what is the meaning of delta sigma noise in time domian?

[pianomania]

in text book , the delta sigma modulator can be analyzed in frequency domain.
for example , assume it is white noise,
secondly, the power spectrum density is within -fs/2 <f< fs/2, fs means sampling freq , but why ?

what is the physical meaning or phenomenon in time domain when the noise has higher frequency ?
if the higher freqency quantization noise means the variation rate is higher or the signal is higher frequency ?

 

[pmr9]

High freq quantization noise means variation of signal after and before quantizer in time domain is higher and not signal is high freq.
After quantizer any way we have decimator and filter to cutt all higher freqs.