As I've written before, I felt there was a great need for easy-to-use equipment for monitoring 8-VSB transmissions and ATSC streams. There was no shortage of DTV and 8-VSB test equipment at NAB this year. Prices were low enough. There is now no excuse for not knowing exactly what your station is transmitting on DTV.
I believe many transmitter engineers have a good handle on the RF side of their DTV transmission facilities. Transmitter manufacturers continue to improve adaptive precorrection software in DTV exciters to compensate for system linear and nonlinear distortions.
When the transmitter is installed, filter response checked and antenna return loss measured, the entire system usually has no problem meeting the recommended minimum signal-to-noise ratio (SNR) of 27 dB. Many DTV transmitters now include the ability to monitor DTV transmission characteristics and most of them include a Web interface that displays alarms if something goes outside preset limits.
JITTER
Unfortunately, in the ATSC DTV environment, it is not sufficient to deliver a strong signal. As more people buy sets or set-tops with ATSC tuners, some engineers may be surprised that viewers are having trouble receiving their signal, even when SNR and signal level is equivalent to that of other stations in the market.
In these cases, the problem can often be traced to the SMPTE-310 signal delivered to the transmitter's modulator. I'll discuss this in detail in a future column, but I want to mention one piece of equipment TV stations may want to consider purchasing--a jitter analyzer.
Belar was showing a prototype instrument at NAB that measures the jitter in the DTV symbol clock. The DTV symbol clock analyzer I saw had few adjustments and a single digital readout showing the jitter in nanoseconds. The price hasn't been finalized, but I was told it would be less than $6,000. In early May, I could not find any information about it on www.belar.com Symbol clock jitter will affect different manufacturers' ATSC receivers differently and can be very difficult to diagnose without an instrument like this.
PSIP
Manufacturer-specific problems can also be traced to incorrect PSIP tables. If your station doesn't have an ATSC stream analyzer, check out a system I saw that could be put together for not much more than petty cash and a Windows XP laptop. Even if you already have an ATSC stream analyzer, I suspect you will be interested in a simple portable unit costing less than $400!
The system consists of the V@Box USB-A 3560 ATSC terrestrial receiver combined with Rod Hewitt's excellent TSReader software. Rod demonstrated the system to Gary Sgrignoli and me in a Nextel conference room at NAB, using a ThinkPad T40 laptop and a Silver Sensor antenna. We had no problem picking up the Las Vegas DTV stations and looking at their transport streams and PSIP data, including closed-captioning.
The software identified scrambled USDTV program streams and he was able to grab a file being transmitted on the PBS station's DTV signal.
I'll have more on this inexpensive PSIP monitoring setup in a future column. If you can't wait until then, you can learn more about TSReader at www.coolstf. com/tsreader/. The software works with a range of hardware and can be used to analyze ASI, DVB-T, and DVB-S signals as well as ATSC, depending on the hardware installed in the PC. For information on the V@Box USB ATSC tuner, see www.vboxcomm.com You'll notice they also have PCI cards for DVB-T and ATSC reception on desktop computers. The V@Box cards and USB receiver were being shown at NAB by Optibase (www. optibase.com), and I also found them being sold at www.visiblelight.com
8-VSB
While transmitter adaptive precorrection systems can do a good job making sure an 8-VSB signal is as good as it can be, they aren't of much use if the a problem occurs in the hardware demodulating the RF sample from the transmitter. Most stations will want some signal quality monitoring capability in their master control.
Modulation Sciences (MSI) is well known for its BTSC stereo generator and audio monitoring equipment, as well as its precision NTSC VHF/UHF demodulator.
This year, MSI showed the new MSI-4400 8-VSB analyzer. The MSI-4400 provides basic RF analysis--constellation diagram, eye diagram, signal-to-noise ratio, error vector magnitude and tap equalizer graphs. Alarms can be set on bit error rate and other parameters. It looked like a good system for master control DTV monitoring.
If the master control is unattended, the MSI-4400 can e-mail alarm information. The displays are available to anyone on the network or the Internet via a built-in Web server. Cost is less than $10,000, not counting the PC. Eric Small said MSI is developing a transmitter-oriented version of the box that can display the 8-VSB channel spectrum with enough dynamic range for shoulder measurements, pilot phase and frequency measurements using a GPS reference.
Sencore showed its DTU234 RF Probe, which is capable of receiving VSB, QAM64, QAM256 and NTSC signals. When combined with Sencore RF Expert software, it can show the signal's eye diagram, constellation and channel spectrum. Combined with the stream analysis software Sencore offers, it can also do ATSC/ASI stream analysis. Cost of the complete package, minus the computer, should drop below $4,000.
This only a small sample of the excellent, reasonably priced DTV test equipment shown at NAB. Take a look at the new products from Z Technology, known for signal strength field survey equipment, Pixelmetrix, Thales, Tektronix and others.
ENTICING T&M GEAR
If someone gave me the option to pick a product at NAB I could take home, the Rohde & Schwarz FSH3-TV would have been at the top of my list. The display at the booth was enticing, but many of the features of this handheld wonder weren't revealed until I started playing with it and reviewing the specifications. The FSH3-TV analyzer combines a spectrum analyzer, DTV receiver (4QAM through 256QAM and 8-VSB), analog TV receiver, spectrum analyzer, scalar and vector transmission and reflection analyzer, field-strength meter and tracking generator.
Among the measurements it can handle are carrier-to-noise, symbol rate, error rate, channel power, shoulder attenuation to 80 dB, EMC precompliance--and that's not all. It can also do power measurements when used with Rohde & Schwarz power sensors. Lock it to an external 10 MHz reference and use it as a frequency counter. Data can be exported in ASCII or Microsoft Excel format. The only thing I found it couldn't do was FM modulation measurements. Since the FSH3-TV can demodulate FM, it shouldn't be difficult to add measurement capability, which is present on many digital spectrum analyzers.
I was told an FSH3-TV with a preselector (needed for off-air measurements), and all digital formats costs around $16,000. The transmission/reflection measurement package was quoted at around 3,000 euros.
TDR capability is also an option, as is the "receiver mode." While the upper frequency limit of the analyzer is only 3,000 MHz, this is sufficient to handle the broadcast TV spectrum, the L-band spectrum Crown Castle will be using for their TV-to-handheld transmissions, the satellite radio spectrum and Broadband Radio Services spectrum.
While one of the most expensive products I've described so far, if everything works as advertised, it may be one of the best RF test equipment bargains!
NEXT MONTH
If you receive my weekly RF Report newsletter (see www.tvtechnology.com to sign up), you may have seen my short article on Doug Vernier making shuttle radar topography mission (SRTM) terrain data available for use in his popular V-Soft coverage software. At NAB, I had a chance to see the software in action and compare DTV coverage in the New York City area from two sites using USGS terrain data and the V-Soft version of the USGS SRTM data. I'll have more on this in next month's column, but you may be amazed at the difference in signal strengths and coverage predicted using the SRTM terrain database, which includes the height of buildings and dense forest areas, as well as the standard USGS terrain database based on bare earth. n
As always, comments and questions are always welcome. Drop me a note at dlung@transmitter.com
