By dictionary
Anarogu=analog: (adj.)
using a continuously changing range of physical quantities to
measure or store data. (noun) a thing that is similar to
another thing. Dejitaru=digital: (adj.) using a system of
receiving and sending information as a series of the numbers
one and zero, showing that an electronic signal is there or
is not there.
Burodobando=broadband: I can understand "broadband"
is a compound word of "broad" and "band",
but cannot understand its technical meaning.
By manual
I could not understand the meanings of the above three terms, especially “broadband,” well. Therefore I read some manuals and understood them a little. I would like to explain the digitization of analog data "sound" using my new knowledge.
The vibration which
travels through the elastic body, such as air (gas) and iron
(solid), is called sound waves. Usually sound waves arrive at
our ears as the compressional waves of air. The arrived sound
waves vibrate our eardrums. Then we perceive the vibration as
sound.
When we continuously measure the sound intensity at a point
and plot it against time, a graph like figure 1 is obtained.
Figure 1 is analogous with the real sound.
Fig.1. Sound Waves
At the microphone and the transmitter, sound waves are converted to the analogous voltage/current waves as accurately as possible. The electric signals have the following convenient features, that is, they are;
Storable on the magnetic devices
Able to send to near places
Able to return to sound waves
Able to change to louder/lower sound waves
So this conversion is
very useful.
However, this conversion cannot keep the perfect similarity
between sound waves and electric voltage/current waves. In
other words, the handling of analog waves always accompanies
the deformation of the waves. In the case of long-distance
communication, the waves are gradually deformed by electrical
resistance and noise. The restoration of the deformed waves
is impossible. It is the weakest point of the analog process.
To prevent the deformation of original wave shape, the digitization technique is used. When an analog wave is shown in figure 2, the values of the graph are intermittently sampled at regular intervals; For example, (0.00, 0.00), (0.05, 0.31), (0.10, 0.59), ... .
where (time, value)
the time and value in the parentheses are finite figures: in this case 3 figures.
Fig. 2 Digitization (Sampling)
After sampling, the original wave data are handled as the numerical data (The decimal digits are used here but the numerical data are basically handled with the binary digits in the digital devices.)
It is said that the digitized data are resistible against the deformation but the digitized data also deteriorate like analog data. In the computer and its peripherals, the binary digits, 0 and 1, are frequently substituted with 0 volt and 5 volts. When the signals of 0 and 5 volts changed to 0.1 and 4.8 volts because of noise, the system can automatically restore them to the original values, 0 volt and 5 volts. In such meaning, a little deterioration of the binary data is restorable.
Because the deterioration of quality of sound occurs only with A/D and D/A conversions and analog data handling, it is desirable to digitize the analog data (A/D conversion) at the first step of the data processing and to return them to the analog data (D/A conversion) at the final step.
We can reduce the deterioration by using the large digit number. But the large digit number raises the cost and lowers the speed of the data handling. So the compromise between the cost and the speed is necessary. I suppose that the image data handling is almost equal to the sound data handling even if the devices used differ.
About 50 years ago, I could hardly hear the phone call from the next prefecture, Tokushima. But now I can hear the phone call from Europe as clearly as the call from the next-door. I think I have been using the same analog phone in my house, but maybe most of apparatuses between my house and the party's phone has digitized already. Of course, the analog process must have also been improved very much.
Although two kinds of
voltage signals are used in the computer and its close
peripherals, the sine curve signals are used on the long
distance communication.
Practically many kinds of signals are created by changing the
three parameters (amplitude, frequency, and phase) of the
sine curve. As the simplest example, AM modulation uses two
sine signals: amplitude=0 and amplitude=1. The
transmission/reception speed of this two-signal system is
slow because the system transmits (or receives) only 1 figure
of binary digit at a time.
To improve this weak point, more than two signals are used. When using 16-signal system, we can send/receive 1 figure of hexadecimal digit at one time. Because the 1 figure of the hexadecimal notation equals to the 4 figures of the binary notation, sending/receiving 1 figure of hexadecimal notation means sending/receiving 4 figures of binary notation. That is, the speed of the 16-signal system becomes four times as fast as the speed of 2-signal system.
The Amplitude-Phase modulation, which uses two kinds of amplitude and eight phases, is practically used to increase the communication speed. I heard that the system that uses much amplitude is impractical because the system is weak in noise.
I will try to explain about broad band using ADSL(Asymmetric Digital Subscriber Line) which is most popular now. Figure 3 shows ADSL.
upper left: personal computer lower left: telephone
Fig. 3 ADSL
When ADSL is used, the high speed communication between computers and the traditional speed communication between analog phones are simultaneously done on the traditional telephone line. Practically, the splitters and modems are attached at the home side and the exchange side. At the receiver side splitter, sound and computer data are separated, and at the sender side splitter they are mixed.
The computer data are demodulated at the receiver side modem and modulated at the sender side modem. The band between 0 and 4 kHz is used for sound; the band between 26 and 138 kHz is used for the digitized up data; the band between 138 and 1104 kHz is used for the digitized down data. The ADSL uses the broader frequency band than the traditional telephone used. So this is a kind of broad band communication.
The up and down bands
are both subdivided to sub-channels of 4 kHz band. Each
sub-channel can independently send or receive signals.
Because the down stream has more sub-channels than the up
stream, the down stream is faster than the up stream.
Assuming that one cycle is used as one signal, the
communication speed on the high frequency band is faster than
the speed on the low frequency band. (cf. Figure 4: the time
required to send one 1-Hz signal is equal to the time to send
two 2-Hz signals.)
Fig. 4 Different Frequency Waves
I think that the broadband communication using the phone subscriber's line (ADSL) has the same principle as the communication using the optical fiber and radio.
I think I wrote about what I was able to understand, but maybe it had some misunderstandings. I will correct them if I find them.