Fundamentals - The MPEG - page 41

can be read or written. The size of the chip package restricts the number of pins available, so that large memories
use the same address pins more than once. The bits are arranged internally as rows and columns, and the row
address and the column address are specified sequentially on the same pins.
Figure 2.32: Digital semiconductor memory types. In (a), one data bit can be stored in a simple set-reset latch,
which has little application because the D-type latch in (b) can store the state of the single data input when the
clock occurs. These devices can be implemented with bipolar transistors of FETs, and are called static memories
because they can store indefinitely. They consume a lot of power. In (c), a bit is stored as the charge in a potential
well in the substrate of a chip. It is accessed by connecting the bit line with the field effect from the word line. The
single well where the two lines cross can then be written or read. These devices are called dynamic RAMs because
the charge decays, and they must be read and rewritten (refreshed) periodically.
Just like recording devices, electronic data storage devices come in many varieties. The basic volatile RAM will
lose data if power is interrupted. However, there are also non-volatile RAMS or NVRAMs which retain the data in
the absence of power. A type of memory which is written once is called a read-only-memory or ROM. Some of
these are programmed by using a high current which permanently vaporizes conductors in each location so that
the data are fixed. Other types can be written electrically, but cannot be erased electrically. These need to be
erased by exposure to ultraviolet light and are called UVROMS. Once erased they can be reprogrammed with new
data. Another type of ROM can be rewritten electrically a limited number of times. These are known as electric
alterable ROMs or EAROMS.
2.17 Binary coding
In many cases a binary code is used to represent a sample of an audio or video waveform. Practical digital
hardware places a limit on the wordlength which in turn limits the range of values available. In the eight-bit samples
used in much digital video equipment, there are 256 different numbers, whereas in the sixteen-bit codes common
in digital audio, there are 65 536 different numbers.
Figure 2.33 (a) shows the result of counting upwards in binary with a fixed wordlength. When the largest possible
value of all ones is reached, adding a further one to the LSB causes it to become zero with a carry-out. This carry
is added to the next bit which becomes zero with a carry-out and so on. The carry will ripple up the word until the
MSB becomes zero and produces a carry-out. This carry-out represents the setting of a bit to the left of the MSB,
which is not present in the hardware and is thus lost. Consequently when the highest value is reached, further