Picture perfect

Picture perfectGood digital terrestrial TV reception requires good receiver design says Oak Technology’s Peter Claydon
Design of consumer quality receivers for the recently introduced digital terrestrial television (DTTV) services requires a good understanding of the transmission channel to achieve maximal coverage area. Published studies in the UHF and VHF frequency ranges have provided descriptions of the multipath nature of the terrestrial channel, but have not investigated in detail the dynamics of the channel. Indeed, the European DVB-T standard provides a channel model consisting of 20 echo paths with no time variation of echo parameters. There is evidence from experience of operating wireless local loop telephone systems that the parameters of the echo paths vary considerably with time, albeit at frequencies in the bands above UHF.
A receiver designed assuming no time variation of the echo paths will perform poorly and give a greatly reduced coverage area compared to one that can adapt to the variations in the channel. This is the situation that exists with all current receivers for the 8-VSB modulation scheme used in the US ATSC standard.
Conversely, although the DVB-T standard has been shown to be inherently more tolerant of dynamic channels, few details about the nature of channel dynamics have been published, which has made the design of DVB-T receivers somewhat hit-and-miss.
The main areas in the design of DTTV receivers that are affected by the channel propagation characteristics are synchronisation and, particularly, channel equalisation. The channel model developed at Oak Technology allowed a highly optimised DVB-T architecture to be developed, which has since been proven in silicon.
It has been found that channel equalisers, which assume the amplitude and phase of multipath echoes are constants, with no time variation, perform inadequately for commercial use.
Using information from a field survey (see panel), it has been possible to develop algorithms, particularly for equalisers, which allow adaptation on a short time scale to variations in the channel. These algorithms give considerable improvement in decoding performance when challenged with off-air captured data.
Measurements have been made from off-air captured data of DTTV transmissions that clearly demonstrated time variation of individual channel echo paths. The mechanisms for this variation are most likely to be due to motion of reflectors in the transmission path.
The results of the survey have been used to optimise the receiver algorithms, particularly for channel equalisation. The expectation is that this optimisation will result in DTTV receivers that are capable of providing a high level of signal coverage.
Peter Claydon is from Oak Technology email: petec@oaktech.com DTT receiver design puts theory into practice
Oak Technology in Bristol has first silicon of its single-chip demodulator design for DVB-T markets.
A conventional design approach would have been to start with a system solution and then to create a prototype using FPGA’s, microprocessors, memory and discrete logic. This prototype would then be tested in the lab and in the field.
The major disadvantage of this approach is that it does not directly yield information about the nature of the terrestrial broadcast channel. All it would tell us is how well the prototype performs in this channel, leading to a hit-and-miss design process.
The alternative design approach taken at Oak was to develop a model of the channel based on detailed information gathered from an extensive field survey. This model was then used in simulations which allowed an optimised architecture to be created, prior to silicon implementation.
To make measurements in the field, a large off-road vehicle was used with a 10m pump-up mast fitted with a typical domestic 12-element Yagi antenna. An electronic data capture system was developed which allowed storage of samples from a 12-bit analogue-to-digital converter whose input was the intermediate frequency of a DTT tuner module.
Data was captured initially from DTT test transmission, but more recently from public DTT transmissions at more than 50 locations around London, Bristol and Bath. At each location a separate survey was performed, which allowed us to account for differences in reception around a local area and at different heights.
The captured data was processed using a software demodulator which allowed decoding of the DVB-T signal and extraction of the channel impulse response as a function of time.
The results of the field survey showed that despite the use of a highly directional antenna, most sites exhibited a significant level of multipath. At some sites the channel showed considerable time variation.
These included sites where there was not at first sight any obvious cause of the time variation, such as passing vehicles.

Leave a Reply

Your email address will not be published. Required fields are marked *