In the future, Hirschmann plans to integrate other digital standards, such as ISDB T (Japan) and ATSC (USA) into its systems. Hirschmann is also exploring the concept of an «antenna base receiver," in which the receiver is located at the base of the antenna. This arrangement makes it possible to eliminate RF lines and amplifiers, and promises to bring further improvement to the reception quality. By shifting from hardware to software, a receiver is to be developed that can be configured for the various broadcast services via software. Such a software radio is based on a uniform hardware platform upon which a whole series of receiver variants can be adjusted, configured, and used by software across a wide range of frequencies. The software radio concept calls for scanning of the broadcast signal directly at the antenna, and then performing all further processing in the digital domain. At present, it is not yet possible to realize a software radio based on this definition, because the required system processing performance and, above all, the very high sampling frequencies needed for the ADC are not yet affordable. Nevertheless, a subset of software radios that can operate with as few intermediate frequencies as possible can be implemented. Such radios are likely to find their way into new car infotainment systems.

Emerging mobile TV

The original transmitter planning for analogue television was intended for stationary home reception, where clear reception was achievable using directional antennas at roof-top height. Antennas used for TV reception in automobiles, however, are usually mounted at much lower levels. Not only is it impossible to build antennas with the same elevation, but directional antennas cannot be used in such applications because the direction from which the transmission signal is received continuously varies as the vehicle travels down the road. In addition, the signals that are received become superimposed due to scattering and reflections from buildings and other vehicles. This results in a heavy signal loss (Rayleigh amplitude distribution). The first DVB T network in Germany (Berlin) is designed for portable reception, but not for mobile reception. As a result, ensuring acceptable in-vehicle TV reception requires «diversity" systems using several antennas. With «scan diversity," for example, the system switches between the various antennas it has at its disposal, selecting the antenna that delivers the highest reception level. For analogue television, such switching is done separately for video and voice since the carrier waves for each are spaced more than 5MHz apart and will suffer interference from different sources. Hirschmann's receiver features an improved proprietary procedure for analogue television that constructively superimposes the received signals.The signals are first scanned and then synchronized for time alignment. The quality of each individual signal is then assessed. Using this assessment, «weighting factors" are determined. These weighting factors will be used to multiply each of the received signals that are being received. The resulting sum of signals produced by this procedure delivers a better signal-to-noise ratio than is possible using only the best signal received. MRC (Maximum Ratio Combining) diversity is used for DVB T receiver antennas. Usising this approach, the output signal is formed by combining the best individual carrier signals, making it possible to ensure good reception in automotive TVs, even when the vehicle is moving at speeds over 100km/h. Frequency diversity works independently of antenna diversity: During reception, the Hirschmann hybrid TV receiver scans the transmitter landscape without requiring any action on the part of the user. If a channel is found with the same content but better reception quality, the device automatically shifts to this new channel, enabling users to watch a program continuously throughout the trip without having to make manual changes as the vehicle moves beyond the range of a particular transmission station.

Design options

Hirschmann first investigated the option of developing a mixed-signal ASIC for fulfilling the needs of the hybrid TV receiver. For the projected volume of an automotive TV receiver, an ASIC would provide the best long-term cost structure. However, designing such a large mixed-signal device in an ASIC would have involved non-recurring engineering in the millions of dollars, in addition to the development time involved designing the device from the ground up. Even after that time and expense there still would have been no absolute certainty until first silicon that the device would actually function as intended.By contrast, there are low-cost FPGAs and advanced development tools available today that make it possible to develop and implement the desired functionality in less time, with the ability to easily test the performance of the design without having to wait six to eight weeks for first silicon to appear, positively impacting time to market. What's more, with the availability of mature microprocessor intellectual property for FPGAs to deliver advanced control functions, the FPGA implementation becomes even more attractive.

Receiver architecture

The block circuit diagram of figure 2 shows an example of the Hirschmann hybrid TV receiver using three reception paths. Although the system is scalable in terms of the number of reception paths possible, two to four paths are generally recommended. The input signal used by the tuner is an RF signal received from integrated antennas, generally disk antennas connected to individual tuners via controlled amplifiers. The tuners are hybrids tuners configured to receive both analogue signals, regardless of standard, and DVB T signals. For analogue reception, the corresponding demodulators are integrated into the tuner cases. During analogue reception, analogue-to-digital converters scan the analogue video and audio signals and deliver them to the hybrid receiver's Cyclone FPGA device. During DVB-T reception, the Coded Orthogonal Frequency Division Multiplexer (COFDM) demodulators deliver an MPEG transport stream. The individual COFDM demodulators are interconnected, establishing the required MRC diversity, which optimizes the MPEG transport stream. The MPEG decoder supplies the video, audio, and data signals contained in the MPEG transport stream. These signals are then also delivered to the Cyclone device.

DSP functions in FPGA

All the digital signal processing, including antenna diversity of analogue video and sound, baseband sound processing, teletext decoding and other data content such as Video Program System (VPS), managing and storing this information in the connected SDRAM memory, and communicating with the control host via a serial interface, is carried out in an Altera Cyclone EP1C12 FPGA. This low cost device offers up to 234Kbits of embedded memory, distributed in multiple 4,608-bit memory blocks, each supporting various configurations such as dual-port and single-port RAM, ROM, and FIFO buffers. Hence the RAM blocks are also used for functions such as FIFO storage. The designers of Hirschmann's diversity receiver leveraged Altera's development tools to create complex units required to support system functions. The finite impulse response (FIR) filters, for example, were automatically generated using the Altera's FIR compiler, and a controller system was produced using the SOPC Builder tool. This controller unit consists of the Nios embedded processor as well as RAM, a serial interface, and an SDRAM controller. The FPGA's configuration data is located in the EPCS4 serial flash memory and is loaded onto the FPGA when the receiver is switched on. The graphical data processed in the Cyclone device is sent to a DAC which creates the analogue output signal shown on TV monitor. The audio signals from the Cyclone device are transferred digitally to a Media Orientated Systems Transport (MOST) transceiver, which sends them to the MOST bus. When a user enters control instructions via an MMI, the instructions are also sent to the transceiver via the MOST bus. The host microcontroller communicates with the receiver's configurable elements to manage all the system's control functions. Even though this article illustrates the implementation of one specific automotive application, the capabilities embedded in the Cyclone devices, combined with System-on-a-Programmable Chip building tools, are e well suited for the implementation of other telematic or entertainment applications (as shown in figure 3).