HQP and NOS DACs

Yes, images always happen around multiples of final (output) sampling rate. Only difference with hires is that the images are already further away from audio band at the beginning.

If you properly upsample for example to DSD256, then images theoretically repeat at every multiple of 11.3 MHz sampling rate. But since DSD has high frequency noise increasing as function of frequency, the analog reconstruction filter needs to start rolling off earlier, for example at 150 kHz, and because there’s a huge amount of space between 150 kHz and 11.3 MHz, even relatively gentle analog filter is able to cut out everything way before the first image frequencies around 11.3 MHz. So with proper upsampling to DSD the image problem doesn’t exist.

Note however, that since upsampling to such rates requires fairly large amount of processing power, many delta-sigma DACs don’t manage to properly upsample that high, only to 352.8/384k (8x) or 705.6/768k (16x) rates and then go on by just copying same sample multiple times. Meaning that images still exist at multiples of those rates. Doing things properly is not a problem for modern computers though.

1 Like