DE VELO PMENT INFOTAINMENT
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DVB-T2 Reception in Vehicles
AUTHORS
With television in HD quality, more robust signal transmission and an expanded range of channels, German TV audiences will soon be able to enjoy the advantages of “Digital Video Broadcasting – Second Generation Terrestrial” (DVB-T2 for short). After an initial test phase in spring 2016, normal
Steffen Lang works in the Product Management at the Hirschmann Car Communication GmbH in Neckartenzlingen (Germany).
DVB-T2 operation will gradually replace the existing DVB-T standard. The changeover in the larger conurbation areas will be followed, over the course of 2017, by the changeover in the medium-sized conurbation areas. Extensive coverage should largely be achieved in 2019. Similar to some neighbouring countries, private channels will be encrypted based on CI+
Christian Schwarz is responsable for Standards & Technology at the Hirschmann Car Communication GmbH in Neckartenzlingen (Germany).
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(Common Interface Plus). For the original equipment of current new vehicles, this means that the TV tuners used must be DVB-T- and DVB-T2-capable in order to ensure a seamless transition, as Hirschmann describes.
NEW GENERATION
In comparison to its predecessor, DVB-T2 is characterised above all by more efficient utilisation of the given bandwidth, i.e. a higher data transmission rate (Mbits/second). The changeover to the new video codec HEVC (H.265) will take place at the same time as the introduction of DVB-T2 in Germany. Overall, viewers will be able to benefit from better picture quality and a wider selection of channels. For the original equipment of current new vehicles, this means that the TV tuners used must be DVB-T- and DVB-T2-capable in order to ensure a seamless transition. With precisely this objective in mind, Hirschmann already started working on the development of a DVB-T2 receiver three years ago. This has been fully integrated as original equipment in the vehicle’s operating environment. The TV receiver is also operated via the familiar user interface of the infotainment system. In principle, the way to select broadcasting stations or display the electronic programme guide (EPG) is similar to using a set-top box in your living room. Depending on the OEM requirements, a diversity system with up to 4 receiving antennas can be implemented, TABLE 1. DVB-T2 CHALLENGES
A number of challenges concerning DVBT2 reception in vehicles had to be solved during the development of this TV receiver. These ranged from the mobile, robust reception to the development of processors suitable for use in motor vehicles and the integration of CI+. Compared with DVB-T, DVB-T2 is significantly better in terms of robustness and the available data rate. These two
FIGURE 1 Increase in reception stability due to number of antennas in the diversity system (© Hirschmann)
performance features are not congruent, however, as the available data rate declines with maximum robustness and vice versa. A look at the existing DVB-T2 broadcasts in the different countries shows that the selected DVB-T2 configuration usually favours a high data rate. Besides the fading and multipath reception effects, the Doppler effect is particularly noticeable in the case of mobile reception. With increasing speed – and the associated increasing Doppler effect – the C/N required for flawless reception increases significantly. With an increasing number of receiving antennas and suitable diversity algorithms, it is also possible to achieve C/N values for DVBT2 that ensure stable reception even at higher speeds, FIGURE 1. PROCESSORS SUITABLE FOR T V RECEPTION IN MOTOR VEHICLES
New standards often necessitate new hardware. This applies for DVB-T2 as
well. For example, DVB-T2 increases the modulation complexity (256 QAM, 32k FFT) and the video decoder must support the new codec HEVC. The core functions of any TV tuner are the tuning, demodulation and decoding of video and audio. Even though high-performance, specialised modules are available from entertainment electronics, they are not suitable for the vehicle environment. For this reason, Hirschmann has – in close cooperation with the respective semiconductor manufacturer – developed two chips to the AEC-Q qualification standard that are suitable for use in vehicles. The channel decoder, which processes RF signals in a frequency-selective manner and converts them into a digital transport stream (TS), comes from an Israeli chip manufacturer. The core of the TV tuner is formed by an SoC (system on a chip) from Fujitsu that supports all relevant codecs (video: HEVC, H.264, MPEG-2, AVS; audio: MPEG, HE-AAC, Dolby) and on which the main application runs in a 4-core processor.
