Basic knowledge of digital signals in telecommunication systems

Basic knowledge of digital signals in telecommunication systems

The digital signal consists of 1 and 0. But this statement is also a bit confusing: how can a person insert "1" or "0" into the line? Unlike the varying levels used in analog transmission, the way of signals in digital circuits is very simple. Two different levels are sent to represent 1 (eg, 3V), and no voltage represents 0. This is called a unipolar signal. The receiving device measures the level every bit time.

One bit time is the time required to send one bit. For example, at 9600 bits per second, one bit time is equal to 1/9600 seconds (about 104um). The measurement is carried out at the center of one bit of time, at which time it is most likely to obtain accurate electrons.

There are actually two types of digital signals. The previous description is a kind of signal used for local (that is, non-telephone company) digital signal mode. The method of AMI or bipolar signal is slightly more mature and can be used for wide-area digital transmission.

Because the essential characteristics of speech are analog (that is, it changes continuously, rather than being generated in a discrete, digital manner), if you want to digitally transmit speech, you must do analog-to-digital (A / D) conversion. The following paragraphs will describe how to convert analog signals into digital form. But first we must answer an important question: "Why is it so troublesome to convert to numbers?" The answer is high quality.

If the analog signal path is very long, an amplifier must be used. But the amplifier retains or even enhances the noise in the conversation, so the more amplifiers used in the transmission path, the more noisy the background. So when you want to use analog lines to wish your grandfather in an ancient village a happy birthday, the luckiest thing is that you can understand each other under the background noise. In order to solve this problem in transoceanic (or long-distance) calls, we can use an old-fashioned method: shout desperately!

Overcoming the noise by yelling at the phone will not solve many problems. When we increase the volume to a new level, we increase the amplitude. This makes the signal amplitude very strong (stronger than the telephone company expected). The telephone company installed equipment on the line to keep the volume below a certain level. These "attenuators", as their name implies, can detect amplitudes much larger than normal, so they will produce attenuation to reduce the amplitude. This will only aggravate this problem, because the attenuation reduction in decibels (dB) has little effect on noise. Therefore, we can not get obvious effect

It is the repeater that replaces the amplifier in the digital system. We usually call it "crazy cookie monster". why? Let's explain it below.

When digital pulses are added to the line, only 1 (+ 3V) needs to be considered. But as soon as the pulse was applied to the line, it immediately began to propagate along the wire. The wire will also immediately begin to reduce the intensity of the pulse. The result is a weakening electrical pulse. Because the decay process starts immediately, there is a high probability that the pulse will fall into the fuzzy zone. Therefore, "Sweet Story Monster" is placed at a connection point about 5000 to 6000 feet apart. As the pulse intensity becomes weaker, it must maintain enough intensity to be recognized without being mistaken for noise. The strength that the signal should maintain is called the pulse detection threshold. If the signal strength falls within the detection threshold, the pulse will be mistaken for noise and ignored.

When the pulse reaches the repeater, the repeater will act as a cookie monster. It completely absorbs the signal-eat it up and make it disappear. Cookie Monster has completed this task, but at the same time it will be crazy on the other hand. So it will say: "There is a pulse there, so I will be a flattering person. A new pulse will be placed in the original position of the line." Therefore, a new 3V pulse is generated. This signal immediately began to propagate along the next part of the line. And the process started again.

The amplifier can only increase the amplitude of the input signal, and a repeater can detect the original signal and see it in another case to retransmit them as a new signal, especially if any noise between the signal and the repeater is lower than The allowable value will be completely ignored. The signal amplitude between the two repeaters will still be attenuated, and noise will also be generated, but as long as the repeaters can recognize the pulses of the original signal, they are only used to construct the output signal and the input signal. Therefore, if digital technology is used in the communication line, it is entirely possible that the signal received by the final receiver is the same as the signal of the first digital transmitter on the line. The duration of the pulse is very short, and the energy value is discrete, there is no continuously changing level, so more repeaters are needed in the book office. In the local loop, if the lines of the same distance use digital communication, the telephone company needs three to four times the equipment.

Take note of what was said in the previous paragraph: "... the first digital transmitter on the line ...". Because voice is an analog signal, many places need to be converted. From the speech source to the first transition point, noise will be introduced and added to the signal. Obviously, the closer the A / D conversion is to the voice source, the better the effect.

If you talk to people in other states or other countries, and the effect is as clear as you talk face-to-face, then you can be sure that most, if not all, of the technology is digital rather than analog. This call quality makes customers happier, the transmission is clearer, data communications are improved, and carrier companies are happier. This is why there is a frenzy of interest in the transmission of Shengzi in industry.

The use of digital instead of analog transmission has another advantage: digital technology makes it easier to combine and operate signals, and it also gives carrier companies greater flexibility in the way they move signals from sources to destinations. Of course, this mainly benefits the service provider directly, without too much impact on customers.