Analog Video Comb Filter

Analog-Video-Comb-Filter

Analog Composite Video to S-Video comb filter board using SAA4960, SAA4961 or SAA4963 integrated circuit.

WARNING

This project uses end-of-life components that are no longer manufactured. If you want to build this, make sure that you can obtain all components before starting. Used or new-old-stock parts can sometimes be found online.

Description

This is a fully analog alternative to digital comb filters such as Extron YCS Transcoder or YCS 100. That doesn't mean it will give better results though. I just made it because I didn't find any fully analog device like that on the market and I wanted one.

The comb filter is used to convert a composite video signal into an S-Video (Y/C) signal. Comb filters are much better than passive LC filters usually built into composite video displays, as they can preserve the frequencies of luminance that overlap with chrominance, providing higher image detail while removing dot crawl, and remove those overlapping luminance frequencies from the chrominance signal which reduces color artifacts.

When using the SAA4960 integrated circuit, PAL B, D, G, H and I systems are supported.

SAA4961 provides aditional support for PAL M, N and NTSC M systems.

When using the SAA4963 integrated circuit, only the NTSC M system is supported.

The board is designed to fit inside a KRADEX Z-76 enclosure.

Circuit description

A composite video signal is fed in through an RCA connector (J4). The signal is terminated with a 75 ohm resistor and fed to U3 (or U7), U4 and U6 through 100 nF capacitors.

The LM1881 sync separator (U6) uses the composite video signal to generate a burst gate signal. The burst gate signal passes through a 74HC04 inverter (U5) to the MC44144 subcarrier PLL (U4).

U4 uses the composite video signal and the burst gate signal to generate a subcarrier frequency synchronized to the colorburst of the composite video signal. This subcarrier signal is then passed to the FSC input of U3/U7.

The SAA4960/61/63 comb filter (U3 or U7) is fed with the composite video signal and the synchronized subcarrier signal. The jumpers SYS1 and SYS2 set the video standard, and the jumper LPF can be used to disable the input low-pass filter. This circuit outputs filtered luminance and chrominance signals and a delayed composite video passthrough signal (when using SAA4963, the CVBYP jumper has to be shorted to allow non-delayed composite video passthrough). Those signals are then fed to the output amplifiers.

The AD813 triple op-amp (U2) is used for the output amplifiers. The output signals from the SAA4960/61¹ are DC-biased by around 1V, so a non-rail-to-rail op-amp like the AD813 is viable. The gain can be adjusted from approx. 1,5x to 2,5x with RV1, RV2 and RV3.

Pictures

Assembled rev.2 devices (PAL and NTSC)	Completed, operating device in enclosure

Image comparison - no filter, LC filter and comb filters

No filter	LC filter
Luminance detail is preserved, but dot crawl is visible across the entire picture.	Dot crawl is mostly removed, but luminance loses some sharpness, and higher frequency luminance components (responsible for sharpness) are left in chrominance causing color artifacts.
This analog comb filter	MC141627 digital comb filter (for comparison, Extron YCS transcoder)
Dot crawl is mostly removed and luminance sharpness is mostly preserved. Higher frequency luminance components are properly separated from chrominance.	More effective than the analog comb filter.

Monochrome image comparison

No filter	LC filter
Luminance detail is preserved, but dot crawl is visible in colored sections.	Dot crawl is mostly removed, but luminance loses some sharpness.
This analog comb filter	MC141627 digital comb filter (for comparison, Extron YCS transcoder)
Dot crawl is mostly removed and luminance sharpness is mostly preserved.	More effective than the analog comb filter.

LEDs

D1 shows that the device is receiving power.

D2 shows that the comb filter is operating (U3 is in COMB mode). I'm not sure if U3 will ever not be in COMB mode when used in this circuit, so this LED might be unnecessary. SAA4963 does not have the output needed by this LED, so if it is used, then R19, R20, D2 and Q1 can be left unpopulated.

