Echo Cancellation in Telephony

Echo in a telephone network can be classified as 1.Acoustic echo and 2. Hybrid or network echo .Acoustic echo occurs widely in digital applications and it may be due to the coupling of the received voice signal and the mouthpiece of a handset or due to the coupling of the speaker and microphone of a hands-free phone. It is more complex than the hybrid or network echo, and the delays that occur due to this acoustic echo are much longer. Hybrid or network echo that occurs on the public switched telephone network (PSTN) is caused by the four-wire to two-wire impedance mismatch. This impedance mismatch causes unwanted reflection of the transmitted signals back towards the speaker or the source. In order to minimize the effect of this echo, the telephone networks are equipped with Echo Cancellers. There are many algorithms available to design these echo cancellers. Some of the popular algorithms used in designing a Echo Canceller are

1. Log Likelihood Ratio Test (LRT)
2. Least Mean Square Algorithm (LMS)
3. Normalized LMS Algorithm.

An Echo Canceller consists of the following blocks 1.Main Filter 2. Control Logic 3. Shadow Filter and 4. Parameter Estimator. There are two main design problems for acoustic and network echo cancellation. They are

1. Choice of adaptation algorithm
2. Control Logic for adaptation

Based on these design problems a particular algorithm is chosen to design a Echo Canceller.

VLSI in MOBILE Communication

The demand for mobile operation in electronic circuits has become significant, in recent years. By merely providing the same functionality of a desktop system in a mobile unit, a significant market advantage can be obtained for any electronic function. Consumer electronics companies have been successful by creating portable televisions, video tape recorders and compact disk players, as well as, cordless phones -with limited mobile access for voice communications - and cellular phones for unlimited access. Aggressively, laptops, notebooks and palmtops are becoming the fastest growing computer products. In the future, several portable products will emerge to satisfy consumer demands for mobile communications of voice and video with much higher capacity, quality and bandwidth than the currently available features.

The integrated circuits and node architectures that will be needed for video coding will be presented. Digital Modem Circuits are embedded to support adaptive features in a multimedia mobile communications environment, digital modems are required where the data rate and noise immunity can be programmed dynamically to adapt to varying channel conditions, available bandwidth and traffic demands. Spread- spectrum based CDMA modems can naturally support this adaptively by changing codes and processing gain. Architectures and circuits are described for direct-sequence spread-spectrum transceivers that can achieve low power using voltage scaling and direct conversion to simplify the receiver, and robustness through coherent demodulation and pulse shaping. Node Architecture: Conventional techniques for integrating multimedia processing, wireless modems and laptops rely on shared-bus oriented architectures that cause data throughput losses due to time spent in moving the data between the host CPU, system and the end user. Thus the VLSI chips improve the performance of mobile communication.

Color Television Signal Transmission

The three primary colors Red, Blue and Green are mixed in right proportion to get all the other colors patterns. The same principle is applied in color television .Video camera tubes like vidicon, image orthicon and plumbicon are used to capture a picture. The image thus recorded is converted into a video signal either in terms of voltage variation or current variation by the video camera .The amplitude of the video signal depends on the light intensity of the scene. The video signal thus obtained is modulated using a suitable color carrier before transmission. The color signal received should be able to reproduce a black and white picture using a monochrome receiver. Similarly a color television receiver should be able to reproduce a monochrome picture .In a monochrome signal the brightness information alone is transmitted while in a color signal we need both the brightness information and the color information .Hence the color sub carrier is included along with the monochrome signal .The color sub carrier frequency is about 4.43MHz which is included in the total bandwidth of MHz .This modulated color composite signal is transmitted over an antenna .There are different types of color TV systems followed by different countries .

1. NTSC -- America 2. SECAM -- France 3. PAL -- India, Australia

Microwave Oven - Working Principle

Microwave Oven is a kitchen appliance that cooks or heat food by the application of microwave signal. It uses microwave radiation to heat water and the polarized water molecules, within the food .The heating of food using microwave radiation is fairly uniform which cannot be obtained using any other heating techniques . Microwave oven heats food quickly, efficiently, and safely, but do not brown or bake food in the same way as conventional ovens .A Microwave Oven mainly consists of five parts. They are: High Voltage Transformer, Magnetron Source, Magnetron Control Circuit, Wave guide, Cooking Chamber. The Magnetron Oscillator is a transmitter that generates microwave frequencies around 2.45 GHz. Food has a high percentage of water, and water is famously H2O. The molecule of water has oxygen in the middle and two hydrogen atoms stuck on it, behaving as a polarized molecule. Polarized molecules try to line themselves up with the electrical field, like a magnetic compass needle trying to point at North. But the electrical field is changing 2,450 million times a second due to very high frequency; the molecules do not have time to line up one way before they have to try to line up the other way. So, anything with water in it has all these molecules being moved by the electrical field and heated up. Thus the food inside a Microwave Oven in cooked.

Buck Boost Converter

Buck-boost converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is a switch mode power supply with a similar circuit topology to the boost converter and the buck converter. The output voltage is adjustable based on the duty cycle of the switching transistor. One possible drawback of this converter is that the switch does not have a terminal at ground. This complicates the driving circuitry. Also, the polarity of the output voltage is opposite to the input voltage. Neither drawback is of any consequence if the power source is isolated from the load circuit as the source and diode can simply be reversed and the switch moved to the ground side. DC-DC converters have a wide range of uses today and are becoming increasingly more important in every day use. DC power supplies are probably the largest use of the converters and are much more compact and efficient than the old method of conversion with transformers. These converters can have an output of any range. For instance, one can run logic gates or large dc motor drives with a simple converter. The efficiencies were not where they were expected to be and this is because the inputs of the converter were very small compared to the output requirements. The efficiency increases with increasing input voltage and decreasing duty ratio.

Signal Generating Device For Emergency Situations

With the recent developments in embedded and GPS systems in mind, this is an innovative device that could save many lives that are killed in accidents. GPS enabled devices will have a wide range of applications that the consumer market can benefit from. For example, this device can be mounted on various types of commercial goods such as on a car, a helmet of motorcyclists, and even children can use this device on their miniature helmets while riding their bicycles in crowded city neighborhoods. Furthermore, there are many other perfectly great places where this device can come in handy. How better to explain this device than to illustrate a scenario, let us suppose a car is driving at night on a hilly curved road and all of a sudden the car hits a ditch and falls off the road and into the nearby valley. Obviously the passengers in the car are not able to get any sort of help, but our product can come in handy because it will send an SOS signal to the emergency facilities so they can be reached for their help.

Another excellent use of this device is for people who play extreme sports. For example, let us suppose a skier is skiing down a hill and falls into a ditch or get hit by a tree this GPS enabled system can signal the emergency authorities with the location of this person and the individual can be saved. Moreover, people who live in remote areas such as Arizona and like to mountain hike can use this device if they fall off a rock and break an ankle, or get bit by a rattle snake, and/or out such events. So, as you can see GPS device can be widely use both nationally and internationally. I think this device could have saved some crocodile hunters had they bought our device in time. If it is an accident, the device automatically sends a signal to relief squads such as 911 facilities and lets them know the location of the accident. This is going to basically occur through a