[Tianming]

I have a HifimeDIY U2 DAC (ES9023) and a laptop with Ubuntu 14.04 and Windows 8 dual boot. Through the same USB port, in Ubuntu, the volume out of the DAC is significantly quieter than in Windows. I don't worry a lot about the volume because I have an amp, but does this difference affects sound quality?

 

[RRod]

  I have a HifimeDIY U2 DAC (ES9023) and a laptop with Ubuntu 14.04 and Windows 8 dual boot. Through the same USB port, in Ubuntu, the volume out of the DAC is significantly quieter than in Windows. I don't worry a lot about the volume because I have an amp, but does this difference affects sound quality?

 
If you're on a typical 14.04 install, then you are running sound via Pulseaudio. First check that the mixer volume is all the way up on all your devices, sound sinks, and programs in Pulseaudio:
1) install pavucontrol (if you haven't already)
sudo apt-get install pavucontrol
2) run pavucontrol (it will be in the apps list or you can just type "pavucontrol" into console)
3) Make sure everything is set to 100% on the output volume; do this while playing music so you can check your program volume too.
 
If you aren't running Pulseaudio, then check ALSA sound levels via alsa-mixer or gnome-alsamixer.
 
Sound quality will only be affected if you go really low in the volume %. There was a thread here on the performance of a typical 32- or 64-bit software volume control… search for it, but I'll post it here if I can find it.

 

[Tianming]

   
If you're on a typical 14.04 install, then you are running sound via Pulseaudio. First check that the mixer volume is all the way up on all your devices, sound sinks, and programs in Pulseaudio:
1) install pavucontrol (if you haven't already)
sudo apt-get install pavucontrol
2) run pavucontrol (it will be in the apps list or you can just type "pavucontrol" into console)
3) Make sure everything is set to 100% on the output volume; do this while playing music so you can check your program volume too.
 
If you aren't running Pulseaudio, then check ALSA sound levels via alsa-mixer or gnome-alsamixer.
 
Sound quality will only be affected if you go really low in the volume %. There was a thread here on the performance of a typical 32- or 64-bit software volume control… search for it, but I'll post it here if I can find it.

I read from http://superuser.com/questions/492281/from-a-quality-perspective-what-is-better-turning-volume-up-in-the-software-i that reducing volume in software reduces bit depth and is harmful to SQ. I wonder if this is happening to my DAC in Ubuntu now.

 

[Sleep Mode zZ]

Yes it is happening to your DAC now. The fact that you are asking about it indicates that you can't hear the difference though. Reduced bit depth means higher noise floor. If you do not hear any extra noise in the background of your music, there really is not a problem. But do, as RRod suggested, check that all your volume controls are at 100% (0 dB). There should then be no difference in volume levels in Windows and Ubuntu.

 

[RRod]

  I read from http://superuser.com/questions/492281/from-a-quality-perspective-what-is-better-turning-volume-up-in-the-software-i that reducing volume in software reduces bit depth and is harmful to SQ. I wonder if this is happening to my DAC in Ubuntu now.

 
Most music people listen to doesn't come anywhere near needing a full 16-bits for its dynamic range, and is usually normalized to have 0dB peaks. Thus you will always have some amount of wiggle room for volume control. I'm not sure how Pulseaudio implements its digital control; for peace of mind, you can set it to run at 24-bits. See the man page for "pulse-daemon.conf."

 

[nepherte]

You can use alsa for bitperfect exclusive mode playback. As far as I know, pulse audio will mostly resample your music.

 

[RRod]

  You can use alsa for bitperfect exclusive mode playback. As far as I know, pulse audio will mostly resample your music.

 
If most of your music is 44.1 and your PA default is 44.1, then it's not resampling. ALSA still has digital volume control, otherwise people without an analog control would be SOL 

 

[nepherte]

That is indeed true. Pulseaudio will only resample incompatible sample rates. And yes, ALSA still has digital volume control (like some external DACs). Then again, simply set the volume to 100% and you bypass the digital volume control altogether.
 
 Pulseaudio is quite convenient in that it won't lock your audio device for exclusive usage, which may or may not (dmix) be the case with ALSA.

 

[Sleep Mode zZ]

Pulse audio, in its default configuration, lets through 44.1kHz and 48kHz streams without resampling. Resampling can happen in some situations though. For example, you start watching a video with 48kHz soundtrack, and then you start playing a 44.1kHz audio file. In this situation Pulse has to resample the latter to 48kHz because it can only give the hardware one audio stream of in one sample rate at any one time,
 
In this situation, if you stop the 48kHz video soundstream, the 44.1kHz audio file will still be resampled to 48kHz. It is not possible for Pulse to change the sample rate without stopping the audio. Stop the 44.1 stream and start it again: now it will not be resampled anymore.
 
In its default configuration Pulse will not let higher sample rates through but will downsample them to 44.1kHz or 48kHz. It is possible to change the settings, but for hi-res music it might be more convenient to use alsa directly - and to make sure that nothing else is using or needing the audio hardware at the time.

 

[Tianming]

  Yes it is happening to your DAC now. The fact that you are asking about it indicates that you can't hear the difference though. Reduced bit depth means higher noise floor. If you do not hear any extra noise in the background of your music, there really is not a problem. But do, as RRod suggested, check that all your volume controls are at 100% (0 dB). There should then be no difference in volume levels in Windows and Ubuntu.

I just checked that both left and right channel are set at 0db(100%), but it's still different volume compared to Windows. In Ubuntu I can turn the volume knob to about middle, but in Windows that position will be deafening.

 

[RRod]

Spoiler: Way more than you asked for, OP

Because I have no life:
 
Here** are the results of taking a -0.5dB 10kHz sine wave @ 16/48000 and passing it through Pulseaudio at both 16/48000 and 24/48000 at 35% (-27.29dB) volume. You can see that at 16bits it does appear that PA is dithering the output, and the highest distortion product is about 10dB higher than at 24 bits.
 
**L-to-R: original, 16, 24. dB levels are relative to the max within the file, so subtract 27.29dB from the levels of the volume-controlled files to get an absolute scale.

 

[RRod]

  I just checked that both left and right channel are set at 0db(100%), but it's still different volume compared to Windows. In Ubuntu I can turn the volume knob to about middle, but in Windows that position will be deafening.

 
Make sure to check the volume in your player application, as well as the Pulse volume associated with the output from the player. Then do the same in Windows, though someone else will have to help you with how it handles these things.

 

[Tianming]

   
Make sure to check the volume in your player application, as well as the Pulse volume associated with the output from the player. Than do the same in Windows, though someone else will have to help you with how it handles these things.

I opened up alsamixer and found the volume is not set at 0db. I changed it to 0db and now it's as loud as in Windows. I don't remember doing anything audio config but for some reason it seems it's using alsa, not Pulseaudio. Actually it's kind of too loud for my efficient HD 558

 

[RRod]

  I opened up alsamixer and found the volume is not set at 0db. I changed it to 0db and now it's as loud as in Windows. I don't remember doing anything audio config but for some reason it seems it's using alsa, not Pulseaudio. Actually it's kind of too loud for my efficient HD 558

 
Great! Well, maybe not great, but glad you found the source of the difference. Pulse runs on top of ALSA as a means of making everything the same sample spec. ALSA is actually what talks to the sound cards.

 

[Tianming]

Thanks a lot for your help, folks. I now need to read more about digital volume control and SQ and figure out what setting to use. Everything at 100% is too loud for my HD 558. I have to set my amp at very low to not deafen my ears. Actually I think I don't need an amp for HD 558, but somehow I got one (O2).