DVB-T
DVB-T2
NEW HEVC CODEC
FEC
Convolutional coding and Reed-Solomon codes
LDPC + BCH
FFT size
2k, 8k
1k, 2k, 4k, 8k, 16k, 32k
Bandwidth
6, 7, 8 MHz
1.7, 5, 6, 7, 8, 10 MHz
Standard data rate (UK)
24 Mbit/s
40 Mbit/s
Max. data rate (@20 dB C/N)
29 Mbit/s
47.8 Mbit/s
HEVC compresses the video data even more efficiently than all previous codecs. In comparison to H.264, the data volume can be reduced by around half. Even though HEVC was only officially standardised in 2013, Hirschmann already integrated this latest video codec into a fit-for-production TV tuner in 2015.
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TABLE 1 DVB-T and DVB-T2 characteristics
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DE VELO PMENT INFOTAINMENT
FIGURE 2 Switchable filters for suppressing LTE signals (© Hirschmann)
To date, solely the standard resolution (SD) with a maximum of 720x576 pixels is transmitted via DVB-T in Germany. The German TV stations ZDF and RTL have announced that they will broadcast their channels via DVB-T2 in full HD resolution with 1080 p/50 (1920x1080 pixels). This is a higher resolution than the HDTV that is nowadays transmitted via satellite. The higher transmission capacity of DVB-T2 as well as the increased coding efficiency of the HEVC video codec allow a greater number of channels to be offered – up to seven channels per frequency multiplex – and higher picture quality. DIGITAL DIVIDEND REQUIRES SWITCHABLE FILTERS
With Digital Dividend II, which was introduced by the German Federal Network Agency, the 700–800 MHz frequency range will now also be assigned to the LTE mobile communication network providers. Thus only a reduced frequency range is available for the introduction of DVB-T2 digital terrestrial television in Germany. For TV receivers, this means that – depending on the point in time and country – they can use this frequency range for DVB-T/T2 reception or they must suppress it as far as possible. For this reason, the Hirschmann TV receiver is equipped with software-controlled filters that can be switched for each antenna path. The automotive manufacturer is responsible for determining the switching criteria, FIGURE 2. ENCRYPTED CONTENT VIA DVB-T2
As things stand at the moment, the public service broadcasters in Germany will be free-to-air, while the private broadcasters will encrypt their content. The encrypted content can be viewed by purchasing a CAM (Conditional Access Module) from the corresponding provider. In comparison: All DVB-T2
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FIGURE 3 CA module with protruding CI slot (© Hirschmann)
channels are encrypted in Austria – whereas all channels are free-to-air in England. The CAM is connected to the TV receiver via the standardised “Common Interface Plus” interface, CI+ for short, FIGURE 3. The challenge for automotive manufacturers now lies in positioning the receiver, or rather the CAM unit, so that it is accessible for end customers and this in multiple model series. For this reason, Hirschmann offers both a CAM unit integrated in the TV receiver and an external CAM unit that is connected to the receiver via USB. This makes it easier for the OEMs to find the necessary installation space for both devices, as solely the CAM unit has to be accessible for customers. Another advantage: The CAM reader is now optional and does not have to be ordered in countries such as England if not desired. It must also be taken into account that the HD channel content is not allowed to be further distributed in the vehicle after decryption. The TV receiver usually communicates with a head unit, which manages all audio and video sources and actuates the display. The data decrypted by the CAM is locally encrypted on its way through the vehicle and is thus protected against possible illegal access. The encryption methods used depend on the bus system used. DTCP is specified for Most, while HDCP is used for automotive Ethernet.
