Outline:
However, a unifying feature of modern computing systems are that they represent information using physical means which generally can be in one of a fixed number of distinct states. We refer to such representations as digital.
In particular, almost all modern systems rely on a physical means with exactly one of two distinct states. Such a two-state mechanism can be manifested by means of:
No matter which of the physical means is being used, we think of such a two-state system as an abstraction called the "bit" (binary digit).
Often we think of as
So, one bit can be in either of two distinct states.
If you look at two distinct bits together, you will find that the pair can be set in one of four distinct states (00, 01, 10, 11).
If you look at three distinct bits, you can set them in one of eight distinct states.
In general, a series of n bits can be set in any of 2ndistinct patterns.
Common quantities:
Each cell is assigned a unique name, called its "address" (e.g., cell #0, #1, #2, #3, ...)
RAM == Random Access Memory Allows "random access" ('arbitrary' access is more fitting term) (Note difference between DVD vs. VHS, or CD vs. Audio Tape)
Memory might be "read-only" or may allow both read/write.
Relevant times:
Has single track that spirals. (to transfer data at constant rate, disk must actually be rotated at varying speeds)
Downside is that you have to fast-forward/rewind to get to a desired part of the tape.
We will study the details of this next time...
Text is relatively easy to represent, because an alphabet is already a discrete collection of symbols. We can encode text digitally by using a different pattern of bits in place of each alphabet character of the given text.
Example: Morse Code [ alphabet, MP3 sample, Real Audio sample, demonstration Applet, another Applet ]
If all characters are encoded using the same number of bits, this is a fixed-length encoding. (Note: Morse Code is not fixed-length; commonly used letters were assigned shorter codes)
Most important is that both the creator and the reader use the same conventions for numbering the different symbols.
To achieve acceptable quality for human perception, there are two issues:
Based on above, 1 second of music in stereo requires over 1 million bits.
Can try to use compression techniques to reduce the space requirements. e.g MP3 (MPEG-1 Audio Layer-3) can achieve 12:1 compression ratios.
Video track can save space by taking advantage both of image compression and temporal similarity from one frame to the next.
For those interested, read second full paragraph on p. 159 [Br], though we will not cover this topic in this course.