Task 4. Переведите на русский язык.

You must insert the second PAM channel in the gap between the original channels.

2. Can a receiver transmit pulses over wires?

We do not have to change a sampling rate every time.

You should know the difference between PAM with PCM.

5. May I connect these two wires and switch the device?

Task 5. Продолжи предложения, содержащие конструкцию would like.

I would like (to)- Я бы хотел….

I would like….

Would you like…?

I would not like….

 

 

TEXT

What is PCM?

 

A:I’d like to ask you some questions about PCM or pulse code modulation. Could you give some background information to start with?

 

B:Of course. As you know, in the simplest telephone system, speech, in the form of airpressure waves, in other words mechanical energy, is converted into electrical energy by the telephone microphone. This electrical energy travels along a pair of wires to a receiver which converts the electrical energy back into speech. Now, as you can see here in Figure 2, in a traditional transmission system, the current is an analogue of the speech.

 

A: Sorry, what do you mean by analogue?

 

B: Well, that the current has the same wave form as the variations in air pressure. This iswhy we call this method “analogue transmission”. As you can see on the graph, the vertical axis is either current or air pressure.

 

A: So, what’s the difference with PCM?

 

B: Well, I was just coming to that. Let’s look at this graph. If the height of the waveform is sampled and these samples are transmitted, then the original waveform can be reconstituted at the receiver end.

 

A: I see. How often does it need to be sampled?

 

B: Good question. The sampling rate must be at least double the highest frequency

component present in the waveform.

 

A:So, what’s a typical sampling rate?

 

B: For telephony, in which frequencies are limited to 3400 Hz, a sampling rate of 8000 samples per second is used.

 

A:So, that’s what PCM is.

 

B: No, that’s not the whole story. This sampling process is known as pulse amplitude modulation. PAM for short. You’ll notice the gaps between the pulses- these can be used for other channels. On this graph, Figure 2i, a second PAM channel, shown in black, has been inserted between the original PAM channels. This now forms a simple time division multiplex system, or TDM.

 

A:So, how many channels can you put on a line?

 

B: Well, the 24- channel system is widely used but you can have more. However, thesePAM pulses are not transmitted directly over lines, since they would become distorted. Instead, height of the pulse is measured and given a binary code. Each code is then transmitted as a train of pulses within the sample time or time slot. The process of approximating the sample value to a coded level is called “quantization” and the whole process of sampling and coding is known as pulse code modulation or PCM.

 

A: Thank you. That’s a lot clearer. Could you now explain the 24- channel system in more detail?

 

B: Yes, let’s look at this block schematic diagram…

 

 

Task 4. Ответьте на вопросы по тексту.

 

1. What does the telephone microphone convert in the simplest telephone system?

1. What function does the receiver have in this system?
2. What happens at the at the receiver end?
3. What sampling rate is used in telephony?
4. What is PAM?
5. What is PCM?
6. What is quantization?

 

 

Task 5. Найдите эквиваленты следующих выражений и составьте с ними собственные предложения:

 

а) Я бы хотел; как вы знаете; я понимаю; однако; известен как; более детально; давайте посмотрим;

в) ток, провод, частоты, речь, передавать, широко применяется, цепочка импульсов,

ограничен, в секунду.

 

Task 6. Подставьте most, the most или most of.

 

1. … students can make or repair receivers in the laboratories of this college.
2. … the components are made of silica.
3. … useful and expensive thing in my pocket is a cellular phone.
4. I can change channels with the help of … modern control unit.

 

 

Unit 3