Ensuring CD-ROM Digital Audio Quality under Windows 98
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This article provides technical implementation and testing guidelines for OEMs and IHVs who provide CD-ROM devices, to help ensure that end users have a good experience when digital audio playback is enabled under Microsoft Windows 98.
Registry Entry for Digital Audio Playback
Windows 98 supports digital audio playback from CD-ROM devices. This feature requires that the CD-ROM device must have the ability to play digital audio. Many CD-ROM drives do not, especially earlier models. Microsoft has defined a binary value in the registry called "DigitalAudioPlay" for use by the device driver to define whether a CD-ROM device is capable of playing digital audio and whether the user can turn on digital playback for the device.
The value is located in the Software registry key for the device, which is always under the CDROM key. For example: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\CDROM\0000 "DigitalAudioPlay"
The key 0000 refers to a specific CD-ROM drive; the next CD-ROM drive would be 0001. These key names are incremented every time a CD-ROM drive is added, no matter what the total number of CD-ROM drives installed in the system is at any time. On a clean installation with one CD-ROM drive, the only key under the CDROM key would be 0000. Within this key is the "DigitalAudioPlay" value.
The values of the bits in the "DigitalAudioPlay" registry value control various aspects of the Multimedia Control Panel applet and CD-ROM playback. These values are set by the Mscdrom.inf file, which sets the value during Windows 98 Setup or when the device is first identified through Plug and Play mechanisms. These values are as follows:
Reserved | Known Device | | Device May Support Digital Playback | | | Enable Digital Playback | | | | v v v v 0 0 0 0
Default Values for Device Capabilities
| Default value | Device represented |
7 (0111) | A device that supports digital playback |
6 (0110) | A device that may support digital playback, depending on the firmware revision |
4 (0100) | A device that does not support digital playback |
2 (0010) | An unknown device |
The bits that an OEM should be concerned with are:
0 | Enable Digital Playback. This bit determines whether the CD-ROM will attempt to play back digitally if digital-capable output is available. This should only be enabled by default if the CD-ROM drive is known to be capable of playing back digital audio. |
1 | Device May Support Digital Playback. This bit determines whether the user can set and clear the option titled "Whenever possible use digital playback on this device" in the Multimedia Control Panel applet. |
| • | Setting this to 1 allows the user to toggle the Enable Digital Playback by checking the related option in the applet. |
| • | Setting this bit to 0 causes the option titled "Whenever possible use digital playback on this device" to be dimmed in the Multimedia Control Panel applet. |
Note: If a WDM audio playback device is not the Preferred device in Multimedia Properties, the user cannot toggle the Enable Digital Playback bit regardless of how this bit is set. This bit should never be set if it is known that the drive is not capable of digital playback. This should always be set if the Enable Digital Playback bit is set by default.
2 | Known Device. The OEM must not set this bit. |
3 | Reserved. The OEM must not set this bit. |
Improving digital audio quality through system settings
Using direct memory access (DMA) mode transfers rather than programmed I/O (PIO) mode can improve digital CD playback quality. DMA mode transfer is faster and uses fewer processor cycles than PIO mode, which often means increased sound quality when playing CDs digitally. DMA mode transfer also increases performance for data transfers. DMA transfer mode can be enabled on the Settings tab on CD-ROM properties in Device Manager in System Properties. However, only controllers and CD-ROM drives that support DMA transfer without data loss or system instability should be enabled.
WDM audio on Windows 98 and Windows 2000 provides support for digital audio on personal computers, which facilitates a model where all audio mixing and routing is done in the digital domain. One of the primary benefits of WDM is the ability to accommodate numerous streams of real-time audio with the high quality possible with digital mixing, processing, and transmission. Under the new system architecture, it is possible to perform audio management using a software-controlled model, with multiple benefits. For an overview of the issues and benefits related to the digital audio initiative at Microsoft, see Introduction to Digital Audio.
For more information about digital audio playback versus Red Book audio, see Windows and Red Book Audio.
Testing CD-ROM Digital Audio Quality
Several companies worked together to develop a standardized testing procedure for measuring audio performance, titled "Personal Computer Audio Quality Measurement Definitions," available from the web site at http://www-test.cirrus.com/en/pubs/whitePaper/meas100.pdf 
. Microsoft test procedures, which are based on this methodology, are available at http://www.microsoft.com/whdc/whql/default.mspx. To test the quality of your device's digital audio playback, you can supplement these tests by using Rbtest.exe from the Windows 98 Beta 2 DDK.
The most important additional element to test is to listen for skips or clicks for a period of 20 seconds using USB speakers when playing audio tracks on the CD-ROM device.
Revising Device IDs after Resolving Problems
Notice that if you revise your device's firmware to solve digital audio playback problems or for other reasons, you must also provide a new Plug and Play device ID. This is an important implementation issue that ensures the operating system can distinguish the updated device from previous implementations.
Call to Action for Digital Audio Quality on CD-ROM Drives
| • | CD-ROM vendors should use Rbtest.exe or other test tools to verify the quality of audio playback on their drives when the option titled "Whenever possible use digital playback on this device" is enabled for CD Music in the beta release of Windows 98. |
| • | CD-ROM and controller vendors should test their CD-ROM drives with DMA transfer enabled. |
| • | If necessary, revise your firmware to ensure a good end-user experience when digital audio playback is enabled when running Windows 98. |