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Originally Posted by jmac698
rfmmars:
This makes sense so far...
You've lost me here - the Y cable is on a component out, which does not rely on phase shift for color information. If an impedance problem causes the levels to be different, it doesn't matter, because each Pb/Pr is affected in the same way. The only problem is the relative difference between Y+Px and Y itself. This can be corrected in software with a multiplication factor. Did you think I was talking about svideo?
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I'm sure Richard was refering to impedance mismatches and other "strangness" that can happen when you tie two fairly broadband amplifiers together in an unitended manner. Depending on the circuit design, the change in impedance seen by the output amplifier or the input amplifiers could possibly affect the circuits response over its range of frequencies introducing phase delays, which could appear as color offsets or smearing.
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From the Connexant 878a datasheet:
Several graphs show -3dB @ 6MHz. That's pretty good! And the flash ADC is directly on the chip, so even the cheapest boards have the same excellent specs.
I have no doubt that what you were saying is valid for purely analog circuits, but digital oversampling filters can be almost perfect right to the edge of operation. There is also a "sharpness' register in the chip to let even higher frequencies through. Read a review of the Creative X-Fi soundcard, you will see +-.1dB all the way to Nyquist.
As for your other question, please see the thread in general discussion.
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The Conexant 878 is a next generation device that uses 2x oversampling and is earmarked for the HDTV market. I have assumed that in all of these discussions we have been refering to your ordinary BT848 or CX23881 devices - they are not quite in the same class
I believe that you are confused by the 2x oversampling nomenclature - what they are doing is using a device that is capable of sampling at 2x faster than the highest frequency that they anticipate (i.e. input << Nyquist for that device) and then bandwidth limit the result in the digital domain. The Fusion 878 is sampling at 28MHz whilst your run of the mill BT848's (and co.) are sampling at 14MHz. This is no great revolution in technology, just that the ecomomies of scale allow for manufacturers now to do so at a price point at which they can make a profit. I'd be willing to bet that Conexant put a filter on each of the inputs to ensure no aliasing would be happening - the goal these days to make these devices idiot proof and minimize external components - that data sheet quote is misleading since the filtering MUST be there, it is just not external to the device.
Oh, BTW, the sharpness register on the BT chip sets only boosts gain of the higher frequencies by changing the taps in the bandpass filter - is does not change the cutoff, just the shape of the rolloff (think peaking just before rolloff)
T