NAB RF Reflections: A-VSB and DTx
June 14, 2006
At NAB this year, walking around booths displaying products on video-over-IP and video to handheld devices, it was easy to forget that some people still watch TV from high-power transmitters.
Of course, there were new products and technologies being shown for free TV broadcasting. This month I'll focus on three of the technologies I found most interesting at this year's NAB.
Rohde & Schwarz and Samsung had an impressive display showing the capabilities of the proposed A-VSB enhancement to the ATSC standard. In the simplest terms, the improved reception demonstrated at the DTV Hot Spot comes from the addition of special reference signal to the 8-VSB signal to make it easier for the Samsung receiver to lock to the VSB signal and provide a more robust data stream.
The demonstration setup used two of the new Samsung Gemini ATSC receivers and the Rohde & Schwarz patented Maximum Ratio Combining technology to improve reception.
In conventional ATSC exciters, a trellis coder is used to map forward error-corrected data into the 8-VSB "constellation." The exciter randomly selects which packet in the input transport stream starts the data field from the trellis coder.
In an A-VSB exciter, the data randomizer, the data interleaver and the trellis coder are all locked to the start of the data field. Providing this level of control over the transmitted ATSC signal allows A-VSB systems to provide virtual training signals to the equalizer to improve dynamic multipath tracking, and it allows the use of layered (hierarchical) modulation.
It also allows what Rohde & Schwarz calls "Deterministic Frame Intelligent TDM" and frame slicing for new applications.
TDM (time division multiplex) and frame slicing are essential if very small handheld devices are used to receive ATSC signals. Current ATSC USB tuners stress the output current capability of my notebook's USB 2.0 port. Power consumption has to be reduced dramatically before ATSC receivers can be built into cell phones.
DVB-H and Qualcomm's MediaFLO use TDM to allow the receiver to turn on, grab a big chunk of data, and then go to sleep. A-VSB not only gives the ATSC system TDM and frame slicing capability, its virtual reference signal provides a way for the receiver's equalizer to lock on to the signal fast and preserve precious receiver power.
The performance of the system under the rapidly changing multipath conditions present during mobile reception (the demonstration simulated reception at speeds over 120 mph), the potential for use of TDM, and use of known symbol patterns to improve smart antenna operation and diversity reception have led some to say A-VSB could allow the ATSC standard to compete with DVB-T and DVB-H for transmission of programming and data to portable, mobile and handheld devices.
While there is still a lot of work to be done on the standard and designing affordable equipment to implement it, I feel the support of a consumer electronics company as large as Samsung gives this technology a better chance of succeeding in the marketplace than E-VSB. Support from broadcasters, of course, will be essential.
Although the A-VSB proposal has been accepted by the ATSC, it is still being refined and has not reached the candidate standard stage. The Rohde & Schwarz and Samsung representatives at the display were actively encouraging companies interested in the technology to participate in the ATSC process and suggest improvements.
Ideally, other consumer electronics companies and chip manufacturers will participate in the process and give U.S. broadcasters many of the capabilities broadcasters in countries using COFDM-based systems (ISDB-T, DVB-T, DVB-H) have when targeting mobile, portable and even handheld TV receivers.
I'll cover A-VSB signal generation, multiplexing, and bandwidth/performance trade-offs in a future column.
While distributed transmission systems based on ATSC Standard A/110 can be effective in providing the strong DTV signals needed for ATSC reception with indoor antennas and portable devices, many broadcasters would be happy with a simpler solution for improving signals in small areas blocked by terrain or buildings. At NAB, I saw two systems that could provide that solution.
K-Tech introduced an on-channel booster with minimal throughput delay a few years ago. These boosters can work well where there is enough isolation between the receive and transmit antennas. However, this isn't practical in many locations. At NAB this year, Steve Kuh explained he was able to create a synchronized transmission system using his new 8-VSB modulators and microwave links with much less complexity than an A/110 system. As currently designed, the system does not have the ability to adjust the timing at each of the transmission sites, but Steve said that shouldn't be difficult to add. Without the ability to adjust timing, the system has to be designed to avoid creating new dead spots where the time delay between signals from two or more transmitters is more than the receiver's equalizer can handle, and the relative signal strengths from the transmitters are close enough to prevent reception. The microwave distribution system will add delay, which could help or hurt the situation. This system should work well for filling in coverage in isolated communities where the booster or synchronized transmitter can be located in line with the main transmitter.
The simplest system I saw was from HiWave. Using technology developed at ETRI, this EDOCR (Equalization Digital On Channel Repeater) eliminates many of the problems seen with conventional on-channel repeaters. Specifically, it includes an equalizer that suppresses the intersymbol interference in the received signal, minimizing the feedback effect created when the receiver picks up the signal transmitted from the repeater.
The system is designed to keep processing time under 5 µs, which makes it ideal for improving signals in shadowed areas and perhaps even urban environments. One drawback of any ATSC transmitter system using multiple on-channel transmitters is that older receivers will have difficulty receiving the DTV signal in areas where the EDOCR signal is within 20 dB of the main transmitter's signal, and the weaker signal is received more than a few microseconds earlier in time than the dominant signal.
Axcera demonstrated the latest version of its DTV exciter and distributed transmission system equipment at the ATSC Hot Spot. However, the feature that interested me most about Axcera's new DTV exciter was what I'll call a SMPTE 310M time base corrector.
Broadcasters are starting to recognize that an unstable transport stream clock rate can cause problems with some receivers. The potential for problems with distributed transmission systems should be obvious. Some ASI-to-SMPTE-310M converters, while generating the correct frequency when averaged over a long time period, have been found to slowly oscillate above and below this frequency. I've also seen cases where the frequency drifts slowly, then suddenly snaps back to the correct frequency.
The new Axcera Axciter is able to correct for drift in the SMPTE 310M clock rate whether generated at the studio, microwave or fiber system or in a converter at the input to the Axciter. I suggested that given the extent of this problem, Axcera produce a standalone SMPTE 310M corrector for stations without Axcera ATSC modulators.
Next month's RF Technology column will focus on towers. Rearranging antennas after analog transmission ends won't be easy!
Your comments and questions on any RF topic are always welcome. Drop me an e-mail at [email protected].