In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted. Most radio systems in the 20th century used frequency modulation (FM) or amplitude modulation (AM) for radio broadcast.

A modulator (Digital modulator & Analog modulator) is a device that performs modulation. Following we will explain the difference among the DVBC（J.83A、J.83B）、DVB-T 、DTMB、DVBC、ISDB and ATSC:

DVB-C

DVB-C stands for "Digital Video Broadcasting - Cable" and it is the DVB European consortium standard for the broadcast transmission of digital television over cable. This system transmits an MPEG-2 or MPEG-4 family digital audio/digital video stream, using a QAM modulationwith channel coding. The standard was first published by the ETSI in 1994, and subsequently became the most widely used transmission system for digital cable television in Europe, Asia and South America.It is deployed worldwide in systems ranging from the larger cable television networks (CATV) down to smaller satellite master antenna TV (SMATV) systems.

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DVB-C transmitter

With reference to the figure, a short description of the single processing blocks follows.

· Source coding and MPEG-2 multiplexing (MUX): video, audio, and data streams are multiplexed into an MPEG program stream (MPEG-PS). One or more MPEG-PSs are joined together into an MPEG transport stream (MPEG-TS). This is the basic digital stream which is being transmitted and received by home set top boxes (STB) or relevant integrable decoder (e.g.Conax) module. Allowed bitrates for the transported MPEG-2 depend on a number of modulation parameters: it can range from about 6 to about 64 Mbit/s (see the bottom figure for a complete listing).

· MUX adaptation and energy dispersal: the MPEG-TS is identified as a sequence of data packets, of fixed length (188 bytes). With a technique called energy dispersal, the byte sequence is decorrelated.

· External encoder: a first level of protection is applied to the transmitted data, using a nonbinary block code, a Reed-Solomon RS (204, 188) code, allowing the correction of up to a maximum of 8 wrong bytes for each 188-byte packet.

· External interleaver: convolutional interleaving is used to rearrange the transmitted data sequence, such way it becomes more rugged to long sequences of errors.

· Byte/m-tuple conversion: data bytes are encoded into bit m-tuples (m = 4, 5, 6, 7, or 8).

· Differential coding: In order to get a rotation-invariant constellation, this unit shall apply a differential encoding of the two Most Significant Bits (MSBs) of each symbol.

· QAM Mapper: the bit sequence is mapped into a base-band digital sequence of complex symbols. There are 5 allowed modulation modes: 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM.

· Base-band shaping: the QAM signal is filtered with a raised-cosine shaped filter, in order to remove mutual signal interference at the receiving side.

· DAC and front-end: the digital signal is transformed into an analog signal, with a digital-to-analog converter (DAC), and then modulated to radio frequency by the RF front-end.

Available bitrates for a DVB-C system (Mbit/s)
Modulation	Bandwidth (MHz)
Modulation	2	4	6	8	10
16-QAM	6.41	12.82	19.23	25.64	32.05
32-QAM	8.01	16.03	24.04	32.05	40.07
64-QAM	9.62	19.23	28.85	38.47	48.08
128-QAM	11.22	22.44	33.66	44.88	56.10
256-QAM	12.82	25.64	38.47	51.29	64.11

DVB-T

DVB-T is an abbreviation for "Digital Video Broadcasting — Terrestrial"; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998. This system transmits compressed digital audio, digital video and other data in an MPEG transport stream, using coded orthogonal frequency-division multiplexing(COFDM or OFDM) modulation. It is also the format widely used worldwide (including North America) for Electronic News Gathering for transmission of video and audio from a mobile newsgathering vehicle to a central receive point.

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Basics

Rather than carrying one data carrier on a single radio frequency (RF) channel, COFDM works by splitting the digital data stream into a large number of slower digital streams, each of which digitally modulates a set of closely spaced adjacent sub-carrier frequencies. In the case of DVB-T, there are two choices for the number of carriers known as 2K-mode or 8K-mode. These are actually 1,705 or 6,817 sub-carriers that are approximately 4 kHz or 1 kHz apart.

DVB-T offers three different modulation schemes (QPSK, 16QAM, 64QAM).

DVB-T has been adopted or proposed for digital television broadcasting by many countries (see map), using mainly VHF 7 MHz and UHF 8 MHz channels whereas Taiwan, Colombia, Panama and Trinidad and Tobago use 6 MHz channels. Examples include the UK's Freeview.

