[mythtv-users] Image Quality PVR-500

[Brian Wood]

On Feb 20, 2007, at 7:55 PM, Brian Wood wrote:

>
> On Feb 20, 2007, at 5:48 PM, David Brodbeck wrote:
>
>> Brian Wood wrote:
>>> Some people have said that the tuners on the capture cards are
>>> pickier about levels than the ones in TV sets, but I'm not really
>>> sure I believe that, because I would think they are using the same
>>> tuner modules that are used in TV sets, why design a totally
>>> different unit?
>>
>> I've always suspected the problem is a tuner card lives in a very
>> noisy
>> environment.  That limits the amount of amplification (and hence AGC)
>> they can put on the card, because it would end up amplifying noise
>> from
>> all the other components in the computer.
>
> Well the RF can is supposed to be shielded, but it has a press-on
> lid. I've said here before that I have had in the past problems that
> were solved by a dab of solder on each side of the can top.
>
> Of course the bottom of the can is open to the board, and unless
> there is a god design, like using one of multiple layers as a ground
> plane, crap can get in that way.
>
> But noise can certainly get in after the RF can, in the baseband
> sections of the card. You really can't expect great design for $50,
> and as you said the inside of a PC case is one of the most
> horrendously noisy environments that can be conceived.

So I pulled out a card and looked at it, the RF can is actually all- 
around.

But my comments still apply, good design requires a lot of bypass  
capacitors and/or series inductances, both of which are expensive,  
and the makers want to cheap out any way they can.

I never thought I'd see the day when a million-transistor device was  
"cheap", and a lousy capacitor was costly, but that's the way it is  
today.