@gdhal
The logic for upsampling Redbook is well documented and justified. Redbook requires a Nyquist filter at 22KHz to prevent aliasing or image noise from 22 to 44 KHz from being included in the analog output signal. Unfortunately a sharp filter at 22 KHz is guaranteed to affect the audible sound due to the proximity of this sharp filter to the audio band. By upsampling you preserve the entire original digital information contained in redbook but you push the Nyquist far up to 96KHz for a 192KHz sample rate. This makes filtering so much easier and entirely benign. Going up to 24 bit also preserves the entire Redbook signal.
The key understanding is that upsampling preserves ALL the original information and so does an increase in bit depth from 16 to 24.
This is very different from down sampling which can alter the original audio (stuff is thrown away) and normally down sampling requires careful processing and filtering to preserve the audio (dithering being a very important processing step in down sampling, as simple truncation is highly distorting)
The key to understand is that upsampling is benign but it makes the job of filtering aliased or ghost frequencies much much more accurate. Every DAC must filter out these “aliasing” artifacts as it is inherent in sampling theory (to accurately capture a signal you need a minimum of two samples per wavelength) - anything that is sampled less than twice will appear as “ghosts” or aliasing artifacts (and these artifacts sit at frequencies above the Nyquist which is half the sample rate frequency). The sharp filters at Nyquist are essential to ensure these “ghosts” do not appear in the reconstructed analog output, however, a sharp filter close to the audible range is less than ideal as it affects the audible range also!
Apart from this sharp near audio band filtering issue due to the low sample rate, CD Redbook is a near perfect audio distribution format - especially when carefully produced. Only recently has analog equipment achieved the equivalent of 21 bit resolution making 16 bits a further limitation of CD redbook.
The logic for upsampling Redbook is well documented and justified. Redbook requires a Nyquist filter at 22KHz to prevent aliasing or image noise from 22 to 44 KHz from being included in the analog output signal. Unfortunately a sharp filter at 22 KHz is guaranteed to affect the audible sound due to the proximity of this sharp filter to the audio band. By upsampling you preserve the entire original digital information contained in redbook but you push the Nyquist far up to 96KHz for a 192KHz sample rate. This makes filtering so much easier and entirely benign. Going up to 24 bit also preserves the entire Redbook signal.
The key understanding is that upsampling preserves ALL the original information and so does an increase in bit depth from 16 to 24.
This is very different from down sampling which can alter the original audio (stuff is thrown away) and normally down sampling requires careful processing and filtering to preserve the audio (dithering being a very important processing step in down sampling, as simple truncation is highly distorting)
The key to understand is that upsampling is benign but it makes the job of filtering aliased or ghost frequencies much much more accurate. Every DAC must filter out these “aliasing” artifacts as it is inherent in sampling theory (to accurately capture a signal you need a minimum of two samples per wavelength) - anything that is sampled less than twice will appear as “ghosts” or aliasing artifacts (and these artifacts sit at frequencies above the Nyquist which is half the sample rate frequency). The sharp filters at Nyquist are essential to ensure these “ghosts” do not appear in the reconstructed analog output, however, a sharp filter close to the audible range is less than ideal as it affects the audible range also!
Apart from this sharp near audio band filtering issue due to the low sample rate, CD Redbook is a near perfect audio distribution format - especially when carefully produced. Only recently has analog equipment achieved the equivalent of 21 bit resolution making 16 bits a further limitation of CD redbook.