Why the high had roll off?
The standard DAC design must had the analog filter.
For NOS DACs, they without digital filter, so they must design a much roll off analog filter, usually had -6 to -10DB at 20KHz, otherwise users maybe can hear the white noise " Sa..." at high .
In oversample DACs, nowdays the digital filter already had quite high performance, can reduced the noise to -110DB or more. But the D/A parts is working after the digital filter, the D/A parts bring the new noise ( I can't remember the noise name in English but who had interesting can find the article in web), in particular the ΣΔ D/A chips , the noise is worst than the R2R D/A chips.
So for reduce the D/A noise, want to design a analog filter after the D/A.
The analog filter can't as the characteristic of digital filter, it usually had -6DB/oct or -12,-24DB/oct. Nowdays usually applied -12DB/oct.
For the nice of frequency band, -12DB/oct can had less roll off at 20KHz but the phase characteristic is worst.
We like applied -6DB/oct for best phase characteristic but not best specs.
The standard DAC design must had the analog filter.
For NOS DACs, they without digital filter, so they must design a much roll off analog filter, usually had -6 to -10DB at 20KHz, otherwise users maybe can hear the white noise " Sa..." at high .
In oversample DACs, nowdays the digital filter already had quite high performance, can reduced the noise to -110DB or more. But the D/A parts is working after the digital filter, the D/A parts bring the new noise ( I can't remember the noise name in English but who had interesting can find the article in web), in particular the ΣΔ D/A chips , the noise is worst than the R2R D/A chips.
So for reduce the D/A noise, want to design a analog filter after the D/A.
The analog filter can't as the characteristic of digital filter, it usually had -6DB/oct or -12,-24DB/oct. Nowdays usually applied -12DB/oct.
For the nice of frequency band, -12DB/oct can had less roll off at 20KHz but the phase characteristic is worst.
We like applied -6DB/oct for best phase characteristic but not best specs.