卫星电视
卫星电视（简称卫视）即通过接收人造卫星转播过来的电视信号节目的电视广播方式。卫星电视的传输过程一般为：通过卫星将地面基站发射的微波信号远距离传输，最终用户使用定向天线将接收的信号通过解码器解码后输出到电视终端收视的一整套过程。其他常见的广播方式有地面电视和有线电视。
早期的微波信号为模拟信号，常常需要很大的天线来接收。90年代后期起数字信号渐渐成为主流，现在模拟信号大多已经淘汰。而数字信号又分为C,Ku两种波段传输，因为很多原因现在越来越多的运营商更趋于选择Ku段。而加密方式各运营商也有不同选择，华人运营商多选择法国电信、Conax、南瓜Nagra等加密方式。
此外，常常同波束的信号不仅负载视频信号，还会负载音频，数据（远程教育，远程网络传输），还会用作即时视讯传输等多种信号。
目录
1接收
1.1天线
1.2解码器
1.3卫视数量
2优劣
2.1成本
2.2接收问题
2.3法律问题
3华人社会的卫星电视
4相关条目
5参见
接收[编辑]天线[编辑]
个人可以花费高一点装设私人的卫星接收碟，另一种廉价方式是透过业者公共卫星碟接收卫星电视再用电缆传送到用户，称为有线电视，在人口密集地区很普遍。
卫星电视的天线，俗称“锅”或“碟”，用于接收卫星传输的电视节目，凹面设计的原理能将卫星信号聚焦到高频头上，再通过电缆传送到解码器。常用的卫星天线可分为两种：正馈天线、偏馈天线，正馈天线对于接收C波段信号最佳，偏馈天线对于接收KU波段信号最佳。由于两种天线的形状虽然差不多，但在弧度却很大分别。所以有需要接收卫星电视时，天线一定要根据何种波段而选择。
解码器[编辑]
和有线数字电视一样，由于卫星所传输的节目是加密的，需要专用的解码器（即机顶盒）才能正常收视，高清信号（一般具备DVB-S2制式）则需要高清解码器接收。提供卫星节目的机构会为付费用户提供收视帐号密码，同时定期修改密码以打击非法收视用户。
卫视数量[编辑]
不同的电视卫星处于东南西北等不同的方位上，所以一只天线只能接收有限的卫视数量。小型的天线若接收的卫视处于相同或相近的方位则可实现“一锅多星”，然而越大的卫星天线能接收的卫视数量则越多。所以有高手的高超技术、高手的耐心与接收地区的信号场强结合起来，可以用一个直径两米的正馈天线做到“一锅十星”。
优劣[编辑]成本[编辑]
虽然卫星电视有覆盖面广，维护成本低的优点，但是卫星的发射成本却比地面电视或有线电视都要高很多倍，而且非法盗看或“一号多机”（即一个帐号多人使用）都使卫视商损失严重。对于用户来说，卫星电视尽管成本较高（若小区或大厦分摊则非常低），但维护费低、选择多、节目更多元化。
接收问题[编辑]
采用卫星传送微波信号存在着一个缺点：每当遇到天气不稳定（如厚云、台风等）的时候，信号受阻的情况很常见。原因是信号由起点（转发器）出发，途中遇上厚云、雷雨云带、金属飞行物体（如飞机）等障碍物，信号不能穿透或受到干扰，以致发生雨衰导致信号中断而不能到达终点（接收器）那处，使到用户不能收受原有的视听效果。这一点在接收Ku波段卫星电视信号时尤为明显。
法律问题[编辑]
有些国家或地区（如中国大陆、新加坡），在政策上不允许或是限制私人安装卫星电视接收器，使卫星电视无法全面覆盖。但中国法律并没有明文禁止，故实际上中国大陆社会（尤其是农村地区）售卖、安装卫星接收器的现象较为普遍，但卫星接收器大多只用来接收境内电视台的讯号，和极少数境外讯号，如“村村通”卫星电视。一些地方的广电部门会安装卫星电视信号干扰设备，以阻止民众私自接收卫星电视节目，但这在中国内地属于违法行为，可能受到无线电管理部门的查处。也有部分频道含有色情等当地法律所禁止的内容，部分地区收看卫星电视受到了限制。
华人社会的卫星电视[编辑]
现役的华人社会卫星电视节目主要有如下几个运营商：
中国国家广电总局中国省级卫星节目平台- 央视各节目及中国地方省台。节目集中在中星6B、中星6A上。
