It might be stating the obvious, but the absence of information and knowledge does not help to solve practical problems. There is a bug in OSS v.4.2. This bug spoils sound quality, and, therefore, it should be fixed as soon as possible.
Some soundcards may not be severely affected, but certain soundcards are in trouble (I have already tested some of them).
The problem is sample rate conversion. You want to have a kind of a virtual mixer (for practical purposes) and you have it (vmix). All the signals are to be converted to a single rate. This rate, by default, is 48kHz. A low quality algorithm of sample rate conversion would degrade the sound quality of 44100 material played through a mixed device, and produce
harmonic distortion. A large majority of users may not notice the difference, but the difference might be obvious for all, when certain special test files are played.
With OSS v.4.1 you have these options for resampling:
vmix0-src <Fast|High|High+|Production|OFF> (currently Production)
With OSS v.4.2 you have only a few options for resampling:
vmix0-src <Fast|High|OFF> (currently High)
"Fast resampling" is something really very bad.
"High quality resampling" seems to be a very rudimentary linear interpolation algorithm. It produces strong harmonic distortions.
High+ is not much better than High (they seem to be about equivalent).
Of course, I do not know precisely what that "High quality" algorithm is, but the sound of harmonic distortions is exactly the same as that of "rudimentary linear interpolation algorithm". I made various tests. In particular, another sound system was installed on the same computer and various resampling algorithms were tested.
"Production quality" algorithm does not produce very strong harmonic distortions, but it is not likely to be something very sophisticated.
Which soundcards are in trouble?
According to Dr. Google, certain soundcards resample everything to 48khz themselves : AC'97 onboard chipsets, all Creative Live and Audigy series, Hercules Fortissimo I/II/III ( not the IV ). Due to their limited power and flexibility, they perform quite crappy resampling on non-48khz material.
I tested some old AC'97 soundcards:
00:11.5 Multimedia audio controller: VIA Technologies, Inc. VT8233/A/8235/8237 AC97 Audio Controller (rev 30)
00:1f.5 Multimedia audio controller: Intel Corporation 82801DB/DBL/DBM (ICH4/ICH4-L/ICH4-M) AC'97 Audio Controller (rev 01)
For such old AC'97 soundcards, the effect of bad resampling is pretty obvious.
You can play the test wave of 44100Hz directly (with vmix disabled), or through vmix (with 48kHz default).
The best quality is produced by vmix with "Production quality" algorithm.
The worst quality is produced by vmix with "Fast" algorithm.
The quality of direct playback (without vmix) is comparable to "High quality" algorithm (which is, presumably, a simple linear algorithm).
There is a reason to believe that Intel HDA ICH8 and the like may have similar problems, although the quality of sound might be better.
Generally speaking, we have two cases:
1. "Production quality" is better than the quality of resampling made by the soundcard itself (e.g. old AC'97 onboard chipsets).
2. "Production quality" is worse than the quality of resampling made by the soundcard itself (some new sophisticated soundcards).
In the first case, you may want to have at least "Production quality".
In the second case, you may want to have something better than "Production quality".
How to use test files?
There are many different test files, but the idea is about the same. Such files contains certain inaudible frequencies.
If there is something wrong with resampling algorithm, it produces audible artefacts (harmonic distortions).
I created a test file 10Hz + 20kHz, as described above.
PRECAUTIONS: It is advisable not to use sophisticated equipment (speakers) for testing sound.
I do not pretend to be a specialist in such matters, and I tend to use very cheap (PC) speakers for testing sound quality (no special filters, etc.). On the other hand, simple speakers may not produce marginal frequencies (and this is good, perhaps).
For those, who want to test sound quality, there are instructions in the net.
This one was found with Google (for a test file: 1kHz + 20kHz):http://www.hydrogenaudio.org/forums/ind ... 2&hl=udial
For everybody fighting with occasional clipping (or perfectionists wanting to test everything) you should try the attached sample, udial.
play the sample in your favourite player (decoded or not, having disabled all DSPs and EQs that can interfere)
if the output sounds weird at any time you should:
1. lower the output vol. in your soundcard config (should be the speaker icon in your system tray).
2. For some soundcards 48kHz is better (audigy 2 etc.).
3. lower the vol. till the sample doesn't sound weird at any time.
The output will sound really weird if your settings clip.
Your output can occasional clip without you knowing/hearing it. But for us perfectionists this will theoretically give a better quality output.
ATTENTION: Play this sample at a low volume anytime, even if you hear nothing special! It can be very harmful to equipment and/or your ears.
It's strongly recommended to use some very cheap (PC) speakers if you want to test this,
otherwise you might really ruin your tweeters (it has happened several times already).
WARNING: Do not play arbitrary test files downloaded from the net. You may better open them with Audacity (drag-and-drop them into Audacity) and study the spectrum.
NOTE: Clipping also produces artefacts (harmonic distortions) somewhat similar to those produced by low quality resampling.