Systems for Audio and Video Broadcasting (part 2 of 2)

Transcription

1 Systems for Audio and Video Broadcasting (part 2 of 2) Ing. Karel Ulovec, Ph.D. CTU in Prague, Faculty of Electrical Engineering xulovec@fel.cvut.cz Only for study purposes for students of the! 1/30 Systems for Audio and Video Broadcasting (Part 2 of 2) 2/30

2 3/30 Digital Video Broadcasting (DVB) System for digital television transmission Development started in 1991 Source coding Audio MPEG Layer 2 (typical bit-rate after compression is 192 kb/s) Video MPEG2 (typical bit-rate after compression is 4 Mb/s) Program multiplex (MUX) = MUX of audio, video (and additional data) streams of one service Transport MUX = MUX of several program MUXs [ EN ] 4/30

3 DVB various versions according to transmission channel Satellite (DVB-S) Standard published in 1993 by ETSI Low transmitted power from satellite (limited energy input) Satellite link without reflected signals Wide bandwidth in satellite frequency bands Cable (DVB-C) Standard published in 1994 by ETSI High-quality cable link without reflected signals Terrestrial (DVB-T) Standard published in 1995 by ETSI Channel with multi path propagation Mobile reception is possible Very robust channel coding needed 5/30 DVB-S conceptual block diagram [ EN ] 6/30

4 DVB-C conceptual block diagram [ EN ] 7/30 DVB-T conceptual block diagram [ EN ] 8/30

5 DVB energy dispersal / randomization The same for -S, -C and -T To avoid long sequence of the same value [ EN ] 9/30 DVB outer coder The same for -S, -C and -T Shortened Reed-Solomon code 188 bytes at the input, 204 bytes at the output, 16 control bytes added 8 byte can be repaired DVB outer / convolutional interleaver The same for -S, -C and -T De-interleaving in receiver allows error bursts at the output of inner decoder to be randomized to improve the burst error correction capability of the outer decoder 10/30

6 DVB inner coding The same for -S and -T; not used in -C Covolutional code Puncturing used Possible code rates: 1/2, 2/3, 3/4, 5/6 or 7/8 [ EN ] 11/30 DVB-S modulation QPSK Modulation impulse (baseband shaping) square-root raised cosine filtering, roll-off factor α shall be 0.35 QPSK single carrier modulation Channel bandwidth from 26 to 54 MHz Useful bit-rate up to 68 Mb/s Useful bit-rate values examples for 33 MHz channel as a function of code rate ( after MUX denotes useful bit-rate) [ EN ] 12/30

7 DVB-S carrier frequency Use the same bands as analog satellite TV Example 11/12 GHz [ DVB-S2 2 nd generation system arisen from DVB-S Higher state modulation used 8-PSK, 16-APSK, 32-APSK MPEG-4 AVC used for source coding Better error protection code adopted 13/30 DVB-C modulation Differential QAM Differential encoding in order to get a rotation-invariant constellation Modulation impulse (baseband shaping) square-root raised cosine filtering, roll-off factor α shall be 0.15 QAM single carrier modulation 16, 32, 64, 128, or 256-QAM (64-QAM coax, 256-QAM optical fiber) Channel bandwidth 2, 4, or 8 MHz Useful bit-rate up to 38 Mb/s (for 64-QAM) 14/30

8 DVB-T modulation OFDM used Multi-carrier modulation Mapping Constellation states of all carriers are prepared prior to OFDM modulation Data carriers Video, audio and additional data transmission All data carriers modulated by 64-QAM (maximal bit-rate), or 16-QAM, or QPSK (the most robust transmission) Modes 2k (the number of 1512 data carriers is used) and 8k (6048 data carriers) Pilot carriers Unmodulated carriers (pilot cells) constant amplitude and constant phase (0 or 180 degrees) For synchronization and equalization 45 continual pilot carriers in 2k mode and 177 continual pilot carriers in 8k; also scattered pilot carriers used (see picture on next slide) TPS (Transmission Parameter Signalling) carriers Modulated by differential BPSK (DBPSK) Information for example about data carriers modulation, about the length of GI, about code rate and about mode During one OFDM symbol all TPS sub-carriers are modulated alike and convey only 1 bit of TPS sequence 17 TPS carriers in 2k mode and 68 TPS carriers in 8k 15/30 DVB-T placing of various types of carriers in time frequency grid a) b) Q I c) ~ f (2K mode) (8K mode) ~ t Continual pilot sub-carriers Scattered pilot sub-carriers Data subcarriers TPS sub-carriers a) Measured constella tion diagram for all sub-carriers, b) theoretical constellation diagram, c) placing of sub-carriers in frequency (x axis) and in time (y axis) 16/30

9 DVB-T modulation OFDM Two modes: 2k with 1705 active carriers and 8k with 6817 active carriers (system for handheld devices DVB-H uses mode 4k as a compromise mode for good mobile use and for good SFN possibility) Guard interval possibilities: 1/4, 1/8, 1/16 or 1/32 Useful part of OFDM symbol duration T u = 224 µs n case 2k mode and T u = 896 µs for 8k mode (assuming 8 MHz bandwidth) Channel bandwidth 5, 6, 7, or 8 MHz DVB-T carrier frequency Use the same bands as analog terrestrial TV TV band III (from 174 to 230 MHz) and bands IV/V (from 474 to 860 MHz) 17/30 DVB-T useful bit-rate values [Mbit/s] in dependence on modulation, code rate and guard interval choices (for 8 MHz channel bandwidth) Usual system setting in Czech Republic marked by red ellipse modula tion code rate guard int. 1/4 guard int. 1/8 guard int. 1/16 guard int. 1/32 18/30

