Information Technology Reference
In-Depth Information
The buffer will only function properly if the two addresses are kept apart. This implies that the amount of data read
from the disk over the long term must equal the amount of data used by the MPEG decoders. This is done by
analysing the address relationship of the buffer. If the bit rate from the disk is too high, the write address will move
towards the read address; if it is too low, the write address moves away from the read address. Subtraction of the
two addresses produces an error signal which can be fed to the drive.
In some DVD drives, the disk spins at a relatively high speed, resulting in an excessively high continuous data rate.
However, this is reduced by jumping the pickup back. Repeating a previous track does not produce any new data
and so the average rate falls. As all the disk blocks are labelled it is a simple matter to reassemble the
discontinuous bitstream in memory.
The exact speed of the motor is unimportant. The important factor is that the data rate needed by the decoder is
correct, and the system will skip the pickup as often as necessary so that the buffer neither underflows nor
overflows.
The MPEG-2 decoder will convert the compressed elementary streams into PCM video and audio and place the
pictures and audio blocks into RAM. These will be read out of RAM whenever the time stamps recorded with each
picture or audio block match the state of a time stamp counter. If bidirectional coding is used, the RAM readout
sequence will convert the recorded picture sequence back to the real-time sequence. The time stamp counter is
derived from a crystal oscillator in the player which is divided down to provide the 90 kHz time stamp clock. As a
result the frame rate at which the disk was mastered will be replicated as the pictures are read from RAM. Once a
picture buffer is read out, this will trigger the decoder to decode another picture. It will read data from the buffer
until this has been completed and thus indirectly influence the disk bit rate.
Figure 7.18:
Simple processes required for a DVD player to operate.
Owing to the use of constant linear velocity, the disk speed will be wrong if the pickup is suddenly made to jump to
a different radius using manual search controls. This may force the data separator out of lock, or cause a buffer
overflow and the decoder may freeze briefly until this has been remedied. The control system of a DVD player is
inevitably microprocessor-based, and as such does not differ greatly in hardware terms from any other
microprocessor-controlled device. Operator controls will simply interface to processor input ports and the various
servo systems will be enabled or overridden by output ports. Software, or more correctly firmware, connects the
two. The necessary controls are Play and Eject, with the addition in most players of at least Pause and some
buttons which allow rapid skipping through the program material.
