According to our extensive tests in conjunction with
Philips Consumer Electronics, digital TV (both COFDM
and ATSC) is actually less susceptible to interference
from spread spectrum clocks (ssc) than current analog
TV (both PAL and NTSC).
Philips has stated in a letter to us that they do not
consider spread spectrum clocks to be an interference
threat to digital TV.
One report from the University of Hertfordshire has
stated that digital TV is more susceptible to ssc
than non-modulated clocks, which by itself is true
because of the digital filtering techniques used
in digital TV. However, this report does not look
at the real world potential for interference.
Philips found that when an interfering ssc signal is
present and the digital TV and analog TV are both at
comparable levels (X dB above their respective
threshold of visibility signal strength) it takes
roughly 16 dB higher amplitude ssc signal to disrupt
the digital TV versus the analog TV.
In addition, through many contacts of both television
manufacturers, television broadcasters, and discrete
clock vendors we have found virtually no evidence of
interference complaints due to spread spectrum clocks.
Since there are no real interference problems with
analog TV, and digital TV is less susceptible, there
is no additional interference risk due to ssc. We
are in the process formalizing these results for
publication. Our previously published analog TV
interference study can be found at:
http://www.lexmark.com/sscg
Regards,
Rob Oglesbee
EMC Engineer
Lexmark International, Inc.
rogle...@lexmark.com
John Woodgate <jmw%jmwa.demon.co...@interlock.lexmark.com> on 06/15/2001
06:09:27 PM
Please respond to John Woodgate <jmw%jmwa.demon.co...@interlock.lexmark.com>
To: emc-pstc%majordomo.ieee....@interlock.lexmark.com
cc: (bcc: Rob A Oglesbee/Lex/Lexmark)
Subject: Clock dithering
<3b2a4fda.8c64c...@mediaone.net>, David Heald <davehe...@mediaone.net>
inimitably wrote:
>Con's????? None that I can think of, but...I have seen somewhere (maybe
>here??) that the EU is considering new regulations for modulated clocks,
>but this is in the early stages, so use them now while you can get the
>most benefit from them.
That's correct. Digital TV has proved rather badly susceptible to
interference from dithered clocks.
--
Regards, John Woodgate, OOO - Own Opinions Only. Phone +44 (0)1268 747839
Fax +44 (0)1268 777124. http://www.jmwa.demon.co.uk Why not call a vertically-
applied manulo-pedally-operated quasi-planar chernozem-penetrating and
excavating implement a SPADE?
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