10 DVB-T example of real signal spectrum 19/30 DVB-T comparison of spectra of digital video broadcasting vs. analog television 20/30

11 21/30 Digital Audio Broadcasting (DAB) System for digital audio signal transmission Standard published in 1995 by ETSI Source coding MPEG Audio Layer 2 Sampling frequency 24 or 48 ksa/s Bit-rate after compression from 8 to 384 (typical 192) kb/s Mono, joint stereo, stereo, surround sound (encoded to stereo signal) Channel coding Interleaving (time and frequency) Error protection optional use of RS (204,188) code and convolutional code with code rate in range from 1/4 to 4/5 (eight values) Modulation OFDM / DQPSK (differential QPSK) 22/30

12 DAB Channel coding 4 transmission modes Mode Number of carriers Carrier spacing OFDM symbol duration Guard int. duration Frame duration Intended for: I khz ms 246 µs 96 ms SFN in Bands I, II and III (T-DAB) II khz 312 µs 62 µs 24 ms local services in Bands I, II, III, IV, V and L-band (T-DAB) III KHz 156 µs 31 µs 24 ms frequencies below 3 GHz and cable IV khz 623 µs 123 µs 48 ms local services in Bands I, II, III, IV, V and L-band (for optimum SFN operation in L-band) 23/30 DAB type of logical channel Main Service Channel (MSC) Audio and data service components Time-interleaved logical channel divided into a number of sub-channels which are individually convolutionally coded, with equal or unequal error protection Sub-channel may carry one or more service components Fast Information Channel (FIC) For fast access of information about multiplex configuration Is not time-interleaved with fixed equal error protection Synchronization For frame synchronization, automatic frequency control, channel state estimation and transmitter identification First two OFDM symbols of each transmission frame (null and phase reference symbol) 24/30

13 Digital Radio Mondiale (DRM) Terrestrial digital audio broadcasting in bands below 30 MHz Long waves: from to khz Middle waves: from to khz Short waves: from 2.3 to 27 MHz Standard first published in 2001 by ETSI Designed by DRM Consortium (international organization broadcasters, manufacturers, research institutes, regulatory bodies, ) Arisen from French system SkyWave 2000 DRM source coding Very efficient compression algorithms MPEG4-AAC (Advanced Audio Coding) for audio using 12 or 24 ksa/s sampling freq., typical bit-rate of compressed audio data stream is 20 kb/s MPEG4-CELP (Code Excited Linear Prediction) for speech using 8 or 16 ksa/s sampling freq., typical bitrate of compressed speech data stream is about 10 kb/s, (compression and decompression algorithms are based on speech analysis and synthesis) MPEG4-HVXC (Harmonic Vector Excitation Coding) for speech using 8 ksa/s sampling freq., typical bitrate of compressed speech data stream is 4 kb/s, (compression and decompression algorithms are based on parametric analysis and synthesis of speech signal) Source coding extensions: SBR (Spectral Band Replication) and PS (Parametric Stereo) 25/30 DRM channel coding Interleaving Long (2 s) or short (400 ms) Error protection Convolutional code, many possibilities of code rate values (0.6 is typical) Various services can have chosen error protection differently (unequal error protection UEP) OFDM (from 90 to 460 carriers according to the transmission mode) / QPSK, 16QAM, or 64QAM Gain reference Similar as scattered pilot carriers in DVB-T For equalization of channel transfer function Time reference Reference OFDM symbol For time synchronization 26/30

14 DRM channel coding 4 transmission modes (correspond with robustness): Robustness mode A B C D E T u 24 ms ms ms ms 2.25 ms f Hz Hz Hz Hz Hz T /T u 1/9 1/4 4/11 11/14 1/9 T ms ms ms ms 0.25 ms T s = T + T u ms ms 20 ms ms 2.5 ms Frame duration 400 ms 400 ms 400 ms 400 ms 100 ms Typical propagation conditions Gaussian channels, with minor fading Time and frequency selective channels, with longer delay spread As robustness mode B, but with higher Doppler spread As robustness mode B, but with severe delay and Doppler spread Time and frequency selective channels RF bandwidth 9 or 10 khz (half and double are possible as well) for modes A to D and 100 khz for mode E 27/30 DRM three types of the logical transmission channels Fast Access Channel (FAC) Used for synchronization Carries information about OFDM settings and the configurations of other logical channels Uses QPSK (4-QAM) and fixed code rate of 0.6 for modes A to D and 0.25 for mode E Only at a very few carriers in a very few symbols per frame (leading to very low data rate of 180 bps) Service Description Channel (SDC) Contains information for MSC decoding or as example alternative frequencies of different audio broadcasting systems transmitting the same service (FM, DAB) Uses QPSK or 16-QAM and code rate of 0.5 for modes A to D and 0.5 or 0.25 for mode E At all carriers, in two (or three) symbols per super-frame (super-frame = several frames) Main Service Channel (MSC) Contains audio (and additionally) data of up to 4 services Uses either 4-QAM, 16-QAM (with standard mapping) or 64-QAM (with standard mapping, symmetrical hierarchical or mixed hierarchical mapping) Possible to use equal error protection EEP or unequal error protection UEP for services UEP = multiplex can be split in higher and lower protected data part and each part can use different error protection Typically code rate of 0.6 is chosen (EEP) Cell (time) interleaving of either 2s (long) or 400ms (short) protects the MSC data against error bursts At all carriers, in almost all symbols For typical DRM system settings useful data rate of MSC is about 20 kbps 28/30

15 Systems for Audio and Video Broadcasting (Part 2 of 2) Examination areas: Digital Video Broadcasting (DVB), various versions according to transmission channel, used types of modulation and error protection, examples of useful data bit-rate values 29/30 Thank you for your attention. Questions please 30/30