Various pictures from Arny's posts Reply #25 – 2015-04-03 16:16:50 You'd love the IM I get on my oldest system here using Arny's sample. At moderate levels, the loudness of the IM is unrelated to the setting of the volume control! At volume=0, it's not there because the audio output is muted, but then between volume=1 and 10 (which is a range between almost inaudible and a reasonable listening level for normal audio signals) the loudness of the IM is pretty much constant.This volume control is on a crappy Sony mini system which sits between the audio 2496 sound card and the HD 580 headphones.Would you like me to tell you another secret that I learned a few years back? If you put ultrasonic audio content through something that's also processing analogue SD video, the relationships between the audio, and the harmonics of the video line frequency (15.625kHz, 31.25kHz etc for "PAL"), change the distortion products. Hence, if you don't remember there's also ~15kHz + harmonics already in the system, there will appear to be magic ultrasonic frequencies at which distortion increases and virtually disappears.With all the facts, these things are quite predictable. Without all the facts, the distortion due to ultrasonic signal components can seem quite random and unpredictable. That's why it's good to see an IMD test signal with several different sets of frequencies in it.Cheers,David.