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					Philips Semiconductors                                                                                                          Application note



  Comb filter application


COMB FILTER APPLICATION                                                    The disadvantage of this method is that luminance information that
One of the functions that must be performed by a color video               exists in the frequency domain close to that of the subcarrier is lost.
decoder is to separate the composite video signal into the luminance       The second (and more expensive) method is to use a comb filter.
(brightness) and chrominance (hue and saturation) components.              Comb filters take advantage of the fact that, generally, the
This is generally performed by one of two methods:                         luminance content of a video signal does not change greatly from
A bandpass/bandtrap method which results in removing the                   one line to the next and that the chrominance signal is inverted from
subcarrier frequency portion (and some surrounding frequencies)            one line to the next (this is due to the fact that the subcarrier
from the video signal, see Figure 1.                                       frequency is an odd multiple of the half line frequency).
                                                                           See Figure 2.




                                    LUMINANCE
                                    AMPLITUDE




                                                FREQUENCY                     FSUBCARRIER




                                 CHROMINANCE
                                    AMPLITUDE




                                                FREQUENCY                     FSUBCARRIER
                                                                                                                                        SN00097

                                                                  Figure 1.




 1996 May 31                                                           1
Philips Semiconductors                                                                                                               Application note


  Comb filter application




                             LINE N




                           LINE N+1                                                                         LUMINANCE COMPONENT
                                                                                                            OF VIDEO SIGNAL




                             LINE N




                                                                                                            CHROMINANCE COMPONENT
                                                                                                            OF VIDEO SIGNAL




                           LINE N+1




                                                                                                                                           SN00098

                                                                      Figure 2.

Composite video is passed into the comb filter which stores one line           Two types of comb filters are covered here, an analog glass delay
of video (line N) and then compares the stored line with the next              line type, the CFM300 and switched capacitor delay line type, the
(line N+1).                                                                    SAA4961. Composite video is input to either of these devices and
                                                                               the Y/C output is input to the decoder which is programmed to
Luminance is derived by summing the two lines and dividing by two.
                                                                               process the input as S-Video (the decoder chroma notch is turned
The luminance content is doubled (and subsequently divided by two)
                                                                               off).
and the chrominance cancels out.
                                                                               The CFM300 is an integrated hybrid circuit module which is
Chrominance is derived by inverting one line (line N) and summing
                                                                               adjustment-free. The device consists of a precision-ground glass
lines N and N+1 with the luminance canceling out and the
                                                                               wave guide with two piezo-electric transducers bonded to two
chrominance doubled (then normalized by dividing in half).
                                                                               corners. One of these transducers is a transmitter. It converts the
The advantage is that luminance information that consists of                   electronic video signal to an ultrasonic sound wave which is then
frequencies around that of the subcarrier, is maintained.                      propagated through the glass and received by the other transducer
                                                                               and converted back to video. The length of propagation time from
One drawback to comb filters is that in situations where the video
                                                                               transmitter to receiver is exactly one horizontal period. See Figure 3.
signal does change greatly from one line to the next (as in the case
when a horizontal object is depicted), certain artifacts may be                The reconverted and delayed video signal (line N) is then summed
created. This is especially true with horizontal graphics that are             with the current video signal (line N+1) as described above to
overlayed onto the video because, unlike camera images that exhibit            produce the luminance and chrominance signals. The CFM300 is
some low frequency roll-off (due to the analog nature of the imager),          not adaptive. The advantage of this type of comb filter is that it is
graphics can create changes in the line to line video that are quite           inexpensive and will work with time-variant signals such as VCRs.
abrupt. As a result of the two-line summation/cancellation nature of           The disadvantage is its size. However, a horizontal version is
the comb filter, if there is an abrupt change from one line to the next,       available, called the CFM330.
un-cancelled subcarrier information (and un-cancelled luminance
                                                                               The other type of comb filter, The SAA4961 uses a switched
content) may remain. This can result in a softening of the edge of
                                                                               capacitor delay line to produce the line-length delays required. This
the horizontal structure (due to residual luminance) and a line of
                                                                               capacitor delay line is incremented by a clock running at the
unsuppressed subcarrier at the transition, sometimes called
                                                                               subcarrier frequency. This clock can either be derived from the input
“hanging dots”. Some comb filters minimize this effect by detecting
                                                                               signal (easily done in the case of an analog decoder application, not
these abrupt transitions and switching momentarily from a comb
                                                                               easily done in the case of a digital decoder application) or from an
filter mode to a chroma trap mode (the first separation method
                                                                               oscillator. See Figure 4.
mentioned above). This is known as adaptive comb filtering.




1996 May 31                                                                2
Philips Semiconductors                                                                                                                Application note


  Comb filter application


The SAA4961 will perform comb filtering for either Pal or NTSC (the                Applications in which the video is scaled down for display in a
CFM300 is NTSC only). The subcarrier clock frequency must match                    window or for compression (such as JPEG or MPEG) generally do
that of the standard being combed. The SAA4961 is an adaptive                      not require comb filtering. The added resolution from the comb
comb filter that will switch to a chroma trap mode in the event of a               filtering process would be lost anyway ( in the case of JPEG or
discontinuity from the previous line to the next.                                  MPEG compression, it could actually degrade the compression
                                                                                   ratio). Also, because Pal subcarrier is at a substantially higher
An application schematic for both comb filters is included with the
                                                                                   frequency than that of NTSC (4.43MHz as opposed to 3.58MHz),
DPC71 evaluation board application note (sheet 4 of the DPC71
                                                                                   the chroma notch method has less of a detrimental effect on the
schematics).                                                                       luminance response. Therefore comb filtering is not as necessary
                                                                                   for high luminance response from Pal.



                                                                                            TRANSMITTER TRANSDUCER




                                                                                                       WAVE GUIDE NODE


                                                                                            RECEIVER TRANSDUCER
                                                 GLASS WAVE GUIDE
                                                                                                                                            SN00099

                                                                      Figure 3.


                                SUBCARRIER
                                CLOCK INPUT


                                                            1, 2 OR 4 LINE DELAY                     DELAYED
                                                                                                     SIGNAL
                    COMPOSITE
                    VIDEO IN
                                                                                                                         CHROMA OUT




                                                           INVERTER


                                SUBCARRIER
                                CLOCK INPUT


                                                            1, 2 OR 4 LINE DELAY                     DELAYED
                                                                                                     SIGNAL
                    COMPOSITE
                    VIDEO IN
                                                                                                                         LUMA OUT



                                                                                                                                            SN00100

                                                                      Figure 4.

ORDERING INFORMATION                                                               Technical/Manufacturing Centre
CFM300        4322-027-06182       (vertical mounting)                             Philips Components Blackburn
CFM330        4322-027-06212       (horizontal mounting)                           Philips Road
                                                                                   Blackburn, United Kingdom BB1 5RZ
                                                                                   Tel. +44 1254 55241
CONTACT INFORMATION                                                                Fax. +44 1254 698702, +44 1254 52040
Philips Components
Marketing Communications
                                                                                   U.S.A Only
                                                                                   Customer Service Center
Building BAE-1
                                                                                   Philips Components
5600 MD
                                                                                   1440 W. Indiantown Rd.
Eindhoven, The Netherlands
                                                                                   P.O. Box 689605
Fax. +31 40 27 24547
                                                                                   Jupiter, FL 33468-9605
                                                                                   Tel. +1 561 745 3300
                                                                                   Fax. +1 561 745 3601, +1 561 745 3602


1996 May 31                                                                3

				
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Description: Comb filter application