Subject: Re: sound cards, bit depth and the Windows mixer From: Stephen Neely <neely(at)BOYSTOWN.ORG> Date: Thu, 21 Apr 2005 09:00:14 -0500John, I am not familiar with the specific soundcards that you mentioned, but a few years ago I investigated in detail the ways in which some Turtle Beach soundcards handle sampled data under Windows. Based on that experience, I am fairly certain that almost all mixing and volume control are handled digitally, prior to analog conversion, with the exception being mixing of the CD output. This means that, as you suggested, full D/A converter resolution is only achieved at full volume. Our Research Computing Core group at BTNRH is currently developing a soundcard API to provide a common foundation for software in all of our auditory resarch labs. For perception experiments, we provide an easy way to supply visual cues that are synchronized to an auditory stimulus. For otoacoustic emissions, we provide the ability to synchronize sound input and output. We plan to make our implementation of this API publicly available within a few months. I can provide more details, if you are interested. -- Stephen ================================================================== Stephen T. Neely, D.Sc. OFFICE: +1-402-498-6751 Boys Town National Research Hospital FAX: +1-402-498-6351 555 North 30th Street E-MAIL: Neely(at)BoysTown.ORG Omaha, NE 68131 ================================================================== -----Original Message----- From: AUDITORY Research in Auditory Perception [mailto:AUDITORY(at)LISTS.MCGILL.CA]On Behalf Of John Culling Sent: Thursday, April 21, 2005 4:39 AM To: AUDITORY(at)LISTS.MCGILL.CA Subject: sound cards, bit depth and the Windows mixer Dear list, Does anyone know how the Windows "Volume Control" mixer is implemented. Possibly it is soundcard and driver dependent. My reason for asking... We were recently doing some basic tests on new soundcards (Edirol UA20 USB card, Echo Mia PCI card). Both are nominally 24-bit. We wanted to verify the dynamic range to at least 16 bits, so we prepared 16-bit sound files with 1-kHz pure tones of various bit depths. The ultimate test was a 1 bit (dithered) signal. We viewed the ouput on a 'scope. Both cards passed with flying colours - the 1-bit signal was clearly visible, with little noise apparent - other than the 1-bit dither we added). Then we wondered whether full bit depth was only achieved if the system volume was on full (i.e. does turning down the volume lose bits). Intsallng the Echo Mia "pure wave" driver forced the system volume to full, but for the Edirol it was still adjustable. So we played our 1-bit signal and turned down the volume. The image on the 'scope scaled down nicely, but was eventually overcome by noise. So, no bit depth was being lost for this signal, but of course the soundcard has 24 bits, so the other 8 could be used for implementing digital volume control and mixing. If so, presumably a 16-bit card would not have behaved in this way. Is that the case? I can think of one alternative, but I think it unlikely. Could the card have several DACs for converting the different sources in the mixer (wav, CD, system sounds etc.) and an analogue mixer under control of the computer. This seems relatively complex and less satifying as an engineering solution, because it's generally better to minimise analogue circuitry. Maybe some soundcards do it though. Does anyone know? John. Dr. John F. Culling (Senior Lecturer) School of Psychology, Cardiff University. Tower Building, Park Place, Cardiff CF10 3AT Tel. +44 29 208 74523 FAX +44 29 208 74858 www.cf.ac.uk/psych/home/cullingj1/