中国国家广电总局“村村通”节目平台-节目集中在中星9号上。
中国国家广电总局CBTV境外平台- 播出境外卫星频道。节目集中在亚太VI号上。
中国长城平台- 播出数个精选的大陆、香港、澳门的卫星电视频道。亚洲地区：亚太V号北美地区EchoStar7
台湾华人直播 -亚太7号等
台湾太平洋卫视直播系统 -
贺曼娱乐电视网 -
艺华卫视-亚太V号
香港有线电视-亚太V号
台湾数码天空直播系统 -亚太V号
马来西亚Astro卫星电视 -马星3号
相关条目[编辑]
DirecTV-美国最多用户使用的卫星直播视频服务。
Dish Network- 美国第二大卫星运营商，中国大陆长城平台以及台湾的节目在EchoStar上传输。
视频点播- 强调用户个别性、即时性的视频服务技术。
日本卫星电视-日本的卫星直播视频服务，包括广播卫星（BS）与通信卫星（CS）等两大系统。
星空传媒- 亚洲最大的卫星电视频道经营商。
中星6A所属组织中国航天科技集团公司卫星平台东方红四号卫星平台
（DFH-4 Bus）[1]任务类型通信广播卫星发射时间2010年9月5日0时14分运载火箭长征三号乙运载火箭发射地点西昌卫星发射中心质量5,100千克（11,200英磅）[1]轨道参数轨道类型地球同步轨道静止经度125°E[2]
鑫诺六号通信卫星（英语：SinoSat-6），又叫中星6A通信卫星（英语：Chinasat 6A），是一颗运行在东经126.4度、赤道上空的通信广播卫星[2]，用以接替于2007年7月投入使用的鑫诺三号通信广播卫星[3]。相对于鑫诺三号，鑫诺六号的功率更高、信号覆盖的范围更广，其卫星波束可覆盖中国全境及部分周边的亚太地区，将进一步提高覆盖区内广播电视节目的传输容量和节目收视质量[3]。鑫诺六号的设计寿命为15年，有24个C频段转发器、8个Ku频段转发器和1个S频段转发器[3]，其中12B转发器传输“中央广播电视节目无线数字化覆盖工程”央视节目信号。
搭载鑫诺六号的长征三号乙运载火箭于2010年9月5日凌晨零时14分在西昌卫星发射中心点火发射升空，火箭飞行约26分钟后星箭分离，成功进入近地点高度213公里、远地点高度4万2061公里、轨道倾角25.2度的地球同步转移轨道。之后，西安卫星测控中心及远望号测量船将对卫星实施测量和变轨控制等之遥测作业任务。[4]
发射升空当日凌晨零时25分左右，4枚残骸坠落在贵州省镇远县境内，其中1枚火箭外壳残骸分布在羊场镇及都坪镇境内，而另外3枚助推器残骸分别落在都坪镇天印村、地花村、大坝村境内。羊场镇龙洞村一村民目睹了残骸坠落的过程。残骸落地时的一瞬间产生极大气浪，造成60米直径范围内的植物枯萎，但没有造成人员伤亡和重大财产损失[5]。当天14时，卫星整流罩残骸坠落于江西省遂川县汤湖镇南屏村[6]。
故障传言[编辑]
由于鑫诺六号迟迟不宣布定点成功，有传言称“鑫诺六号”卫星氦增压系统发生泄漏，有可能将运行寿命由设计寿命15年减少至10年。
中星6B是中国直播卫星公司拥有并运营的一颗通信卫星。该卫星由法国泰雷兹·阿莱尼亚宇航公司制造，卫星平台为SB4000C2，带有38个C波段转发器。
中星6B通信卫星于2007年7月5日在西昌卫星发射中心由长征三号乙运载火箭发射成功，卫星定位于东经115.5度，预计使用寿命为15年。卫星信号覆盖范围包括中国大陆、香港、台湾、蒙古、朝鲜半岛、日本、东南亚、澳大利亚、中南半岛及缅甸与印度的部分地区。目前卫星转发器的客户主要为中国国内的电视频道供应商。2009年2月9日，中星6B发生故障，导致所传输的所有电视节目信号中断47分钟，稍后恢复，故障原因中国直播卫星公司没有公布[1]。
制造商：法国泰雷兹·阿莱尼亚宇航公司
轨道位置：东经115.5°
发射日期：2007年7月5日
转发器数量：C波段 38个
饱和 EIRP 范围：C波段：31 至 42 dBW