FIGURE 4 Database and distribution point for recorded reception signals in the Hirschmann laboratory (© Hirschmann)
TESTING IN THE LABORATORY AND ON THE ROAD
FIGURE 5 Type and number of integrated antennas using the example of a rear window (© Hirschmann)
The most important prerequisite for the reception tests of the TV receiver in the laboratory is a radio-frequency input signal that corresponds to the signal transmitted by TV towers. However, with the exception of test broadcasts in Munich and Berlin, DVB-T2 is not yet off the ground in Germany. Hirschmann has been developing TV tuners for the different TV standards for over ten years, and these tuners are used worldwide. Besides DVB-T for Europe, this includes ISDB-T for Japan and T-DMB for South Korea, for example. As part of this, an extensive database and signal distribution have been created with which real signal combinations that exist in the field can be simulated in the laboratory. In the case of DVB-T2, the database was supplied with information from different sources, FIGURE 4. On the one hand with recordings from the DVB-T2 field tests in Germany. On the other hand with recordings from countries that already broadcast DVB-T2 in regular operation. Using this data base, Hirschmann was able to test the DVBT2 TV receiver at an early stage in the laboratory before time-consuming test drives were necessary in the field. Test drives in different countries are, nevertheless, necessary and helpful, as it is only there that the real transmission and reception conditions exist, as experienced by the end customers in their vehicles. Test drives in countries such as Italy, Austria, Russia and Great Britain allowed Hirschmann to successfully test mobile DVB-T2 reception with live signals. The tests also involved checking that the various codec variants of each country decoded the signals correctly. Country-specific platform models also need to be taken into account and/or integrated, such as the simpliTV channel package in Austria, which is broadcast in encrypted format. Tests with CAMs from leading manufacturers confirmed the functional capability. 05I2015
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STABLE T V RECEPTION IN VEHICLES
Multiple antennas and multiple reception paths (individual tuners) are required so that in-car TV can become a real joy to use for end users. A diversity system based on MRC (Maximum Ratio Combining) with two to three reception paths is used for stable reception. The number of antennas and their configuration are important factors for optimum reception quality. For effective diversity reception, the antennas should be decorrelated to the maximum extent possible, i.e. differ as much as possible in terms of their reception properties (directional pattern). Integrated window antennas are usually used due to the specifications of the automotive manufacturers regarding the design and installation space. Only the rear window is usually available for this purpose in saloon cars. Sufficiently decorrelated antennas can, however, be achieved even under these conditions via a suitable antenna design, FIGURE 5. The permanent reception of numerous TV channels and seamless zapping between them is enabled by means of separate tuners that continuously scan the current channel landscape via a background search. Two or three tuners receive the current channel, while one or two more check in the background to see which alternatives are available and generate the channel list for the selection menu via the user interface (UI). This ensures on the one hand that an up-todate list of the channels available at the
respective reception located is always displayed even during a longer journey. On the other hand, this also facilitates automatic broadcasting station tracking. Once selected, a channel can be tracked during the entire journey, as the TV receiver automatically selects the channel’s applicable reception frequency in each case. BUS CONNECTION ACCORDING TO OEM SPECIFICATIONS
The TV receiver must, of course, also comply with the stipulations from the requirement specifications of the automotive manufacturer. An important point in this connection is the bus system via which the TV receiver is connected. A large bandwidth for the signal distribution in the vehicle is required in order to maintain the HD picture quality of the channels transmitted via DVB-T2. There are currently two different approaches here. Both MOST150 with the Video-over-MOST feature and automotive Ethernet are suited for this purpose. Hirschmann is already using both systems and can implement them for a vehicle platform in accordance with the OEM’s requirements. The further development steps will be shaped by the widespread introduction of LTE; something that can enable TV reception by means of online streaming. However, in-car DVB-T2 will be affected by this in the medium term rather than the short term, as LTE is not yet universally available and streaming at higher data rates is currently still too expensive for end customers.
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