The DVB-T Standard is published as EN 300 744, Framing structure, channel coding and modulation for digital terrestrial television. This is available from the ETSI website, as is ETSI TS 101 154, Specification for the use of Video and Audio Coding in Broadcasting Applications based on the MPEG-2 Transport Stream, which gives details of the DVB use of source coding methods for MPEG-2 and, more recently, H.264/MPEG-4 AVC as well as audio encoding systems. Many countries that have adopted DVB-T have published standards for their implementation. These include the D-book in the UK, the Italian DGTVi, the ETSI E-Book and the Nordic countries and Ireland NorDig.

DVB-T has been further developed into newer standards such as DVB-H (Handheld), which was a commercial failure and is no longer in operation, and DVB-T2, which was initially finalised in August 2011.

DVB-T as a digital transmission delivers data in a series of discrete blocks at the symbol rate. DVB-T is a COFDM transmission technique which includes the use of a Guard Interval. It allows the receiver to cope with strong multipath situations. Within a geographical area, DVB-T also allows single-frequency network (SFN) operation, where two or more transmitters carrying the same data operate on the same frequency. In such cases the signals from each transmitter in the SFN needs to be accurately time-aligned, which is done by sync information in the stream and timing at each transmitter referenced to GPS.

The length of the Guard Interval can be chosen. It is a trade-off between data rate and SFN capability. The longer the guard interval the larger is the potential SFN area without creating intersymbol interference (ISI). It is possible to operate SFNs which do not fulfill the guard interval condition if the self-interference is properly planned and monitored.

DTMB

DTMB (Digital Terrestrial Multimedia Broadcast) is the TV standard for mobile and fixed terminals used in the People's Republic of China, Cuba, Hong Kong and Macau.

Previously known as DMB-T/H (Digital Multimedia Broadcast-Terrestrial/Handheld), the DTMB is a merger of the standards ADTB-T (developed by the Shanghai Jiao Tong University), DMB-T (developed by Tsinghua University) and TiMi (Terrestrial Interactive Multiservice Infrastructure); this last one is the standard proposed by the Academy of Broadcasting Science in 2002.

At first, neither Shanghai Jiao Tong University nor Tsinghua had enough political strength to make their own technology become the unique standard, so the final decision was to opt for a double standard, merged with the TIMI 3 standard, responding to a need for backward compatibility.

The DTMB was created in 2004 and finally became an official DTT standard in 2006.

DVB-C

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ISDB

The Integrated Services Digital Broadcasting (ISDB; Japanese: 統合デジタル放送サービス, Tōgō dejitaru hōsō sābisu) is a Japanese standard for digital television (DTV) and digital radio used by the country's radio and television networks. ISDB replaced NTSC-J analog television system and the previously used MUSE Hi-vision analogue HDTV system in Japan, and will be replacing NTSC, PAL-M and PAL-N in South America and the Philippines. Digital Terrestrial Television Broadcasting (DTTB) services using ISDB-T started in Japan in December 2003 and in Brazil in December 2007 as a trial. Since then, many countries have adopted ISDB over other digital broadcasting standards.

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ATSC standards

Advanced Television Systems Committee (ATSC) standards are a set of standards for digital television transmission over terrestrial, cable, and satellite networks. It is largely a replacement for the analog NTSC standard, and like that standard, used mostly in the United States, Mexico and Canada. Other former users of NTSC, like Japan, have not used ATSC during their digital television transition because they adopted their own system called ISDB.

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The ATSC standards were developed in the early 1990s by the Grand Alliance, a consortium of electronics and telecommunications companies that assembled to develop a specification for what is now known as HDTV. The standard is now administered by the Advanced Television Systems Committee. The standard includes a number of patented elements, and licensing is required for devices that use these parts of the standard. Key among these is the 8VSB modulation system used for over-the-air broadcasts.

ATSC includes two primary high definition video formats, 1080i and 720p. It also includes standard-definition formats, although initially only HDTV services were launched in the digital format. ATSC can carry multiple channels of information on a single stream, and it is common for there to be a single high-definition signal and several standard-definition signals carried on a single (former) NTSC channel allocation.

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