Introduction:
PULSE-CODE MODULATION (PCM) refers to a system in which the standard values of a QUANTIZED WAVE are indicated by a series of coded pulses. There are numerous ways to quantize an incoming wave. When these pulses are decoded, they indicate the standard values of the original quantified wave. The study of PCM trainer is all about these methods. These codes are binary. The entire range of amplitude (frequency or phase) values of the analog wave can be arbitrarily divided into a series of standard values. Each pulse of a pulse train takes the standard value nearest its actual value when modulated. Then the modulating wave can be faithfully reproduced. Each pulse is given whatever standard value is nearest its actual instantaneous value, and the same amplitude range has been divided into 256 standard levels using 8 element binary code. The system does, of course, have some distortion introduced by quantizing the signal. Both the standard values selected and the sampling interval tend to make the reconstructed wave depart from the original. This distortion, called QUANTIZING NOISE, is initially introduced at the quantizing and coding modulator and remains fixed throughout the transmission and retransmission processes. Making the standard quantizing levels closer together can reduce its magnitude. About the system:
The purpose of this trainer Model FOT-7 is to introduce you to the Pulse Code Modulation (PCM) and demodulation technique. A multimode Fiber Optic cable is used for data transmission and reception. In this trainer, you will understand and investigate the following processes. All the major blocks of this trainer are designed from fundamental building blocks instead of using a dedicated CODEC. As a result, all the blocks of CODEC are dissected and are available as separate building blocks for study. This facilitates the student to understand the quantization process and view the quantified and encoded data using 8 bit LED displays at the transmission side. Observe the serial transmission and reception of this quantified decoded data at a test point, observe the decoded 8 bit received data on LED display etc. This data is further reconstructed and the original signal is available at an output terminal for analysis. As a result of this design, the student can see various stages of the PCM transmission and reception processes for himself. Features of PCM blocks are - Digitization of an analog signal,
- Observe the quantization (encoding) of incoming signal,
- Measurement of quantization noise,
- Serialization of quantized signal for onward transmission,
- Serial data capture,
- Decoding of quantified signal,
- Observation of received signal,
- Reconstruction of received signal, and
- Study of aliasing.
In order to achieve the above, the trainer has necessary built in input signal sources.
(a) Continuously variable analog input in the range of 0 to 5 VDC,
(b) Function generator with Sine, and Square wave inputs in the range of 200Hz to 20KHz
(c) Microphone input after proper signal conditioning, for voice transmission and
(d) Provision to connect an external signal source. The output of the transmitted and received signals, decoded signal and finally the reconstructed outputs can be observed on the oscilloscope. In order to observe the sequence of events taking place, there are two types of clocks provided one operating at 200KHz, the other at 1Hz. During slow clocking it is possible to observe the signal flow at various test points as detailed below. (a) Quantization process
(b) Serial transmission
(c) Decoded output and
(d) Reconstructed output.
(e) A power amplifier with speaker to hear the voice reception ( when sampled at 200KHz) This trainer comes with completely self-contained systems with built-in power supplies, signal sources, input output test points for signal observations, a microphone with proper signal condition, a power amplifier with speaker etc. This requires an optional external Dual beam oscilloscope to observe the PCM signal transmission at different stages of modulation and demodulation processes. Specifications:-
Transmitter : Fiber optic InfraRed LED 820nm peak wavelength housed in FC adapter.
Receiver : Photodiode 400 to 1100nm.
Frame size : 8 Bit with LED indicator for frame marker.
Sampling rate : 250KHz.
Connection : Plastic fiber optic package, suitable for PMMA cable,
housed in FC Adapter
On board signal generator : Variable SINE, SQUARE @ 200Hz
Fiber connector : Industry standard FC ? FC connector
Fiber optics cable type : Plastic optical cable, step index, multimode.
Core refractive index : 1.492
Clad refractive index : 1.406
Numerical aperture : 0.5
Acceptance angle : 60 degrees
Core diameter : 1000mm
Fiber diameter : 2.2mm
Fiber length supplied : 1 meter and 3 meter
Power supply : 5V and 12V Input sources:
1. Built-in Function generator with SINE, and SQUARE waveforms.
2. Frequency: Variable frequency of 250Hz to 2KHz
3. Amplitude: Variable in the range of 0 to 5 Volts
4. Continuously variable DC source : 0 to 5V DC
5. Provision for connecting external signal source
6. Microphone with proper signal conditioner Output:
1. 8 bit LEDs to display Quantized data
2. 8 bit LEDs to display decoded data
3. Power amplifier with speaker
4. Frame timing LED to display clock signal
5. Synchronizing signal LED
6. Speaker assembly with suitable power amplifier.
Built-in power supplies. All the above are available in an ergonomically designed cabinet. |
