20 bit players vs. 24 bit players

What's the 'sonic advantage going to a 24bit if most redbook cd's are only 16bit??..
Limited advantage unless some digital processing is done - in which case the extra bits help.

Main advantage is in upsampling - this helps improve filtering and can reduce jitter if done well.
"if most redbook cd's are only 16 bit??" - what do you mean by "most". All redbook CDs are 16 bit. Sony-Philips never created any other definition. Even HDCD is recorded in 16 bit format where bit 16 (least significant) switches dynamic range of music encoded in 15 remaining bits.
Ok' ALL Redbook cd's
Yeh but what's the SONIC Advantage??
Extra bits come from digital filtering - pretty much useless information. Sonic benefit is, as Shardone explained already, improved filtering with possibility of reduced jitter. Improved more accurate filtering in digital filters gives smoother more coherent sound while reduced jitter lowers noise floor.

I would not put too much attention to number of bits or specifications in general.
The difference between 16 and 20 bit quantization is an expansion of dynamic range, as well as increased low-level detail resolution. 20 bit resolution actually represents the state-of-the-art with regards to digital audio reproduction. Try some XRCD material to experience 20 bit resolution on your Redbook player. Some DACs have 24 bit capacity, but the last 4 bits aren't precise enough to encode music accurately. Some advertisers claim 24 bit resolution as a marketing ploy. The short version: There is no sonic advantage between 20 and 24 bit(yet).
To look at these D/A converter chipsets (from just their bit quantization i.e. 16, 20, 24 bits) is a VERY simplistic way of understanding these converter types. Generally speaking, the 24 bit chipsets will have the latest greatest internal noise shaping architecture as well as other advancements that chipset manufacturers develop and implement as they release new chipsets to the marketplace year over year....and this is just the tip of the iceberg of what is within a D/A chipset! The fact that 6 different manufacturers' 16 bit D/A chipsets can all sound very different proves that it's much more than just looking at the bit quantization label. Typically, with chipset development, sonic progress moves forward with the latest advancements in D/A chipsets. Each manufacturing chipset company attempts to attain greater S.N ratios and other "tech measurables" with proprietary exotic architecture within (these are VERY complicated pieces of silicon with MANY DIFFERENT ways to achieve the end results.) Whether it be Wolfson or Burr Brown or Analog Devices or AKM or a slew of other chipset offerings, each has it's own unique proprietary method of achieving it's way to handle the digital data and convert in on silicon to an analog voltage or current output. Since there is a boatload of money tied up in creating a new chipset alone, the later generation chipsets are almost always a higher bit rate of 24 or even higher bit quantization levels. Many of these D/A chipsets are made to handle a slew of applications in many marketplaces - from home theater players to commercial music studio equipment and then other industries altogether like aerospace to you name it (we high end audio guys are just a drop in the bucket with single digit percentage marketshare comparably). Again though, the quantization rate alone will not necessarily give you a good measure of a D/A chipset's true sonic capability. If it was only it was this simple ;-)
Thanks for the responce Ehider..Your right if it was only this simple.

"Shardone explained already, improved filtering with possibility of reduced jitter. Improved more accurate filtering in digital filters gives smoother more coherent sound while reduced jitter lowers noise floor."

This is true.
My Arcam CD192, with the four Wolfson dac's and upsampling sounds head and shoulders better than the NAD C542 I had owned,and liked.
Ehider - All 24-bit DAC are sigma-delta. Traditional architecture reaches only 18 maybe 20 bit. There are many reasons for that and one of them is immunity of sigma-delta converters to power supply noise. Sound of these converters is quite different. Quantization noise is higher but moved above audible range. SACD is pretty much the same as byproduct of sigma-delta before filtering and new studio DSD recorders are wide/mulitibit version of SACD. Texas Instruments released even DAC that is a combination of both technologies. Analog Device converted to Sigma-Delta. I absolutely agree that the sound is very different and not to everybody liking. Many people don't like sound oversampling converters. To them sound of traditional DAC is more organic with better reverberation. Sigma-delta is, in my opinion, more smooth and accurate.