Ultrasonic noise in some high resolution downloads

[scan80269]

I'm probably late to the party, but I've been discovering significant non-musical ultrasonic noise embedded into certain high resolution PCM music downloads.

 

For example, the 2L.no test bench site carries evaluation samples of recordings at various resolutions & formats.  I ran MusicScope on three files for the 2L-106 Arnesen Magnificat album:

 

DXD (2L-106_stereo_DXD-352k_MAGNIFICAT_04.flac)

24/192K (2L-106_stereo_PCM-192k_MAGNIFICAT_04.flac)

24/96K (2L-106_stereo_PCM-96K_MAGNIFICAT_04.flac)

 

Attached screen shots are with MusicScope stopped at the last second of the tracks, where the music has faded to quiet.

 

Among these three, the DXD file has the largest hump of persistent ultrasonic noise, going up to around -60dBfs at 176KHz.  The 24/192K file has a smaller hump peaking at 86KHz.  Only the 24/96K file appears devoid of any significant non-musical ultrasonic noise.

 

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.

 

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.

 

I have discovered for some time that inserting a Jensen ISO-MAX PI-2XX XLR balanced isolation transformer between my DAC and headphone amp can reduce the harshness & brittleness of treble when playing some high-res PCM files.  The transformer acts as a low-pass filter to attenuate the amount of ultrasonics reaching the headphone amp.  This finding leads me to suspect any ultrasonics embedded in high resolution PCM recordings may not be totally harmless.  In other words, the ultrasonics in these files may be preventing them from sounding their best in my system.

 

As a separate ongoing experiment, I've been running XiVero's XiFEO FLAC entropy reducer on a number of 2L sample recordings (w/ DXD master) that have the ultrasonic humps.  XiFEO can reduce encoded noise below the noise floor (by LSB truncations & flat dithering) and apply LPF to band-limit the content to achieve higher FLAC compression ratio.  In some cases the FLAC file output by XiFEO is <20% the size of the input FLAC!  A quick A/B listening test at a friend's place resulted in the processed FLAC being declared better sounding than the original FLAC.  The tested system benefits from the offline removal of embedded ultrasonics in such files.  Once XiFEO is enhanced to support manual setting for the LPF cut-off frequency, I plan to re-process my ultrasonic-carrying high-res PCM files followed by some serious listening to compare with the original 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.

[scan80269]

1 hour ago, mansr said:

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.

 

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.

Thanks for the clarification, mansr!

 

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 suppose doing the master recording at DXD is also intended to support conversion to DSD for release?

 

Your advice is excellent indeed and I will heed it.  Even Barry Diament's fantastic recordings released at 24/192K show very little musical energy above 30kHz or so.  For a handful of 2L recordings I bought previously at DXD resolution, I will do offline LPF to get rid of the ultrasonic noise.

 

[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.

 

[The_K-Man]

This should clear up a lot of misconceptions about 'high res' audio.

 

 

[asdf1000]

Just discovered this incredible works and based on this old thread, I went with buying the (cheaper) 24bit/96kHz version.

 

Perfect sound forever indeed @mansr 

 

[audiventory]

On 20.08.2017 at 1:34 AM, scan80269 said:

I'm probably late to the party, but I've been discovering significant non-musical ultrasonic noise embedded into certain high resolution PCM music downloads.

 

It's DSD legacy noise. In general, it is harmless.

The noise can:

• cause audible products at some audio systems;
• spent part of dynamic range.

But when audible noise present during playback, it is recommended to cut the ultrasound.

[Torgny]

I am also very impressed with the XiFEO software and have run it on a number of 24/96 and 24/192 downloads with excellent results.  Often the output is truncated by 2-3 or more bits, and frequencies above 30-40 kHz filtered out.  Sometimes your're even left with just straight 16/44, at least some of the tracks!  But the sound quality is almost always improved even when file size is cut in half.  Is XiFEO revealing truth in hi-rez files?  Possibly.  Unfortunately, on a few occasions when there has been no effect at all on the output file size , the sound quality is slightly reduced so in such cases I retain the input.

[mkrzych]

On 20 sierpnia 2017 at 1:54 AM, mansr said:

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.

 

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.

 

Totally agree with above comment - in PCM 24/96 is what I mostly buying. One comment maybe that having 24/192 version may bypass some of your DAC filter stages/upsampling stages as I understood if you have DAC capable of true 24/192 - don't know to be honest in details if that's true and audible at all? 

 

 

[oso]

The reason 2L records in 352(DXD) is indeed because it has somehow become accepted to transfer those to DSD.

 Native DSD who were famous for only having DSD files, are selling 352 PCM files now... goes by the name DXD?

 

But has anybody here actually damaged their equipment by playing DXD files?

 

I had no problems after having compared  these Sound Liaison files, 1 song in 11 different formats;