Chances are there is filtering going on, just not explicitly... Can happen in the output buffer, cables, tweeters, ears... One of the "non-filtered" outputs I saw years back had a transformer-coupled output. Hmph. Look up how inductors pass high frequencies... It is also possible the images create distortion elsewhere in the system (including the tweeters) that is modulated down in frequency and "fills in" the audible band.
FWIWFM, I use the term aliasing to refer to signals that are aliased to baseband around the sampling frequency and its multiples. An anti-alias filter at the input of an ADC is used to suppress signals that would otherwise be aliased. The output of a DAC produces images at multiples of the sampling frequency so the term I use for the filter after the DAC's output is anti-imaging filter, or just image filter.
My 0.000001 cent (microcent) - Don
Certainly agree that anything with transformers in will be filtering the high frequencies, still don't see that £200,000 is justified for a NOS DAC with no output filter, even if the coupling transformers are silver
and isn't the first significan peak due to no filter is at 66kHz. the 3rd harmonic of the sampling frequency?
I can see the logic in calling the DAC output filter an image filter though here in the EU reconstruction filter is the commonly used term for audio IME (I was taken to task about the term by a Dutch engineer).
I was the first person to use a digital recorder to measure data on a Formula 1 car, in 1982 iirc. Tape recorders were not much good because of the environment and I was keen to know what was going on. I was not trying to record things at high frequencies and had never read anything about digital recording of AC, my ignorance about aliasing was embarassingly total in retrospect, I was just wanting to measure suspension travel, throttle opening and output shaft speed. All this was very low frequency but I never converted back to an ac signal, just looked at sample values.
In fact the first commercially available data recorder for racing cars, which made the man who did it very rich eventually, had no provision to deal with aliasing for several generations.
My custom first in-house designed recorder, started in 1986, was actually developed as a controller for the Williams Active Suspension which I had designed, the recording aspect was just a secondary feature. By then removing aliasing effects had become essential, of course.
All this had to be do-it-yourself since there was zero suitable commercially available kit. And the fastest data transfer we had was wired RS232 when the car was in the pits and a 256kB memory chip was about £1000!