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What is the simplest way of detecting the C signal?

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pankaj

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Detection of Chroma

Hello,

I have to design a circuit which detects the presence of Y and C on a S-Video Connection.
Detection of Y signal is easy since it contains the sync information and it can be passed through a sync detector and the presence of video can be detected.
But what could be the simplest way of detecting the C signal


Pankaj
 

MonkeyBusiness

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A simple approach would be to use a filter matched for the C signal bandwidth, and detect signal energy after the filter. You can use a squaring or an ABS value for this purpose, or even a simple diode detector, depending on the available S/N.
 

TurboPC

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What do you want to compare the C signal to? Something compared to nothing? Or chroma compared to Y?

A chroma signal contains generally only frequencies in the range of the subcarrier frequency: 3.58 MHz in NTSC or 4.43 MHz in PAL.

If it is only signal presence, you could probably integrate the color signal using a diode and a "capacitor with a resistor in parallel" and control the time constant. Then use a comparator to detect the presence.
 

pankaj

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Yes, I just want to detect the presence of the signal and not compare it to anything.




pankaj
 

cbsirio

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try to make an amplifier that fed the signal to an LC filter centered on 4.43 MHz , then rectify it and use a comparator on the rectified signal.

HTH, Sirio.
 

Tavarnelle

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I use a TBA 395 to detect the presence of color on a circuit.
It is the simplest way:the circuit open a port in "burst" position and control
the presence of a 4.43 signal.It is possible to make the same circuit
with TLL or other similar system(eg a PIC).It is very important to control only the pedestal of syncro,because if you try to find 4.43 signal in the time of image you have the possibility to detect also 4.43 signal in luminance part.
Alberto
 

TurboPC

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Tavarnelle said:
I use a TBA 395 to detect the presence of color on a circuit.
It is the simplest way:the circuit open a port in "burst" position and control
the presence of a 4.43 signal.It is possible to make the same circuit
with TLL or other similar system(eg a PIC).It is very important to control only the pedestal of syncro,because if you try to find 4.43 signal in the time of image you have the possibility to detect also 4.43 signal in luminance part.
Alberto
Good point. He could also pass the luma signal into a sync extractor like the GS1881 or LM1881 and use the "back porch" output pin to trig the detection on the color burst signal at 4.43MHz (or 3.58 in NTSC) from the chroma signal.
 

Tavarnelle

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Exactly:I made a similar circuit with
LM1881.
It gone perfectly.
Alberto.
 

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