Disc Identification
53
SACD Disc Type
DVD Player models
Mirror – Voltage created when the light returned from the disc exceeds a threshold. The light is the strongest when focus is near.
Focus Error – Identifies the exact point of focus. The point of focus will occur within the window created by the MIRROR signal.
The Mirror signal is used for disc detection. The focus error signal is used to verify that the mirror signal is a valid one.
Operation
During focus search, the laser’s objective lens is instructed to move up and down while light reflected back from the disc is analyzed. As the lens moves up during focus search, so does its corresponding focus point.
Light is first returned when the focus point arrives at the bottom of the disc. This creates the first voltage (disc entry) at the mirror output. A second voltage occurs when the information layer is found. A third volt- age occurs if there is a second information layer. The time between the voltages determines the disc type.
For example, in a DVD disc the information layer is closer to the bottom of the disc than a CD. Therefore in a DVD, the second mirror signal will occur sooner than in a CD where the layer is further away.
This disc ID information is initially used to determine the rough speed of the disc. Finally, the disc type is authenticated during playback when the digital data reveals the disc type.
Therefore the first full upward travel of the lens during focus search will identify the disc type.
SACD stands for Super Audio Compact Disc. Although this DVD player can detect the SACD disc, it can only play its normal CD layer (number two). The first layer can contain six channels of uncompressed audio and a PCM audio track. At this time the SACD disc is only available in Japan.