Sound is a single waveform which can be decomposed into pure
frequencies. For example, the music of a rock band is a single
waveform. Physiologically, however, the listener hears the different
instruments and the voice. In its natural state sound is an analog
phenomenon-that is a wave phenomenon. By measuring the single waveform
at frequent intervals and representing each measurement as a binary
number, sound can be digitized.
The quality of digital sound depends on the frequency at which the
sound is measured as well as the size of the word used to represent the
wave form. To convert the voice into a telephone quality binary signal
the voice is sampled 8000 times a second. The measured waveform is
converted into a seven-bit binary number with an eighth bit added for
error checking. This means a telephone conversation is converted into a
stream of 64,000 bits per second. The telephone system is presently
analog between telephones and the switchboard, but an increasing
portion of long distance phone traffic is being sent digitally. The
phone conversation is converted at the telephone exchange (switch)
prior to long distance transmission. In the future all telephone
conversations will be converted to digital at the phone( 10 to 30
years). Corporate internal communications are rapidly becoming digital
so that the corporation can use the installed telephone lines for
simultaneously voice and data transmission. In order to obtain stereo
quality sound you need a larger word size (16 bits) and much more
frequent sampling to capture the high frequency sounds. One scheme for
digital recordings measures the sound 44,000 times a second using a
16-bit( 2 byte) word for 1.4 million bits a second. The new stereo
sound TV samples about 36,000 times a second and used a 14 to 16-bit
word for the measurements.
Computer graphics: The fundamental characteristic which
determines the clarity of a computer screen is the number of picture
elements which are called pixels. On a computer screen these pixels are
arranged in a rectangular grid and the size of the grid varies
considerably among computers. For example, if each character is
represented by a rectangle of 9 lines and 9 columns, then an 80
character by 24 line text display would have a total of 216 lines and
720 columns. For graphic pictures from satellites, 256, 512 and 1024
lines and columns are grid sizes frequently used. On computer screens
and television sets there are 4 columns for every 3 lines. In contrast,
the aspect ration of movie screens is 16 to 9 (broader field of
vision). Currently 640 columns and 480 lines is a fairly common size
grid for 14 inch computer screens. For computer assisted design more
detail is desirable; hence, workstation screens are 16 inch or larger
with 1280 columns by 846 lines or greater. For this purpose the new IBM
PC2 offer cards with grids up to 1024 by 1024. The number of bits
associated with each pixel determines the number of colors that can be
represented. For example, black and white, 4, 16, 256, thousands, and
millions of colors would be represented by a pixel of 1,2,4, 8, 16, and
24 bits respectively. There is no point going beyond 24 bits because
the human eye can not distinguish more colors. Computer graphics are
digital; however, as humans prefer an analog signal, it is sometimes
converted to analog just before display.
To avoid flicker on a computer screen the image must be redrawn about
60 times a second. The number of bits of information which must be
processed each second to output to a computer screen is the refresh
rate times the number of columns times the numbers of rows times the
number of bits used to represent each pixel. For current PC's this
number can be as large as 60 x 640 x 480 x 4 = 73,728,000 bits per
second.
The trend in computer screens is a higher and higher resolution and a
greater number of colors. Since for most purposes no more than ten
thousand colors are required for quality pictures, this limit may
become common. Currently television-type monitors are cheaper and
better than flat screen displays, such as those in notebook computers.
In the future quality flat screen displays may displace television-type
monitors. Currently, there are many types of competing flat screen
technologies.
Surf the Internet: Remember that the resolution of a computer screen is
much less than the resolution in a media magazine. A picture in a media
magazine generally has 2600 pixels/inch: whereas a picture on a
computer screen only has 70 pixels/inch. It is a real challenge to make
computer pictures look real. The following list shows examples of
different aspects of art, graphics and pictures on the Net. Surf to
each example.
Commercial TV graphics: TV has a single light waveform (analog).
The single waveform corresponds to the position of the electron beam as
it moves across the TV screen in a 525 (US standard) line zigzag
pattern. On one pass the electron beam draws the odd lines and on the
next the even lines(interlacing). The entire picture is refreshed, that
is redrawn, 30 times a second. Color is represented by adding red,
blue, and green light. Current TV is a low resolution device with about
300 columns and 200 lines. Note that when text is presented on a TV
screen you almost never see more than 20 characters in a line. If they
tried to present 80 characters, all you would see is a blur. TV looks
realistic because it is an analog device which displays millions of
colors.
To digitize a TV picture the signal is sampled at twice the frequency
humans can discriminate, which is about 5 million cycles per second,
and the colors represented by a 24 bit word. The resulting raw digital
signal varies from 90 million bits to 220 million depending on the
standard. The digital signal is converted back to analog for viewing, a
process which functions to smooth out the discrete digital points.
There was a commercial battle between Japan, France and the US for the
next generation of high definition television, Japan and France both
adopted analog standards with approximately double the resolution of
current TV. The US to leapfrog the competition has organized a
competition between major firms. The final standard is digital and will
probalby come online by 2010. To switch to digital TV there must be an
inexpensive box which will convert the digital signal to analog for the
existing TVs. Also the broadcasting industry will have to see new
profit potential to justify the replacement of all their current analog
technology. Also, the consumer must be offered new interesting services
to want to purchase the new TVs.
HDTV Resources