Ultrasonic noise in some high resolution downloads

[mansr]

1 hour ago, scan80269 said:

Any idea as to why a high resolution PCM file would contain such ultrasonic noise?  The noise humps remind me somewhat of DSD, but can they be the result of noise shaping?  If the master recordings were DXD (24/352.8K), it seems to me the dynamic range of the container is more than big enough to capture all the music, and noise shaping or dithering should not be needed.

Virtually all ADCs use a sigma-delta (DSD-ish) front-end followed by a digital low-pass filter. At 176/192 kHz and higher output rates, some modulator noise is usually present at the top end, and this is what you're seeing here.

 

1 hour ago, scan80269 said:

I'm a bit concerned as to how such ultrasonic noise can affect a playback system.  For example, my Auralic Vega DAC performs internal upsampling of incoming PCM to 1.5MHz and uses "gentle" low-pass filters at its outputs.  With a high resolution PCM recording carrying significant ultrasonic noise (like the DXD file above), these's a risk a certain amount of ultrasonics will leak out of the DAC to reach downstream analog equipment like preamp or headphone amplifier.  My Auralic Taurus Mk II headphone amp has a frequency response up to 300KHz (-3dB) so it is capable of being affected by ultrasonics sent from the DAC.  The ultrasonic noise being way above 20KHz is not directly audible, but the intermodulation effects of this noise with the music in the audio spectrum (within 20KHz) is potentially audible to some extent.

This is a valid concern. The obsession with gentle filters combined with the inevitable high-frequency noise often does more harm than good.

 

The main reason for using high sample rates is to ensure any filter artifacts are well removed from the audible range, and 96 kHz is plenty for this purpose. Since most microphones only extend to 50 kHz or so, and even that is uncommon, higher frequencies in a recording have little but unwanted noise.

 

My advice, save your money and buy the 96 kHz version.

[mansr]

10 hours ago, scan80269 said:

I also found some strange behavior regarding ultrasonics in the first track of this 2L-106 Arnesen album in DXD.  While observing the playback of this track with MusicScope, I noticed the ultrasonic noise dropping off abruptly at the 5:25 mark, then returns for a fraction of a second at the very end of the track.  It looks as if someone played around with LPF controls during the mastering of this track.

I suspect the recording was done using multiple microphones and ADCs with varying noise characteristics. The music shifts from vocals to organ around the time you mention, so they probably cut out one or more inputs to the mixer there.