digital tv DTMB

TECHNOLOGY AND STANDARDS OFDIGITAL TELEVISION TERRESTRIAL

MULTIMEDIA BROADCASTING

SHAJANA K BASHEER

S7 ECEB

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ANALOG TELEVISION

• Conventional analog television begins with a video camera

taking pictures of a frame.

• TV camera processes these images in form of pixels.

• Composite video is transmitted.

• A television receiver uses a transmitting tower with an antenna

to distribute signals in a circular pattern.

• Line of sight propagation was only possible.

• UHF and VHF frequencies were used.

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ANALOG TO DIGITAL

• Integrated digital receivers - “set-top-boxes” replaces huge analog

receivers including antennas, dishes, cabling….

• Performance of analog systems tends to deteriorate as the channel

performance deteriorates while digital systems remain as defined.• Ability of digital systems to compress data into a smaller space

• Use of compression coding techniques which allow relatively high

sound and picture quality to be accommodated in a much smaller

channel bandwidth.

• Digital TV have ability to offer the viewer and listener more

services, greater variety and higher technical quality.

• Digital broadcasting techniques can offer credible “single

frequency networks”.3

TV SET AND RECEIVERS

ANALOG DIGITAL

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Analog TV Standards Worldwide

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DTTB- DIGITAL TELEVISION TERRESTRIAL

BROADCASTING• Digital broadcasting is the practice of using digital data rather than

analog waveforms to carry broadcasts over television channels or

assigned radio frequency bands.

• Digital television (DTV) is the transmission of audio and video by

digitally processed and multiplexed signal, in contrast to the totally

analog and channel separated signals used by analog television.

• DTV can support more than one programme in the same channel BW

• Digital broad casting technologies have indeed successfully brought

millions of viewers the excellent watching experience never realized

before from watching analog TV.

• DTV services can be delivered by terrestrial, satellite, cable

broadcasting, or over the internet, and therefore, several transmission

standards exist.

• Digital television terrestrial broadcasting (DTTB) standard which

defines the air-interface of the DTV signals.6

• Three main DTV terrestrial standards existed in the world.

• China started to develop its own DTTB standard in 1994.

• DTMB (Digital Terrestrial Multimedia Broadcast) is the Tv

standard for mobile and fixed terminals used in the china.

• Although at first this standard was called DMB-T/H(Digital

Multimedia Broadcast-Terrestrial/Handheld).

• It decides Frame structure, channel coding and modulation for digital

television terrestrial broadcasting system

• Standard contains many combination of modes

1. Single carrier modulation and Multi-carrier modulation

2. Three frame header (FH) options

3. Three FEC coding rates,

4. Five constellation mappings……

• It results in hundreds of operation modes to support various multi-program

SDTV/HDTV terrestrial broadcasting services.

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SINGLE-FREQUENCY NETWORK (SFN)• Several transmitters simultaneously send the same signal over the same

frequency channel.

• Analog AM and FM radio broadcast networks as well as digital

broadcast networks can operate in this manner.

• Higher number of radio and TV programs transmission in comparison

to traditional multi-frequency network (MFN).

• May result in intersymbol interference.

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COFDM AND OFDM• Self-interference cancellation is facilitated by the OFDM or COFDM

modulation method.

• Orthogonal frequency division multiplexing (OFDM) is a method of

encoding digital data on multiple carrier frequencies.

• A large number of closely spaced orthogonal subcarrier signals are

used to carry data on several parallel data streams or channels.

• Each subcarrier is modulated with a conventional modulation

scheme(QAM/PSK) allow symbol rate, maintaining total data rates

similar to conventional single carrier modulation schemes in same

bandwidth.

• OFDM uses a large number of slow low-bandwidth modulators

instead of one fast wide-band modulator.

• Since each modulator is very slow, we can afford to insert a guard

interval between the symbols, and thus eliminate the ISI.

• COFDM has ability to completely overcome multipath effects

• It uses multiple carriers to transmit the same signal 8

The Moving Picture Experts Group (MPEG)

• Formed by ISO and IEC to set standards for audio and video

compression and transmission.

• MPEG-2 standard supports interlacing and high definition.

• MPEG-2 is considered important because it has been chosen

as the compression scheme for over the air digital television

ATSC, DVB and ISDB, digital satellite TV services like Dish

Network

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DTTB STANDARDSFour competing DTTB standards

1. Advanced Television Systems Committee (ATSC)

2. Digital Video Broadcasting-Terrestrial (DVB-T) standard

3. Terrestrial Integrated Services Digital Broadcasting (ISDB-T)

4. Digital television terrestrial multimedia broadcasting (DTMB)

DETAILS

1. ATSC

• 8 VSB Modulation

• MPEG-2 for coding

• Single carrier modulation

2. DVB-T

• Coded OFDM

• Supports hierarchical transmission

3. ISDB-T

• OFDM and two-dimensional interleaving

• supports hierarchical transmission of up to three layers

• MPEG-2 video and Advanced Audio Coding 11

4.DTMB• TDS OFDM

• Pseudo-random signal frame to serve as the guard interval (GI) of

the OFDM block

• Faster synchronization

• Excellent SFN performance

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DIGITAL TV STANDARDS WORLDWIDE

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DTMB TRANSMISSION SYSTEM

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TECHNICAL DETAILS OF DTMBA. Scrambler

• Pseudo random binary sequence is used to randomize the input MPEG-2 input data to facilitate the system synchronization.

