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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,PART,1,7.Digital Voltmeter,A voltmeter is used to measure the voltage in volts.A voltmeter is always connected across the two point of the device under test.Since voltmeters are always connected in parallel with the component or components under test,any current through the voltmeter will contribute to the overall current in the tested circuit,potentially affecting the voltage being measured.,An electronic digital voltmeter gives a reading on a numerical display(e.g.LED).It eliminates errors due to parallax that can arise in instruments requiring the position of a pointer on a scale to be estimated and also has a very high input resistance(e.g.l0 M,).,The simplified block diagram and the waveforms is shown in Fig.R7-1,it help us to follow the action.The D.C.voltage to be measured is fed to one input of a voltage comparator.The other input of the comparator is supplied by a ramp generator which produces a repeating sawtooth waveform.The output from the comparator is high(1)until the ramp voltage equals the input voltage when it goes low(0).,PART,1,7.Digital Voltmeter,Fig.R7-1 Block Diagram and the Waveforms of Digital Voltmete,PART,1,7.Digital Voltmeter,The comparator output is applied to one input of an AND gate,the other input of the gate being fed by a generator.When both these inputs are high,the gate opensand gives a high output,i.e.a pulse.The number of steady train of pulses from a pulse output pulses so obtained from the AND gate depends on the length of the comparator output pulse,i.e.on the time taken by the ramp voltage to reach the value of the input voltage.If the ramp is linear,this time is proportional to the input voltage.,The output pulses from the AND gate are recorded by a binary counter and then converted into decimal form by a decoder before being passed on to the display.,The whole process commences when the voltmeter is switched on and a pulse from a trigger circuit starts the ramp generator and sets the counter to zero.When the input voltage is of the order of millivolts,it is amplified before being measured.With some additional circuitry the voltmeter can be adapted for use as a multimeter to measure A.C.voltages,current and also resistance.,PART,2,8.Touchsreen,A touchscreen is an intuitive computer input device that works by simply touching the display screen,either by a finger,or with a stylus,rather than typing on a keyboard or pointing with a mouse(Fig.R8-1).,The touchscreen interface,whereby users navigate a computer system by touching icons or links on the screen itself,is the most simple,intuitive,and easiest to learn of all PC input devices and is fast becoming the interface of choice for a wide variety of applications,such as:,Fig.R8-1 Touchscreen,Fig.R8-2 Public Information Systems,PART,2,Any touchscreen system comprises the following three basic components(as Fig.R8-3).,A touchscreen sensor panel,that sits above the display and which generates appropriate voltages according to where,precisely,it is touched.,A touchsreen controller,that processes the signals received from the sensor and translates there into touch event data which is passed to the PCs processor,usually via a serial or USB interface.,A software driver provides an interface to the PCs operating system and which translates the touch event data into mouse events,essentially enabling the sensor panel to emulate a mouse.,There are several types of touchscreens.Here we discuss the infrared touchscreens.Infrared touchscreens are based on light-beam interruption technology.Instead of placing a layer on the display surface,a frame surrounds it.The frame assembly is,8.Touchsreen,Fig.R8-3 Components of Touchscreen System,PART,2,comprised of printed wiring boards on which the opto-electronics are mounted and is concealed behind an IR-transparent bezel.,The frame contains light sources,or light-emitting diodes,on one side,and light detectors-or photosensors,on the opposite side.The effect of this is to create an optical grid across the screen.When any object touches the screen,the invisible light beam is interrupted,causing a drop in the signal received by the photosensors.Based on which photosensors stop receiving the light signals,it is easy to isolate a screen coordinate.,Infrared touch systems are solid state technology and have no moving mechanical parts.As such,they have no physical sensor that can be abraded or worn out with heavy use over time.Furthermore,since they do not require an overlay,which can be broken,they are less vulnerable to vandalism and also extremely tolerant of shock and vibration.,8.Touchsreen,PART,3,HDTV or High Definition TV(as shown in Fig.R9-1)refers to those amazingly realistic audio and video signals received via cable or satellite.Your decoder receives these signals and converts them to high quality audio/video.HDTV implies a larger aspect ratio,wide screen TV in your home theater system,the way movies were always meant to be viewed.HDTV resolution is far superior compared to traditional TV,which is why it is preferred.,A conventional,analog TV uses a cathode ray tube to deliver images to you,which limits the quality of the image.The screen resolution of an analog TV is about 512x400 pixels.HDTV uses a digital display,like your computer monitor,and the screen resolution is at least 1280 x720 pixels,which is comparable to a high-end computer display.A higher screen resolution means a crisper,clearer picture.In addition to dramatically improved picture quality,HDTV also offers a wider format.This makes an HDTV image more like a movie-screen image.The width-to-height ratio,called the aspect ratio,of HDTV is 16:9.Analog TV has an aspect ratio of only 4:3.,The difference in aspect ratio is most noticeable when watching theatrical movies on TV.For analog TV,the movie must be cut down in a process called pan and scan,in which a part of every scene is deleted to fit the lower aspect ratio.The only way to see the entire movie scene on an analog,9.High Definition TV(HDTV),PART,4,Digital audio is the representation of audio information in digital(discrete level)formats.The use of digital audio allows for simple transmission,storage,and processing of audio signals.,Audio Digitization,Audio digitization is the conversion of analog audio sounds into digital form.To convert an analog audio signal to digital form,the analog signal is digitized by using an analog-to-digital(pronounced A to