下行频率波段：C波段：3400 至 4200MHz
上行频率波段：C波段：5850 至 6665MHz
中星九号通信卫星是由法国阿尔卡特设计并制造，由中国直播卫星公司所有的广播卫星，原计划为鑫诺二号的备份卫星。2008年6月9日在西昌卫星发射中心由长征三号乙运载火箭发射升空，定点于东经92.2度赤道上空的轨道。中星9号卫星主要覆盖中国大陆、台湾及香港以及南海北部地区，目前卫星的转发器主要客户为中国境内的电视频道供应商。
中星九号计划与中国空间技术研究院设计制造的鑫诺四号直播卫星一起构建中国第一代电视卫星直播系统[1][2]。鑫诺四号原计划于2008年年底发射，但之后发射被推迟。
广电总局科技司有关人士曾表示：中星九号采取不加密方式传输节目，主要针对偏远的、看不到电视、听不到广播的农村地区。
目录
1概况
2资料
3参考文献
4外部链接
5参见
概况[编辑]
维基文库中相关的原始文献：卫星电视广播地面接收设施管理规定
中星9号由法国阿尔卡特宇航公司制造，SB4000平台，寿命15年，极化方式为双圆极化，定点在赤道上空东经92.2°。采用ABS-S标准传输节目。中星9号通信卫星用于开展中国电视节目直播到户的传输业务，这让中国卫星电视的用户对国务院在1993年10月颁布的129号令解禁产生希望。[3]
2010年1月4日，中九调整播出前端的技术参数（即所谓“升级”），以抑制中国大陆私自安装卫星接收器的用户群的扩展。[4]。不过市面上随后出现所谓“双模机”（即在普通DVB-S标准、MPEG-2接收机内添加中九“一代机”模块的机顶盒，可随时在两种模式间切换）可随卫星自动升级。但之后“一代机”在ABS-S模式下能收看的的节目逐渐减少，从最初的46个电视频道逐渐减少至9个电视频道。2017年2月16日，“一代机”信号全部关闭。
2010年11月10日，中国国家广电总局副局长张海涛在发言中表示，国家广电总局正在对中星9号直播星定位进行重新研究，直播星商业运营或于2011年元旦启动，但初期城市暂被排除在商业化运营范围之外[5]。新的“户户通”平台需使用专用的机顶盒接收，除正常接收的59套电视节目和各省级广播电台主频率外，还可以接收本地地面数字电视节目。此外，运营商还针对部分地区定向传送当地电视台节目。“户户通”专用机顶盒内置定位模块[6]，可通过移动通信基站或北斗导航确定所在位置；开通服务时，每个零售工号只能开通某个区域内（县或村，城区除外）的“户户通”机顶盒（即“白名单”或“县域白名单”），禁止跨区域开户安装；如果机器离开了某一特定区域或进入城区，将自动被锁定，无法收看电视节目。但市场上随后又出现所谓“免定位”（专供渔牧民或无移动基站地区的批量授权机顶盒，或者使用软硬件模拟、锁定或清除基站信息的破解版机顶盒）的“户户通”专用机，用户只要在卫星信号覆盖范围内就可正常收看59套电视节目[7][8]。
资料[编辑]
设计制造：法国泰雷兹·阿莱尼亚宇航公司[9]
卫星平台：Spacebus4000
稳定方式：三轴稳定
极化方式：圆极化
轨道位置：东经92.2°
发射日期：2008年6月9日
转发器数量：Ku波段 22个（18个36MHz带宽，4个54MHz带宽）
饱和 EIRP 范围：Ku波段：49.2 至 57.5 dBW
上行频率波段：KU波段：17.3 至 17.8GHz
下行频率波段：KU波段：11.7 至 12.2GHz
参考文献[编辑]
跳转^鑫诺卫星通信有限公司：鑫诺四号
跳转^人民网：“中星9号”直播通讯卫星简介
跳转^人民网：我国直播卫星发展现状
跳转^中星九号升级影响4000万山寨锅 商家关门等破解
跳转^直播星商业运营或明年元旦启动 城市暂被排除
跳转^山西部署“户户通”建设任务
跳转^“户户通”机顶盒迎洗牌期
跳转^这次动真格了：卫星电视“黑锅”将面临彻底打击. 今日头条-智慧家庭.[2016-07-01].
