iTunes playback quality vs. Logic
Has anyone else noticed that audio played back with iTunes sounds
slightly less good than with Logic ?
This is an issue that has bugged me over the course of several years, using multiple computers, audio interfaces, and apps. Maybe others have noticed, or have suggestions or insight ?
I'm talking about a VERY subtle difference in quality--the kind of thing you probably wouldn't notice except when listening carefully on a good playback system. The difference is subtle enough that I wouldn't even bother trying to hear it with my G4's built-in audio output, but I hear it with my MOTU 828mkII and my Digi 001, and I especially notice when I route digital output from those devices into a better D/A converter (UA 2192).
Here's the strange part: every attempt I've made to measure this difference has been unsuccessful. I've recorded the output of iTunes using AudioHijack, I've routed iTunes into Logic using Jack, and I've recorded the digital outputs of my interfaces into a DAT recorder. In all cases, I get a bit-for-bit identical copy of the starting file, yet I hear the subtle degradation while monitoring the recording (the resulting files sound great again when played back in Logic).
I've listened to this enough to be dead certain that I'm not imagining it, even though the digital data suggests otherwise...
My best guess is that I'm hearing jitter, that somehow, audio routed through the default audio path in OS X picks up a bit of timing jitter before it makes it to the audio driver, and this jitter is enough to degrade D/A performance, although it's not significant enough to cause digital recording mechanisms to glitch.
As best I can tell, this only happens with audio apps that output to the default audio of OS X. When I play back audio using Logic or ProTools (selecting my audio interface directly in the app's preferences), everything sounds great. But when I play back the same file using iTunes or Quicktime, routing it using Audio MIDI Setup or the Sound panel, it sounds slightly flat and grainy by comparison. Again, it's a really subtle difference, and I'm not doing anything overtly dumb like applying eq or "sound enhancement" with iTunes or anything like that--the digital output I capture, again, is identical. (Although, I will say that when I test the output of iTunes, I tend to get one small, inaudible (only noticed by bit-for-bit comparison) digital glitch every 5-10 minutes or so, while the output of Logic is 100% clean. This occasional bit error is clearly not the cause of the continuous sonic difference I'm noticing, but it adds to my suspicion that the overall bitstream is more jittery, which may affect D/A continuously, even though it seldom glitches completely.)
Does anyone know enough about the inner workings of the Apple HAL to know how routing audio through the default output might increase jitter, or otherwise sound slightly less good than audio directed straight to the interface driver via an app's preferences ? Anyone have tips on how to reduce jitter within Mac OS X ? I've tried renicing iTunes and CoreAudio to increase their priority, but it hasn't helped. Anyone know of a good jukebox app that has direct access to audio drivers ?
I'd sure like to be able to enjoy the convenience of iTunes without knowing that what I'm hearing would sound just ever-so-slightly better if I dragged all the files into Logic...
Thanks for indulging my audio neurosis!
James
james@nashvillain.com
Dual 1.25 G4 Mac OS X (10.4.8)
This is an issue that has bugged me over the course of several years, using multiple computers, audio interfaces, and apps. Maybe others have noticed, or have suggestions or insight ?
I'm talking about a VERY subtle difference in quality--the kind of thing you probably wouldn't notice except when listening carefully on a good playback system. The difference is subtle enough that I wouldn't even bother trying to hear it with my G4's built-in audio output, but I hear it with my MOTU 828mkII and my Digi 001, and I especially notice when I route digital output from those devices into a better D/A converter (UA 2192).
Here's the strange part: every attempt I've made to measure this difference has been unsuccessful. I've recorded the output of iTunes using AudioHijack, I've routed iTunes into Logic using Jack, and I've recorded the digital outputs of my interfaces into a DAT recorder. In all cases, I get a bit-for-bit identical copy of the starting file, yet I hear the subtle degradation while monitoring the recording (the resulting files sound great again when played back in Logic).
I've listened to this enough to be dead certain that I'm not imagining it, even though the digital data suggests otherwise...
My best guess is that I'm hearing jitter, that somehow, audio routed through the default audio path in OS X picks up a bit of timing jitter before it makes it to the audio driver, and this jitter is enough to degrade D/A performance, although it's not significant enough to cause digital recording mechanisms to glitch.
As best I can tell, this only happens with audio apps that output to the default audio of OS X. When I play back audio using Logic or ProTools (selecting my audio interface directly in the app's preferences), everything sounds great. But when I play back the same file using iTunes or Quicktime, routing it using Audio MIDI Setup or the Sound panel, it sounds slightly flat and grainy by comparison. Again, it's a really subtle difference, and I'm not doing anything overtly dumb like applying eq or "sound enhancement" with iTunes or anything like that--the digital output I capture, again, is identical. (Although, I will say that when I test the output of iTunes, I tend to get one small, inaudible (only noticed by bit-for-bit comparison) digital glitch every 5-10 minutes or so, while the output of Logic is 100% clean. This occasional bit error is clearly not the cause of the continuous sonic difference I'm noticing, but it adds to my suspicion that the overall bitstream is more jittery, which may affect D/A continuously, even though it seldom glitches completely.)
Does anyone know enough about the inner workings of the Apple HAL to know how routing audio through the default output might increase jitter, or otherwise sound slightly less good than audio directed straight to the interface driver via an app's preferences ? Anyone have tips on how to reduce jitter within Mac OS X ? I've tried renicing iTunes and CoreAudio to increase their priority, but it hasn't helped. Anyone know of a good jukebox app that has direct access to audio drivers ?
I'd sure like to be able to enjoy the convenience of iTunes without knowing that what I'm hearing would sound just ever-so-slightly better if I dragged all the files into Logic...
Thanks for indulging my audio neurosis!
James
james@nashvillain.com
Dual 1.25 G4 Mac OS X (10.4.8)
