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EXTRONEWS JAN/FEB 1999

STEVE SOMERS

Comb Filters:

NTSC Decoding Basics

by Steve Somers, V.P. Engineering

Notch Filter Decoders

(Part 2)

Matrix 50 –

continued from page 3

Those looking for an external control

method will be pleased to know that every

Matrix 50 includes an RS-232/RS-422

serial communications port. This port may

be used in conjunction with Extron’s

Microsoft Windows™ based control

software, which is also included with

every Matrix 50. This package is able to

duplicate all the functionality of the

QuickSwitch™ Front Panel Controller, but

with the convenience of the Windows™

graphical user interface. Third party

packages may also make use of this port

for external control.

THE SWITCHER THAT

GROWS ON YOU

The modular design that makes the Matrix

50 Series so flexible also allows for field-

upgrade-ability between all of the afore-

mentioned options. As your requirements

expand, so can your Matrix 50. This is

accomplished by installing one or more of

the eight different Matrix 50 option kits.

There are two kits for each I/O size, one

for audio capabilities, one for video.

Through these add-on kits, a Matrix 50

sold as an 8 x 8 Composite video and

audio switcher may be upgraded—at any

time—to handle 8 x 8 component video

and audio, simply by adding two 8 x 8

Video Kits. In addition, if your original

number of inputs and outputs are no longer

sufficient, a new Video Kit may be

purchased to replace your existing one for

considerably less than purchasing a new

switcher.

For more information on what we consider

to be “the best unknown switcher on the

market,” please contact your local Extron

Electronics representative at: 714.491.1500

(Extron U.S.); +31.33.453.4040 (Extron

Europe); or +65.226.0015 (Extron Asia).

Also, see the Matrix 50 on the web at

http://

www.extron.com/product/matrix50.stm.

Here we are again. Figure 1 page 8

illustrates the basis of the NTSC

decoder system. Now, we have the

dubious task of wading through the

various methodologies for its disas-

sembly. The most critical portion of

the system is the first box in that

figure… the Y/C Separator. Decoder

quality totally depends on the

approach taken in this important

signal processing function. Some

have devoted their entire career to the

process of Y/C separation… you

know who you are. It is the function

within this one box in which this

entire series of articles is concerned.

There are fundamentally two methods

for separating the luma (Y) informa-

tion from the chroma (C) information.

Both involve filters that are designed

to discern one portion of the signal

from the other. One is called a simple

“notch/bandpass filter” and the second

is called a “comb filter.” The comb

filter variants are numerous and

comprise the successive parts of this

series. In this article, the

notch/bandpass approach is presented.

We learned about the creation of

NTSC in Part 1. Remember that the

visual image is made up of two

components… brightness and detail

information (the higher bandwidth

monochrome portion) and the color

information (low bandwidth portion).

The brightness and detail portion is the

Y channel, which represents the luma

information, and comprises most of the

signal bandwidth. The color informa-

tion, or chroma portion, is the C

channel comprised of less bandwidth.

In the composite signal, the chroma is

superimposed onto the luma channel.

This is accomplished through careful

interleaving of one component over the

other by selection of the chroma

subcarrier frequency… i.e., the 3.58

MHz (the frequency is really

3.579545… MHz).

NTSC DECODING…

IT’S NOTch EASY

Focus one eye on Figure 1. Separating

the luma component from the chroma

component is the main task in that first

box. The designers of the NTSC

system knew theoretically that it would

be possible to properly separate Y and

C, but did not have a cost-effective way

to do it in the early years. In fact, the

more sophisticated methods of separa-

tion through “comb filters” did not

arrive in the market until the late 70’s,

more than 20 years after the system was

adopted.

So, early television receivers used the

Notch/Bandpass Filter system for Y/C

separation because the method has a

low cost and is easily implemented

(continued on page 8 –

See “Notch Filter Decoders”)

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