跳转^中星9号. 中国卫通集团有限公司.[2016-07-01].
List of
digital television
broadcast standards
DVB
standards
(countries)
DVB-T(terrestrial)
DVB-T2
DVB-S(satellite)
DVB-S2
DVB-S2X
DVB-C(cable)
DVB-C2
DVB-H(handheld)
DVB-NGH
DVB-T2-Lite
DVB-SH(satellite)
ATSC
standards
(countries)
ATSC(terrestrial/cable)
ATSC 2.0
ATSC 3.0
ATSC-M/H(mobile/handheld)
ISDB
standards
(countries)
ISDB-T(terrestrial)
ISDB-T InternationalSBTVD(Brazil)
ISDB-S(satellite)
ISDB-C(cable)
1seg(handheld)
DTMB
standards
(countries)
DTMB(terrestrial)
DTMB-A
CMMB(handheld)
DMB
standard
(countries)
T-DMB(terrestrial)
S-DMB(satellite)
Codecs
Video
H.265/MPEG-H HEVC
H.264/MPEG-4 AVC
H.262/MPEG-2 Part 2
VC-1
AVS
Audio
HE-AAC
AAC
E-AC-3
AC-3
MP3
MP2
Frequency bands
VHF
UHF
SHF
v
t
e
Satellite televisionis a service that delivers television programming to viewers by relaying it from acommunications satelliteorbiting the Earth directly to the viewers location.[1]The signals are received via an outdoorparabolic antennacommonly referred to as asatellite dishand alow-noise block downconverter.
A satellite receiver then decodes the desiredtelevision programmefor viewing on atelevision set. Receivers can be externalset-top boxes, or a built-intelevision tuner. Satellite television provides a wide range of channels and services. It is usually the only television available in many remote geographic areas withoutterrestrial televisionorcable televisionservice.
Modern systems signals are relayed from acommunications satelliteon theKubandfrequencies (12–18GHz) requiring only a small dish less than a meter in diameter.[2]The first satellite TV systems were an obsolete type now known astelevision receive-only. These systems received weaker analog signals transmitted in theC-band(4–8GHz) fromFSStype satellites, requiring the use of large 2–3-meter dishes. Consequently, these systems were nicknamed big dish systems, and were more expensive and less popular.[3]
Early systems usedanalog signals, but modern ones usedigital signalswhich allow transmission of the modern television standardhigh-definition television, due to the significantly improvedspectral efficiencyof digital broadcasting. As of 2018,Star One C2from Brazil is the only remaining satellite broadcasting in analog signals, as well as one channel (C-SPAN) onAMC-11from the United States.[4]
Different receivers are required for the two types. Some transmissions and channels are unencrypted and thereforefree-to-airorfree-to-view, while many other channels are transmitted with encryption (pay television), requiring the viewer to subscribe and pay a monthly fee to receive the programming.[5]
Contents
1Technology
1.1Sun outage
2Uses
2.1Direct broadcast via satellite
2.2Television receive-only
3History
3.1Early history
3.2Beginning of the satellite TV industry, 1976–1980
3.3TVRO/C-band satellite era, 1980–1986
3.41990s to present
4See also
5References
6External links
Technology[edit]
Satellite television dishesinMalaysia.
AnInview Neelixset-top box.
Back view of a linear polarised LNB.
Corrugatedfeedhornand LNB on aHughesDirecWaysatellite dish.
The satellites used for broadcasting television are usually in ageostationary orbit37,000km (23,000mi) above the earthsequator. The advantage of this orbit is that the satellites orbital period equals the rotation rate of the Earth, so the satellite appears at a fixed position in the sky. Thus the satellite dish antenna which receives the signal can be aimed permanently at the location of the satellite, and does not have to track a moving satellite. A few systems instead use a highly elliptical orbit withinclinationof +/−63.4 degrees and orbital period of about twelve hours, known as aMolniya orbit.
Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at anuplinkfacility.[6]Uplink satellite dishes are very large, as much as 9 to 12 meters (30 to 40 feet) in diameter.[6]The increased diameter results in more accurate aiming and increased signal strength at the satellite.[6]The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of thetransponderstuned to that frequency range aboard that satellite.[7]The transponder re-transmits the signals back to Earth at a different frequency (a process known as translation, used to avoid interference with the uplink signal), typically in theC-band(4–8GHz),Ku-band(12–18GHz), or both.[6]The leg of the signal path from the satellite to the receiving Earth station is called the downlink.[8]
A typical satellite has up to 32 Ku-band or 24 C-band transponders, or more for Ku/C hybrid satellites.[9]Typical transponders each have a bandwidth between 27 and 50MHz.[9]Each geostationary C-band satellite needs to be spaced 2° longitude from the next satellite to avoid interference; for Kuthe spacing can be 1°.[10]This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites or 360/1 = 360 geostationary Ku-band satellites.[10]C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected byrain(as water is an excellent absorber of microwaves at this particular frequency).[11]The latter is even more adversely affected by ice crystals in thunder clouds.[11]
On occasion,sun outagewill occur when the sun lines up directly behind the geostationary satellite to which the receiving antenna is pointed.[12]The downlink satellite signal, quite weak after traveling the great distance (seeinverse-square law), is collected with aparabolicreceiving dish, which reflects the weak signal to the dishs focal point.[13]Mounted on brackets at the dishs focal point is a device called afeedhornor collector.[14]The feedhorn is a section ofwaveguidewith a flared front-end that gathers the signals at or near the focal point and conducts them to a probe or pickup connected to alow-noise block downconverter(LNB).[15]The LNB amplifies the signals anddownconvertsthem to a lower block ofintermediate frequencies(IF), usually in theL-band.[15]
The original C-band satellite television systems used alow-noise amplifier(LNA) connected to the feedhorn at the focal point of the dish.[16]The amplified signal, still at the higher microwave frequencies, had to be fed via very expensive low-loss 50-ohm impedancegas filled hardlinecoaxial cablewith relatively complexN-connectorsto an indoor receiver or, in other designs, a downconverter (a mixer and a voltage-tuned oscillator with some filter circuitry) for downconversion to an intermediate frequency.[16]The channel selection was controlled typically by a voltage tuned oscillator with the tuning voltage being fed via a separate cable to the headend, but this design evolved.[16]
Designs formicrostrip-based converters foramateur radiofrequencies were adapted for the 4GHz C-band.[17]Central to these designs was concept of block downconversion of a range of frequencies to a lower, more easily handled IF.[17]
The advantages of using an LNB are that cheaper cable can be used to connect the indoor receiver to the satellite television dish and LNB, and that the technology for handling the signal at L-band and UHF was far cheaper than that for handling the signal at C-band frequencies.[18]The shift to cheaper technology from the hardline and N-connectors of the early C-band systems to the cheaper and simpler 75-ohm cable andF-connectorsallowed the early satellite television receivers to use, what were in reality, modifiedUHFtelevision tuners which selected the satellite television channel for down conversion to a lowerintermediate frequencycentered on 70MHz, where it was demodulated.[18]This shift allowed the satellite televisionDTHindustry to change from being a largely hobbyist one where only small numbers of systems costing thousands of US dollars were built, to a far more commercial one of mass production.[18]
In the United States, service providers use theintermediate frequencyranges of 950–2150MHz to carry the signal from the LNBF at the dish down to the receiver. This allows for transmission of UHF signals along the same span of coaxial wire at the same time. In some applications (DirecTVAU9-S and AT-9), ranges of the lowerB-band[ambiguous]and 2250–3000MHz, are used. Newer LNBFs in use by DirecTV, called SWM (Single Wire Multiswitch), are used to implementsingle cable distributionand use a wider frequency range of 2–2150MHz.[citation needed]
