Digital TV (DTV), getting better but a ways to go.

Analog guru Paul Grohe sent a nice link to pictures of the rework at Sutro Tower in San Francisco. Now that the digital television (DTV) transition has taken place and broadcasters don’t have to broadcast both analog and digital signals, they are swapping the digital antennas into the higher locations and boosting power where the FCC lets them. The DTV transition was not as smooth as some hoped and not the disaster some predicted. Like the entire planet, it was analog, some good and some bad. I rescanned my TVs and was amazed to get KRCB channel 22 from Rohnert Park, a good 90 miles from Sunnyvale. Of course the station does not always come in—I cannot generalize when I can receive it and when I get too weak a signal, so I am not sure if it is interference or if they lower their power some times of day. I am lucky to get a nice sight-line across San Francisco Bay, so that explains why I can pull in this station.

Anyone watching a nice (low-compression) high-definition broadcast has to appreciate the good of DTV. However, if your house is in an RF shaded zone, or if you live is a stucco house and are trying to use an indoor antenna, DTV just does not work. We in the Bay Area hope that things will improve at the end of the month when the Sutro Tower refit is complete and power levels are boosted.

The other bad thing about DTV is the interference. Almost every station I watch will have the occasional one-second drop-out, or the video will get all blocky and pixilated once or twice a night. This is with a large outdoor antenna mounted on a boom with a rotator. I guess it will be like cell phones, where young people actually think it is acceptable that you..pho…drop…and…hear…..huh…… Me, I prefer a nice twisted-pair landline current loop. I will concede that VOIP is almost as good but you have to be lucky enough to get a good route so they don’t drop any packets on you.

So I guess we will get used to crappy phones and crappy TVs, just like we got used to crappy American cars. This will help us transition to a century where we can obey our new Chinese or Indian overlords. As goes Egypt, Greece, Rome, and England, so goes the USA.

Real-life TV transmitter engineer Robert Getsla sent me a note and a link:

We are now three months past the DTV transition, and all is not well in the TV broadcasting world.  The following is a link to an ABC News story about how well the transition has worked.  It seems the propagation advantages VHF has enjoyed over UHF have been removed, and many of the TV stations that previously enjoyed the advantages of a VHF channel for analog transmission, are now looking for ways to relocate into the diminished UHF band. It seems small indoor antennas simply do not work well enough for VHF DTV signals, but they are good enough for most UHF DTV signals. 

I wonder when the FCC engineering folks will decide a Federal pre-emptive exemption is needed so everyone can install an outdoor TV antenna, regardless of zoning laws, covenants in deeds, or "association" rules that would prohibit their installation. [Paul’s note: the FFC already protects small antennas.]

It is amusing to see the poor treatment of the VHF bands— At fist the FCC was going to kick everyone off channels 2-13 since the cell phone companies were voraciously trying to consume all the bandwidth that their brown-envelope lobbyists could grab. They had heard about the superior transmission properties of low-frequency signals. Then some braniac realized that channel 2 is 54 MHz and you need a 10-foot antenna to receive it. Since that kind of antenna is unsuitable for mobile applications, where the cell phone companies have inured us to rapacious rates, the cell phone companies no longer had any interest. So the last I heard, the FCC moved all the lower VHF channels, 2-6, the ones under the FM radio band, but they did convert some higher VHF stations to digital. Robert Getsla explained to me that it is much cheaper to run a VHF station, the equipment and electricity cost is much less than a UHF station, but since many consumer TV antenna companies sold “digital TV antennas” that were only UHF, this made receiving VHF signals problematic, even if they do have better propagation characteristics. So now those staid old big-network VHF stations are begging the FCC to put them up in the UHF band where there is some chance of people receiving the stations with a crappy antenna. I love the way consumer antenna companies charged an arm and a leg for these DTV antennas and an old-fashioned loop on the back of your set would work just as well. Before I got my outdoor setup, Paul Grohe showed me how an 18-inch test lead would work as a great antenna for UHF. He just used the alligator clip to grab the center conductor of the coax.

Once of these days I will get my Chilean buddy Ricardo over to the pizza parlor and get a full tutorial for you folks on TV antenna design. Engineers in Chile take broadcast radio and TV seriously and the mountainous terrain provides some real challenges. The quick overview I got from him last month is that you can make your own dipole or Yagi, and that it might be best to have three antennas— upper VHF, UHF, and FM radio. He had all kinds of neat tricks to use stubs to make traps for FM and such. Since these are all home-brew tricks that don’t cost a lot of money, you won’t see them in commercial antenna products. Those outfits want to sell you fancy gizmos with RF “boosters”.

My hat’s off to Channel Master for explaining on their website that beam-head RF amplifiers will not pull in weak signals, they will only make up for losses in long antenna runs. This is because the RF transistor in your TV has all the gain it needs to receive weak signals. If you add a booster amp, all you do is raise the noise floor and bury the weak station deeper “in the grass”. If, however, you have a 50-foot coax, then the attenuation in that cable will drop the signal below the threshold of your TV tuner.

One caveat is to not over-amplify. My buddy Alan got good results with a booster, but then he rotated the antenna away from the strong stations. The booster made up for the drop-off in the strong stations and he could then take advantage of the polar antenna pattern to point his antenna at the weak stations so he could pull them in.

Of course, the hardware folks will come to the rescue again. I got a briefing last month from Silicon Labs on their new Si2170 CMOS TV tuner. This part seems too good to be true. First off it is a monolithic chip, and they claim to have the best sensitivity and selectivity in the market. That was hard to take, especially since I am a fan of silicon-germanium and I always thought that you needed SiGe to get the best noise performance and SNR. The folks at SiLabs assure me that the Si2170 outperforms hybrid tuners with SiGe front ends. I guess I can believe this after seeing Analog Devices do an ultrasound receiver chip all in CMOS. Apparently, if the frequencies are low enough, like ultrasound and TV, you can make up for the poor noise of CMOS by making the transistors bigger (1/f noise) and running more current through them (flatband noise). The bigger transistors have way more capacitance, but if the signals are slow enough, the transistors still will have enough gain at those frequencies to do some good.

Now I wonder if the SiGe hybrid tuner folks can’t use even bigger SiGe transistors and more emitter current-density to improve the performance of their chips. Well, that is the marvel of the invisible hand of capitalism. If there is some way to beat the new Silicon Labs chip, I am sure some clever engineers will find a way in the coming years.