Diapositiva 1

Radio - Televisión Digital
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Un canal de TV analógico tiene una anchura de banda de 5.5MHz en formato entrelazado y tamaño
normal (PAL: 625 líneas).
Un canal de radio tiene una anchura de banda de unos 20 KHz.
Un canal telefónico tiene una anchura de banda de 4 KHZ.
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Radio Televisión Digital
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Tiene todas las ventajas de una señal digital frente al ruido y degradación. Además es susceptible de
compresión en porcentajes elevados (10 % o más)
Transmisión:
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Generación
Digitalización (Conversión A/D o Muestreo)
Compresión (Formato)
Modulación
Emisión
Recepción:
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Captación
Demodulación
Descompresión
(Conversión D/A) ** con dependencia del visualizador
Presentación
DVB (Digital Video Broadcasting)
Los sistemas DVB distribuyen los datos por:
Satélite (DVB-S y DVB-S2)
Cable (DVB-C y DVB-C2)
Televisión terrestre (DVB-T y DVB-T2)
Televisión terrestre para dispositivos portátiles (DVB-H)
Televisión por satelital para dispositivos portátiles (DVB-SH)
Estos estándares definen la capa física y la capa de enlace de datos de un sistema de
distribución.
Todos los datos se transmiten en flujos de transporte MPEG-2 con algunas restricciones
adicionales (DVB-MPEG). Se está experimentando en varios países un estándar para
distribución comprimida en el tiempo (DVB-H) para distribución a dispositivos móviles.
Estos estándares se diferencian principalmente en los tipos de modulación utilizados,
debido a las diferentes restricciones técnicas:
DVB-S (SHF) utiliza QPSK, 8PSK O 16-QAM
DVB-S2 (SHF) utiliza QPSK, 8PSK, 16APSK o 32APSK en los retransmisores.
DVB-C (VHF/UHF) utiliza QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM o 256-QAM (64-QAM
en general)
DVB-T (VHF/UHF) 16-QAM o 64-QAM (o QPSK) en combinación con COFDM
SFN
‘Single frequency network’
Distribución de subportadoras piloto como marcadores de sincronismo
Transformada del Coseno Discreta
DVB-S is an abbreviation for Digital Video Broadcasting — Satellite; it is the original Digital Video
Broadcasting Forward error correction and demodulation standard for Satellite Television and dates
from 1994, in its first release, while development lasted from 1993 to 1997. The first application was
commercially available in France via Canal+, enabling digitally broadcast, satellite-delivered Television
to the public.
It is used via satellites serving every continent of the world. DVB-S is used in both Multiple Channel Per
Carrier (MCPC) and Single channel per carrier modes for Broadcast Network feeds as well as for Direct
Broadcast Satellite services like Sky (UK & Ireland) via Astra in Europe, Dish Network and Globecast in
the U.S. and Bell TV in Canada.
While the actual DVB-S standard only specifies physical link characteristics and framing, the overlaid
transport stream delivered by DVB-S is mandated as MPEG-2, known as MPEG transport stream
(MPEG-TS).
Some amateur television repeaters also use this mode in the 1.2 GHz amateur band.
Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is a digital television broadcast standard that has
been designed as a successor for the popular DVB-S system. It was developed in 2003 by the DVB Project, an
international industry consortium, and ratified by ETSI (EN 302307) in March 2005. The standard is based on, and
improves upon DVB-S and the electronic news-gathering (or Digital Satellite News Gathering) system, used by mobile
units for sending sounds and images from remote locations world-wide back to their home television stations.
DVB-S2 is envisaged for broadcast services including standard and HDTV, interactive services including Internet access,
and (professional) data content distribution. The development of DVB-S2 coincided with the introduction of HDTV and
H.264 (MPEG-4 AVC) video codecs.
Two new key features that were added compared to the DVB-S standard are:
A powerful coding scheme based on a modern LDPC code.
VCM (Variable Coding and Modulation) and ACM (Adaptive Coding and Modulation) modes, which allow optimizing
bandwidth utilization by dynamically changing transmission parameters.
Other features include enhanced modulation schemes up to 32APSK, additional code rates, and the introduction of a
generic transport mechanism for IP packet data including MPEG-4 audio–video streams, while supporting backward
compatibility with existing MPEG-2 TS based transmission.
DVB-S2 achieves a significantly better performance than its predecessors – mainly allowing for an increase of available
bitrate over the same satellite transponder bandwidth. The measured DVB-S2 performance gain over DVB-S is around
30% at the same satellite transponder bandwidth and emitted signal power. When the contribution of improvements in
video compression is added, an (MPEG-4 AVC) HDTV service can now be delivered in the same bandwidth that
supported an early DVB-S based MPEG-2 SDTV service only a decade before.
Basics of DVB-T
Rather than carrying the data on a single radio frequency (RF) carrier, OFDM works by splitting the digital data stream into a large number of
slower digital streams, each of which digitally modulate a set of closely spaced adjacent 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 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, Trinidad and Tobago and the Philippines 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 (European Telecommunications Standards Institute) 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,[3] the ETSI E-Book and Scandivia NorDig.
DVB-T has been further developed into newer standards such as DVB-H (Handheld), now in operation, and DVB-T2, which was recently finalised.
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.