The satellite receiver orset-top boxdemodulates and converts the signals to the desired form (outputs for television, audio, data, etc.).[19]Often, the receiver includes the capability to selectivelyunscrambleordecryptthe received signal to provide premium services to some subscribers; the receiver is then called anintegrated receiver/decoderor IRD.[20]Low-loss cable (e.g.RG-6,RG-11, etc.) is used to connect the receiver to the LNBF or LNB.[15]RG-59is not recommended for this application as it is not technically designed to carry frequencies above 950MHz, but may work in some circumstances, depending on the quality of the coaxial wire, signal levels, cable length, etc.[15]
A practical problem relating to home satellite reception is that an LNB can basically only handle a single receiver.[21]This is because the LNB is translating two differentcircular polarizations(right-hand and left-hand) and, in the case of K-band, two different frequency bands (lower and upper) to the same frequency range on the cable.[21]Depending on which frequency and polarization a transponder is using, the satellite receiver has to switch the LNB into one of four different modes in order to receive a specific channel.[21]This is handled by the receiver using theDiSEqCprotocol to control the LNB mode.[21]If several satellite receivers are to be attached to a single dish, a so-calledmultiswitchwill have to be used in conjunction with a special type of LNB.[21]There are also LNBs available with a multiswitch already integrated.[21]This problem becomes more complicated when several receivers are to use several dishes (or several LNBs mounted in a single dish) pointing to different satellites.[21]
A common solution for consumers wanting to access multiple satellites is to deploy a single dish with a single LNB and to rotate the dish using an electric motor. The axis of rotation has to be set up in the north-south direction and, depending on the geographical location of the dish, have a specific vertical tilt. Set up properly the motorized dish when turned will sweep across all possible positions for satellites lined up along thegeostationary orbitdirectly above the equator. The disk will then be capable of receiving any geostationary satellite that is visible at the specific location, i.e. that is above the horizon. TheDiSEqCprotocol has been extended to encompass commands for steering dish rotors.[citation needed]
There are five major components in a satellite system: the programming source, the broadcast center, the satellite, thesatellite dish, and thereceiver. Direct broadcast satellites used for transmission of satellite television signals are generally ingeostationary orbit37,000km (23,000mi) above the earthsequator.[22]The reason for using this orbit is that the satellite circles the Earth at the same rate as the Earth rotates, so the satellite appears at a fixed point in the sky. Thus satellite dishes can be aimed permanently at that point, and dont need a tracking system to turn to follow a moving satellite. A few satellite TV systems use satellites in aMolniya orbit, a highlyellipticalorbit withinclinationof +/-63.4 degrees and orbital period of about twelve hours.
Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at anuplinkfacility.[22]Uplink facilities transmit the signal to the satellite over a narrow beam ofmicrowaves, typically in theC-bandfrequency range due to its resistance torain fade.[22]Uplink satellite dishes are very large, often as much as 9 to 12 metres (30 to 40 feet) in diameter[22]to achieve accurate aiming and increased signal strength at the satellite, to improve reliability.[22]The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of thetransponderstuned to that frequency range aboard that satellite.[22]The transponder then converts the signals toKuband, a process known as translation, and transmits them back to earth to be received by home satellite stations.[22]
The downlinked satellite signal, weaker after traveling the great distance (seeinverse-square law), is collected by using a rooftopparabolicreceiving dish (satellite dish), which reflects the weak signal to the dishs focal point.[23]Mounted on brackets at the dishsfocal pointis afeedhorn[23]which passes the signals through awaveguideto a device called alow-noise block converter(LNB) or low noise converter (LNC) attached to the horn.[23]The LNB amplifies the weak signals, filters the block of frequencies in which the satellite television signals are transmitted, and converts the block of frequencies to a lower frequency range in theL-bandrange.[23]The signal is then passed through acoaxial cableinto the residence to the satellite television receiver, aset-top boxnext to the television.