B. Forward Error Correction (FEC) Code

• Concatenation of outer BCH(762, 752) and inner low density parity check (LDPC) codes.

• 261 zeros infront of each 752 information bits forms BCH(762, 752)

C. Signal Constellation and Mapping

• The output binary sequence of FEC is converted to MQAM (M-aryQuadrature Amplitude Modulation)

• DTMB supports the following constellations: 64QAM, 32QAM, 16QAM, 4QAM.

D. Time Interleaving

• Convolutional interleaver is utilized across many OFDM Signal Frames with B of 52 as the number of interleaving branches and M as the interleaving depth.

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E. HIERARCHICAL FRAME STRUCTURE

• Each Signal Frame consists of Frame Header (FH) and FB.

• The baseband symbol rate for both FH and FB are the same, which

is 7.56 Mbps.

• FH uses Pseudo-Noise (PN) sequence and that is why the

modulation scheme for multi-carrier mode called Time-domain

Synchronous Orthogonal Frequency Division Multiplexing (TDS-

OFDM).

• There are three different FH lengths of 420, 595, and 945 symbols.

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Hierarchical Frame Structure

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F. System information

• 6-bit physical layer system information in FB

• Give receiver necessary demodulation/decoding information

including :

• Constellation mapping

• LDPC rate

• Interleaving mode

• Carrier mode information

• FH information.

G. Post-Baseband Processing

• Squared root raised cosine filter with roll-off factor α=0.05

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Frame structure comparison between C-OFDM for DVB-T and TDS- OFDM for DTMB

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• TDS-OFDM technology uses PN as the guard interval to achieve much

faster synchronization than DVB-T

• Providing the unique Signal Frame address

NUMBER OF SUBCARRIERS

• There are two options for the number of subcarriers in the

frame body under the same system bandwidth:

• C = 1 (for single-carrier)

• C = 3780 (multicarrier; the subcarrier spacing is 2 kHz

when radio frequency bandwidth is 8 MHz).

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DTV DEPLOYMENT IN HK • Conventional free-to-air analog television transmission has been used in

Hong Kong since 1967

• To verify the suitability of deploying DTMB in the local environment,field trial was conducted which covered reception under a variety ofpropagation conditions, such as line-of-sight, non-line-of-sight.

• The field trial demonstrated the superiority of the DTMB standard overanalog transmission.

• The picture and sound quality delivered by DTMB was satisfactoryunder non-line-of-sight conditions, using only one-tenth of thetransmitting power of its analog counterpart.

• The DTMB standard supported spectrum-efficient operations includingSFN deployment, multichannel programming, and conveyance of HDTVcontent.

• As for mobile reception, a success rate of over 90 percent was recorded .

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• Official commencement of DTV broadcasting began on 2007

• Two versions of set-top boxes used

1. Basic-tier receivers, with basic reception of signals transmitted,

and restricted to standard-definition contents and decoding of MPEG-2

2. Higher-tier receivers, which receive all standard-definition and

high-definition contents.

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OTHER COUNTRIES USING DTMB • Two-thirds of all countries in the world have not decided their

DTTB standards

• From October to December 2008, terrestrial DTV comparison tests

were made in Lima

• The test results show that DTMB system performed better than any

other systems

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ASTRI LABORATORY• Primary contributor to China’s DTMB standard and technology.

• There are five projects under the MBC Joint Research Laboratory

1. DTMB receiver STB and integrated TV development

2. DTMB receiver testing and validation technology

3. DTMB SFN adapter and system development

4. DTMB test equipment development

5. DTMB demodulator IC design

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ASTRI-Tsinghua joint research lab projects.

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CONCLUSION

• Analog to digital transition had brought great changes in TV

transmission technology.

• 4 standards exists for DTV terrestrial transmission.

• DTMB (Digital Terrestrial Multimedia Broadcast) is the Tv

standard for mobile and fixed terminals used in the china.

• DTMB system performed better than any other systems.

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REFERENCES• Chung-yen Ong, “Technology and Standards of Digital Television

Terrestrial Multimedia Broadcasting”, IEEE Trans. Broadcasting,

2011

• J. Song et al., “Technical Review on Chinese Digital Terrestrial

Television Broadcasting Standard and Measurement on Some

Working Modes,” IEEE Trans. Broadcasting, vol. 53, no. 1, 2007,

pp. 1–7.

• W. Zhang et al., “An Introduction of the Chinese DTTB Standard

and Analysis of the PN595 Working Modes,” IEEE Trans.

Broadcasting, vol. 53, no. 1, 2007, pp. 8–15.

• ITU-R Doc. 6A/82-E, “On Transition from Analogue to Digital

Terrestrial Broadcasting,” Oct. 2008.

• Z. Niu et al., “A New Paradigm for Mobile Multimedia

Broadcasting Based on Integrated Communication and Broadcast

Networks,” IEEE Commun. Mag., vol. 46, no. 7, 2008, pp. 126–32.27

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