D)converter.The A/D converter periodically senses(samples)the level of the analog signal and creates a binary number or series of digital pulses that represent the level of the signal(as shown in Fig.R10-1).The typical sampling rate for the conversion of analog audio ranges from 8,000 samples per second(for telephone quality)to 44,000 samples per second(for music quality).,10.Digital Audi,Fig.R10-1 A/D converter and D/A converter,PART,4,10.Digital Audi,Audio Compression,Audio compression is the analysis and processing of digital sound to a form that reduces the space required for transmission or storage.Audio compression coders and decoders(codecs)analyze digital audio signals to remove signal redundancies and sounds that cannot be heard by humans.,Digital audio data is random in nature unlike digital video which has repetitive information that occurs on adjacent image frames.This means that audio signals do not have a high amount of redundancy,making traditional data compression and prediction processes ineffective at compressing digital audio.It is possible to highly compress digital audio by removing sounds that can be heard or perceived by listeners through the process of perceptual coding.,Audio Coding,The type of coder(type of analysis and compression)can dramatically vary and different types of coders may perform better for different types of audio sounds(e.g.speech audio as compared to music).,The MPEG system allows for the use different types of audio coders.The type of coder that is selected can vary based on the application(such as playing music or speech)and the type of device the audio is being played through(such as a television or a battery operated portable media player).The,PART,4,10.Digital Audi,Fig.R10-2 Digital Audio System,PART,5,11.Home Theatre System,Home theater systems are designed to provide the real theater like environment.Besides the visual effects,sound is very crucial to creating the real theater like environment.You must have watched movies in a theater.One thing that always captivates the audience is the sound,the Dolby Surround sound,which makes you feel like you are in middle of the action.So what is Dolby surround sound?The Dolby Surround process involves encoding four channels of information into a two-channel signal.A decoder then decodes the signal and sends it to appropriate destination,Left,Right,Rear,and Phantom Center.In Dolby Surround,the main sounds come from the left and right channels,the vocal or dialog come from the center Phantom channel,and finally the ambience effect is generated from the rear channel.,Dolby surround system was the first to introduce surround sound in home theater system.It evolved into Dolby Surround Pro Logic,Dolby Digital EX,and THX Surround EX.Presently the current standard in surround sound is Dolby Digital 5.1 channel.Dolby Digital is widely used in laser discs,satellite programming,and DVDs.To get Dolby Digital surround sound you should have a discrete Dolby surround sound decoder in the DVD player and Dolby surround sound preamplifier or Dolby digital surround sound receiver.,PART,5,11.Home Theatre System,If you have always coveted a particular type of home theatre system,you can take those home theater pictures when you go shopping for your system.It will also help if you have a professional helping you with the home theater design.If you enjoy do it yourself projects,you can check around and get information on different types of systems.Home theater magazines and home theater system reviews online can help a great deal.,After making your plan for how exactly you would like to set up your home theater,next comes creating the atmosphere with home theater furniture,setting up speakers,wiring,lighting,storage cabinets,etc.Seating is an important aspect of your home theaters.You can have recliners,couches,or just regular chairs.Both type of seating and locating them is an important decision.Depending on how much space you have,you can decide how many seats to install,as you will have to maintain the right angle and distance from the screen.,PART,6,12.Set-top Box(STB),Set-top boxes were also made to enableclosed captioningon older sets in North America,before this became a mandated inclusion in newTV sets.Some have also been produced to mute theaudio(or replace it with noise)whenprofanityis detected in the captioning,where the offensive word is also blocked.Some also include a V-chip that allows only programs of some television content ratings.A function that limits childrens time watching TV or playing video gamesmay also be built in,though some of these work on main electricity rather than the video signal.,Hybrid set-top boxes,such as those used forSmart TVprogramming,enable viewers to access multiple TV delivery methods(including terrestrial,cable,internet,and satellite).Like IPTV boxes,they includevideo on demand,time-shiftingTV,Internet applications,video telephony,surveillance,gaming,shopping,TV-centricelectronic program guides,and e-government.By integrating varying delivery streams,hybrids(sometimes known as TV-centric)enable pay-TV operators more flexible application deployment,which decreases the cost of launching new services,increases speed to market,and limits disruption for consumers.,PART,6,12.Set-top Box(STB),As examples,Hybrid Broadcast Broadband TV(HbbTV)set-top boxes allow traditional TV broadcasts,whether from terrestrial(DTT),satellite,or cable providers,to be brought together with video delivered over the Internet and personal multimedia content.Advanced Digital Broadcast(ADB)launched its first hybrid DTT/IPTV set-top box in 2005,which providedTelefnicawith the digital TV platform for itsMovistar Imagenioservice by the end of that year.,UK basedInview Technologyhas over 8M STBs deployed in the UK for Teletext and an original pushVODservice for Top Up TV.,InIPTVnetworks,the set-top box is a small computer providing two-way communications on anIPnetworkand decoding the videostreaming media.IP set-top boxes have a built-inhome networkinterface that can be Ethernetor one of the existing wire home networking technologies such asHomePNAor theITU-TG.hnstandard,which provides a way to create a high-speed(up to 1Gbit/s)local area networkusing existing home wiring(power lines,phone lines,andcoaxial cables).,In the US and Europe,telephone companies use IPTV(often on ADSL or optical fiber networks)as a means to compete with traditional localcable television monopolies.,
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