REFDL filter capacitors

When using SAA4960 or SAA4961, install C24 and C26, and leave C31 and C32 unpopulated.

When using SAA4963, install C31 and C32, and leave C24 and C26 unpopulated.

Solder jumper settings

Standard selection jumpers (SYS1, SYS2)

SAA4961 only. When using SAA4960, SYS1 and SYS2 jumpers should be left open. When using SAA4963, the jumpers are not connected.

Appropriate crystal for the subcarrier oscillator should be installed depending on the jumper setting. If the standard is going to be changed frequently, it may be possible to install a socket for the crystal, however it should be taken into account that MC44144 is sensitive to additional capacitance at the crystal.

Low pass filter jumper (LPF)

The SAA4960, SAA4961 and SAA4963 integrated circuits have a built-in low-pass filter on the composite video input. With SAA4960 and SAA4961, this filter can be disabled by shorting the LPF jumper, but it's recommended to leave it on. With SAA4963, the jumper is not connected and the filter is always on.

Composite video passthrough jumper (CVBYP)

SAA4963 doesn't have a composite video output, so the CVBYP jumper was provided for composite video passthrough. When the jumper is shorted, the video signal for composite video output is taken from pin 13 (composite video input) of SAA4963. The DC bias for the output amplifier is provided by the clamping circuit inside SAA4963. Do not install R12 to avoid additional load to the clamping circuit. This configuration has not been tested and may not work correctly. Leave the jumper open if shorting it causes issues for the comb filter.

WARNING: This jumper must never be shorted if using SAA4960/61.

Adjustment

After assembly, including setting the jumpers and installing the correct crystal depending on the analog video standard, the variable capacitor C17 will have to be adjusted so that the PLL locks correctly to the color subcarrier and the loop control voltage is centered.

Use the following procedure for adjustment:

1. Connect an EBU (for PAL) or SMPTE (for NTSC) color bar signal from a good quality reference source to the composite video input.
2. Connect a 10x oscillososcope probe to U4 pin 3 (phase detector output loop filter). Use AC coupling, 200 mV/div vertical scale, and 100 µs/div timebase.
3. Here are the reference pictures needed for the following steps:

PAL waveforms

Not locked	Locked, not adjusted	Best adjustment

NTSC waveforms

Not locked	Locked, not adjusted	Best adjustment

4. Follow the flow chart below:

flowchart LR
  A{Able to obtain waveform shown in <b>Best&nbsp;adjustment</b> by adjusting C17?} -- Yes --> B([PLL correctly adjusted.]);
  A -->|No| C{"PLL able to lock (one of <b>Locked,&nbsp;not&nbsp;adjusted</b> waveforms)?"};
  C -->|Yes| D{Does waveform resemble the <b>top</b> or <b>bottom</B> picture in <b>Locked,&nbsp;not&nbsp;adjusted</b>?};
  C -->|No| E["Measure frequency at U4 pin 1 (FSC output), and adjust C17 to get as close as possible to required frequency."];
  E --> F{Is the frequency higher or lower than required?};
  F -->|Higher| G[replace C17 with a higher max. value trim capacitor, or add a parallel capacitor.];
  F -->|Lower| H[replace C17 with a lower min. value trim capacitor.];
  D -->|Top| G;
  D -->|Bottom| H;
  G --> I[Adjust C17 and observe the waveform at U4 pin 3.] --> A;
  H --> I;

5. Set RV1, RV2, RV3 to center position. Connect the luminance, chrominance and composite outputs to an oscilloscope with 75 ohm load resistance. Luminance signal is preferred for oscilloscope trigger. Adjust RV1, RV2, RV3 to obtain the following:

Readjustment is necessary if the crystal and the standard selection jumper settings are changed.

Parts list

Resistors

Capacitors

Inductors

Semiconductors

Integrated circuits

Other

License

This work is licensed under CC BY-SA 4.0 license.