Depending on the type of CD-ROM drive installed, the audio data on a CD-DA disc is either processed by a Digital-to-Analog Converter (DAC) that is built into the drive, or it is moved through the system to a Digital Signal Processor (DSP) on an audio adapter. Some CD-ROM drives can only play CD-DA audio data through the built-in DAC. Others, like the IBM Personal System/2 (PS/2) CD-ROM-II Drive, can play through the DAC, or they can stream data through the audio adapter DSP.
The advantage offered by playing CD-DA through the DAC is that it is a simple operation that greatly reduces system and resource overhead. The advantage gained by streaming data through an audio adapter DSP is that you can potentially enhance the signal beyond the capabilities of the DAC by adding special effects such as reverberation and tremolo, or by modifying treble and bass-capabilities the DAC cannot provide.
The default mode for the CD audio device is playback processed by the DAC through the internal headphones connector. To switch to playback through the audio adapter DSP, the application sends the connector command enabling the cd stream connector.
connector mycd enable type cd stream wait
To resume playback through the internal CD DAC, the application should re-enable the headphones connector. This will automatically disable the streaming connector.
connector mycd enable type headphones wait
Contention for the Amplifier-Mixer Device
Because the audio adapter is modeled as an amplifier-mixer (ampmix) device, the CD audio device will automatically open and connect to its default amplifier-mixer when streaming CD-DA data. If the amplifier-mixer device is currently opened exclusively by another application, enabling the cd stream connector will fail. If the amplifier-mixer is available, then other applications using the same ampmix device might temporarily lose use of the device if the audio adapter cannot simultaneously process all requests. The Media Device Manager (MDM) will handle all resource allocation automatically, however it should be realized that streaming CD-DA data using the CD audio device can effect applications using the waveaudio and sequencer devices as these devices also use the ampmix device. Conversely, a waveaudio device could cause the loss of a streaming CD audio device if the waveaudio device requires the use of the same shared ampmix device.