The reason for using the LNB to do the frequency translation at the dish is so that the signal can be carried into the residence using cheapcoaxial cable. To transport the signal into the house at its original Kubandmicrowavefrequency would require an expensivewaveguide, a metal pipe to carry the radio waves.[24]The cable connecting the receiver to the LNB are of the low loss typeRG-6, quad shield RG-6, or RG-11.[25]RG-59is not recommended for this application as it is not technically designed to carry frequencies above 950MHz, but will work in many circumstances, depending on the quality of the coaxial wire.[25]The shift to more affordable technology from the 50ohm impedance cable andN-connectorsof the early C-band systems to the cheaper 75ohm technology andF-connectorsallowed the early satellite television receivers to use, what were in reality, modifiedUHFtelevision tuners which selected the satellite television channel for down conversion to another lowerintermediate frequencycentered on 70MHz where it was demodulated.[24]
An LNB can only handle a single receiver.[21]This is due to the fact that the LNB is mapping two different circular polarisations – right hand and left hand – and in the case of the Ku-band two different reception bands – lower and upper – to one and the same frequency band on the cable, and is a practical problem for home satellite reception.[21]Depending on which frequency a transponder is transmitting at and on what polarisation it is using, the satellite receiver has to switch the LNB into one of four different modes in order to receive a specific desired program on a specific transponder.[21]The receiver uses theDiSEqCprotocol to control the LNB mode, which handles this.[21]If several satellite receivers are to be attached to a single dish a so-calledmultiswitchmust be used in conjunction with a special type of LNB.[21]There are also LNBs available with a multiswitch already integrated.[21]This problem becomes more complicated when several receivers use several dishes or several LNBs mounted in a single dish are aimed at different satellites.[21]
Theset-top boxselects the channel desired by the user by filtering that channel from the multiple channels received from the satellite, converts the signal to a lowerintermediate frequency,decryptstheencryptedsignal,demodulatesthe radio signal and sends the resulting video signal to the television through a cable.[25]To decrypt the signal the receiver box must be activated by the satellite company. If the customer fails to pay his monthly bill the box is deactivated by a signal from the company, and the system will not work until the company reactivates it. Some receivers are capable ofdecryptingthe received signal itself. These receivers are calledintegrated receiver/decodersor IRDs.[25]
Analog television which was distributed via satellite was usually sent scrambled or unscrambled inNTSC,PAL, orSECAMtelevision broadcast standards. The analog signal isfrequency modulatedand is converted from an FM signal to what is referred to asbaseband. This baseband comprises the video signal and the audio subcarrier(s). The audio subcarrier is further demodulated to provide a raw audio signal.
Later signals were digitized television signal or multiplex of signals, typicallyQPSK. In general, digital television, including that transmitted via satellites, is based on open standards such asMPEGandDVB-S/DVB-S2orISDB-S.[citation needed]
Theconditional accessencryption/scrambling methods includeNDS,BISS,Conax,Digicipher, Irdeto,Cryptoworks,DG Crypt,Beta digital,SECA Mediaguard,Logiways,Nagravision,PowerVu,Viaccess,Videocipher, andVideoGuard. Many conditional access systems have been compromised.
Sun outage[edit]
An event calledsun outageoccurs when the sun lines up directly behind the satellite in the field of view of the receiving satellite dish.[26]This happens for about a 10-minute period daily around midday, twice every year for a two-week period in the spring and fall around theequinox. During this period, the sun is within themain lobeof the dishs reception pattern, so the strong microwavenoiseemitted by the sun on the same frequencies used by the satellites transponders drowns out reception.[26]
卫星电视没有信号怎么办 卫星电视没有信号的原因
打开机顶盒，打开电视，调节到卫星测试界面。
通常会看到一个信号强度，一个是信号质量。我们首先看看信号强度，强度为0，则表示连线有问题，就检查一下，各个接口是否插牢。
如果信号强度不为0，那么就试着转动一下卫星天线的方向。如果信号强度变大，那么恭喜你，信号质量也会随着变大。慢慢转动天线，直到信号质量达到最低限度（一般为30%）为止。
如果怎么转动天线，信号强度和质量都没有变化，那就要考虑其他原因了：是否阴天？否隔着多层玻璃或金属障碍物？是不是气温太低？
东芝电视机怎么连接卫星电视
将卫星接收机的大锅安装在阳台上或者屋顶上。
将卫星接收机连接大锅，并将大锅导出的信号线接入卫星接收机的信号输入接口中。
将卫星接收机与电视连接即可。
电视机是指根据人眼的视觉暂留特性和视觉心理，利用电的方法来传播光学信息的机器。电视机的基本系统由摄像、传输和显像3部分组成。
|卫星电视
东芝电视机怎么连接卫星电视 卫星电视 卫星电视没有信号怎么办 卫星电视没有信号的原因