Bit-mapped graphics
If we go back to the early home
computers the graphics, to say the least, was very primitive. If you wanted to
create a little stick-man figure you would need to work out the binary pattern
for that shape. Something like:
In effect we have bit-mapped
graphics. The binary patterns that are stored in video memory correspond to the
image displayed on screen.
A bitmap is characterised by the
width and height of the image in pixels (the smallest unit on a screen or in
memory) and the number of bits per pixel which determines the number of shades
of grey or colours it can represent. A bitmap representing a coloured image
will usually have pixels with between one and eight bits for each of the red, green,
and blue components, though other colour encodings are also used. See Visual
Basic notes using the RGB colour function.
Vector graphics
Sometimes called
"object-oriented" graphics. Vector graphics deals with separate
shapes such as lines, polygons and text and groups of such objects as opposed
to a painting program which stores only bitmaps. The advantage is that it is
possible to change any element of the picture at any time since each part is
stored as an independent object whereas once something in a bitmap has been
overwritten it cannot in general be retrieved.
At a very simple level the graphics
in Microsoft Paint is bit-mapped whereas the graphics tools in Corel Draw or
even, to a lesser extent, in Microsoft Publisher, use vector graphics.
Sound
In order to understand how sound can
be represented as a bit-pattern we need to consider two different ways in
encounter signals of information.
In our everyday lives we are used to
analogue signals - the temperature in a room, the light levels outside or the
changes in atmospheric pressure are all varying continuously. The temperature
does not suddenly change from one level to another. These continually varying
signals are analogue
signals. However computers do not work with analogue signals - they work with
binary signals which are either on or off. A binary signal is just a particular
example of a digital
signal.
If you look at a digital watch the
smallest interval of time might be 0.1 of a second. There is no value between
this. It is a discrete, digital signal. Time itself is analogue. We can
continually split intervals of time into smaller segments forever (as far as we
can tell).
The difference between analogue and
digital does cause a number of problem. How do we connect analogue devices such
as temperature probes to computers. In effect they are talking a different
language. One way is to approximate the analogue signal over a period of time.
eg
The analogue signal (the thick wavy
line) is converted into digital values (the blocks on the diagram) that
approximate to the “real” system. By reducing the time interval the accuracy
would increase. The digital number, in a computer system would then be converted
into a binary pattern. So, for instance, the highest portion of the graph
would, in 8 bits, be represented by 11111111. The lowest portion might only be
10001000. If the graph went down an analogue value of zero then the binary
pattern would be zero. Obviously the more “bits” in the converter the greater
the accuracy of the pattern to the original. Similarly if the frequency of
conversion (how quickly we sample the sound) is increased we get greater
accuracy.
There is an electronic device known
as an ANALOGUE to DIGITAL CONVERTER (ADC) which can perform this task. If we
are starting with binary or digital patterns then a DAC would be used. A DAC is
used in CD players since the CD is a digital medium and needs to be converted
into something that approximates to an analogue output for the speakers.
Similarly, if you are recording sound from a microphone on to a CD the ADC
process is required. If you can devote more “bits” and/or sample over smaller
periods of time then the quality of out put would improve. Eventually, however,
the human ear will not resolve small changes and so it is not necessary to
increase the quality any further.
So, if we make a new recording of
Dave Berry and the Cruisers, it is more than likely that analogue microphones
would be used. The signals would then be converted into digital ones (binary)
so that they can be processed by computer. The mixing, fading, etc, is all done
on computer and the final output pressed onto a CD. The millions of people who
buy this CD then put this into their CD players where it is converted back into
analogue and outputted to the speakers.
There are alternative ways of
processing sound. It is possible to buy speech chips which have segments of
speech (known as phonemes) built into them. Each phoneme is associated with a
particular binary pattern. So, if you wanted to generate a certain word, a
series of bit patterns would need to be sent to the chip. The final output
would sound like Stephen Hawkins but it would still be comprehensible.
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