下载
AN-945
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Te l: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
System Bandwidth vs. Resolution for Analog Video
by Ferenc Barany
Rev. 0 | Page 1 of 8
INTRODUCTION
Ensuring the best quality picture is the ultimate goal for video
system designers. With the introduction of HDTV, image
resolution as well as video signal bandwidth has increased
dramatically. Because video systems tend to support multiple
resolutions and interfaces, design engineers must carefully
examine the system bandwidth requirements to ensure that
signals are not distorted.
This application note provides a quick overview of the video-
specific terms used to characterize system bandwidth and how
they relate to 3 dB bandwidth.
SYSTEM BANDWIDTH DEFINITIONS
Generally, 3 dB bandwidth is a measure of a system’s band-
width. Assuming a low-pass frequency response, 3 dB
bandwidth is defined as the frequency at which the output
power drops to half of the output power at dc. This translates
into a 29.3% drop in the output voltage compared with dc.
For video applications, 3 dB bandwidth has little meaning
because it doesn’t directly reflect the attenuation of the video
signal components. The video literature mentions the term
gain flatness, which represents the maximum allowable drop in
signal amplitude within the pass band. Tests have revealed that
the human eye is unable to distinguish brightness variations of
less than 1%, which translates into a 0.1 dB signal drop within
the pass band, or put simply, 0.1 dB gain flatness. A more liberal
approach allows about 6% signal drop, which translates into
0.5 dB gain flatness.
The following sections describe the relationship between
the 0.1 dB and 0.5 dB gain flatness specifications and 3 dB
bandwidth. The examples assume a Butterworth frequency
response, which is the most common response.
H(s)
V
OUT
V
IN
75Ω
75Ω
H(s) = VIDEO SYSTEM FREQUENCY RESPONSE
V
IN
= VIDEO INPUT SIGNAL
V
OUT
= VIDEO OUTPUT SIGNAL
07125-007
Figure 1. Video System Frequency Response
