I've asked this question several times and have never gotten a straight answer. Not even from NAD. 1. How does a receiver's low pass filter work? Does it have anything to do with the receiver's DACs? 2. If my cdp is connected via ANALOG, my speakers set to "small" and my sub to "on", is the receiver sending a filtered or full range signal to my speakers? If it were a digital connection then the answer is a definite YES, but here I'm not sure. NAD says full range but I don't think Karen understood my question. She said any analog signal is automatically in bypass mode and sent full range to the speakers and sub. 3. My goal is to connect my cdp via analog to take advantage of its superior DACs but want all signals below 80 sent to my sub. If this is not possible or if the receiver has to put the signal through more conversions in order to redirect (filter) the lower frequencies, then I would just assume use a digital connection.
On a side note: I've concluded that if any of the surround modes are used, then a digital connection is usually better (assuming a decent receiver) thus avoiding the extra conversions.
Well, I thought we'd answered the question for you in a previous thread. Try this.
I'm not sure what you mean by "how does it work", but the probable answer there is simple. Being a low pass filter, it establishes a frequency to use as the "knee" for the roll off. From that frequency point, the signal is rolled down in level as the frequency rises. How quickly the roll off takes effect will be determined by the designer and is typically either -12dB or -24dB per octave. The frequencies below the knee are left intact in level. This would describe a perfect filter, which doesn't exist, but should get you started on what is happening. But, I get the feeling this isn't what you wanted to know.
When you ask whether the filter has anything to do with the receiver's DAC's, you will have to be more specific. As we discussed in a previous thread, there are analog to digital conversions which take place in various areas of the receiver depending on what function you want the receiver to perform. If you feed the receiver an analog signal and want to use digital controls for filtering low/high fequencies (any time you use a digital multichannel playback mode you are running the signal through digital circuits), you will apply one set of DAC's. If you apply a digital signal and want the receiver to do the conversion to an analog signal or pass the signal along in a digital form, you will use a different chip set. This is the general rule and there are more than likely several variations on how the functions are performed depending on the cost and quality of the receiver. Higher cost consumer electronics tend to specialize functions in order to use the "best" parts available for any one function. Obviously, the opposite is true of lower cost electronics.
Your second question can cause confusion due to the number of possible "analog" inputs on many receivers. A two channel analog signal is typically sent through the receiver's digital circuits to allow for filters to remain intact. This means you don't have to reset your receiver every time you switch between a Dolby Digital DVD and a two channel CD. A 5.1 "analog" signal typically bypasses that section of the digital network since the filters are set at your source player. Doubling up on the filters (using both player and receiver) would cause inaccurate frequency response. No two channel player has, to my knowledge, been equipped with any sort of filters for speaker/sub settings. Therefore, it sends a fullrange signal to your receiver. Saying the signal is sent to your speakers "fullrange" when using the 5.1 analog inputs would be somewhat of a misnomer since the player has already determined what filters need to be applied to the signal. The signal that arrives at the 5.1 analog inputs has already been filtered to remove some frequencies, if you've set up the player to establish "sub on" and "small speakers". If you've set the player to "large" speakers all around and "sub off", the player sends a full range signal to the "5" inputs and nothing to the ".1" input(s).
I can't answer your third issue since you haven't stated whether you have a player with 5.1 analog outs. If you are using a two channel only player, I would "assume" you would just "as soon" run a digital line to the receiver if it possesses the better quality DAC's.
However, in the long run, I do think you're making too big of an issue about additional A/D and D/A conversions within the receiver. If you assume the receiver you purchased is a well designed and inplemented unit, you shouldn't have to worry about what happens in these interstage circuits. There are far more important issues to deal with in a typical HT receiver based system. At this point in the game, the digital handling of the signal is rather simple and straight forward.
What exactly are you concerned about when you worry about additional conversions?
How many CDPs have 5.1 analog outputs? Isn't a CD a two-channel format?
Blazer, regarding your second question, simply disconnect the sub and select the highest cutoff frequency that you can. Then alternately select the sub on and off in the receiver setup. You should notice immediately if the sub "on" leads to a filtered signal being sent to the main speakers or not. If it's like my h/k avr-325 receiver, then the CD analog input appears to go through digital conversion even when the speakers are set to large, while the 6/8 channel direct inputs don't. People that don't need a sub for music should try the 6/8 channel inputs to see if they sound better than the CD input.