Originally Posted by Berlioz
Some NOS DAC's ignore the filter entirely, leaving it to the listeners ears to filter the music....
Good that you mentioned it.
It is true that the ear can not hear the image energy. But ALL the makers of DA IC's including devices INCLUDING the old NOS days and up to now want you to use anti imaging filers. One can "take a tour" of the data sheets of DA IC's from AKM, TI, Analog Devices and the other makers, and ALL the application data shows anti imaging filters.
So if we do not hear it, why do we need it? First what is the image energy?
When you look at the low frequency signal right out of a DA IC, say 1KHz sine wave, it looks very similar to what it is supposed to be - a sine wave with some small steps. But look at a 10KHz sine wave, or better yet higher audio frequency such as a 20KHz, and the signal does not look anywhere near the sine wave shape. If you filter it above 20KHz it will look like a desired sine wave but what happens if you leave the filter out? The image energy got its name because the audio energy appears as a near carbon copy (image) around the sample rate, twice the sample rate three times the sample rate and so on. That energy is still high even at the MHz range.
The difference between the direct DA out and the desired signal is all high frequency energy, above audio. If the DA could drive the ear directly, all will be fine. But the DA does not make sound directly. It does not vibrate the air.
One needs to send the DA signal through additional electronic devices. The DA does not have the power to drive a speaker or a headphone. The DA may be driving a power amp followed by a speaker, or a headphone amp followed by a headphone, or a mixer or what not...
So what is the reason we need to remove the high frequency energy? You do not want to burden the electronics DRIVEN BY THE DA with unwanted high frequency that does not belong there.
An audio amp, a power amp, a headphone device and so on are designed to operate well when receiving audio signals. They are not capable of doing a good job of it while being disturbed by signals they are not supposed to process.
I can not think clearly while being hit on the head. One can not enjoy music while being yelled at loudly. The electronics can not process the audio well while disturbed by significant unwanted energy.
A good audio amp, driver, headphone... is not designed to operate under such conditions. It is designed to do audio when there is NO such energy, and that alone is a tough enough task. When there is a lot of high frequency (way over the audio range) riding on the audio signal, the electronics makes distortions in the audible range. There are many mechanisms at play (intermod, slew rate issues and more), but that is too technical for now.
The amplitude of the ultrasonic unwanted energy is pretty low for low audio sound, but at near 20KHz, with NOS, the high frequency amplitude is around the same as the signal itself! That corresponds with what I said earlier - move the filter cutoff to a lower frequency, kill all the highs, and convince some innocent folks that it sounds great.
So we can now put the argument "no need to filter, the ear can not hear it" to rest.