small holes are
ok but slots are bad.
-- Richard A. Smith Bitworks, Inc. rsmith_AT_bitworks.com 501.846.5777 Sr. Design Engineer http://www.bitworks.com
This archive was generated by hypermail 2b28 : Mon Apr 30 2001 - 03:48:31 EEST