Issues Receiving Digital TV in Moving Vehicle

[DylanMadigan]

Long story short my van always had a TV in it that worked fine on the go when everything was analog. Since the digital shift and getting a digital TV in there, I can pick up TV stations just fine (including one just down the road), but when I get over say 5-10MPH I tend to lose the signal almost completely.

When I search around, all I find is that the signal is being blocked as I drive. I know that's not it. Only thing I can think of is the doppler effect but there's no way that can be so significant at such a slow speed right?

 

[dlwtrunked]

I have directly measured Doppler shift on digital TV in a moving car. It can easily be seen with the right settings (zooming in on an AirSpy or SDRplay. At 60 mph, it will be about 1.1 kHz (so at 10 mph will be about 110 Hz). However, due to reflections around you, it will primarily appear as a broadening of the TV pilot (on an ATSC -1 signal, ATSC-3 signals do not have such a TV pilot). I am actually current putting together a pdf talk on this and similar effects.

The attached was done with an expensive spectrum analyzer with its oscilator GPS disciplined so that I could precisely measure the frequenies (Note: Although the AirSpy and some SDRplay models can be GPS disciplined for stability, they tune even then in step, and though stable, are not accurate.)

 

[dlwtrunked]

I have directly measured Doppler shift on digital TV in a moving car. It can easily be seen with the right settings (zooming in on an AirSpy or SDRplay. At 60 mph, it will be about 1.1 kHz (so at 10 mph will be about 110 Hz). However, due to reflections around you, it will primarily appear as a broadening of the TV pilot (on an ATSC -1 signal, ATSC-3 signals do not have such a TV pilot). I am actually current putting together a pdf talk on this and similar effects.

The attached was done with an expensive spectrum analyzer with its oscilator GPS disciplined so that I could precisely measure the frequenies (Note: Although the AirSpy and some SDRplay models can be GPS disciplined for stability, they tune even then in step, and though stable, are not accurate.)

For those wanting a math exercise, my speed in the moving (first image) can be found by calculating (BW/freq)*c/2. where BW is the bandwidth that the original pilot now shows in the moving spectral capture of peak signal levels. (Hint: BW is about 115 Hz and the speed of light is about 186,282 miles per second--be sure to convert your answer to miles per hour.)

 

[dlwtrunked]

Also, here is a paper discussing remedying the problem of mobile digital TV reception HDTV Mobile Reception in Automobiles
but you have to pay to get a copy.

 

[gmclam]

ATSC 1.0 (8VSB) was not designed with mobile reception in mind. Out of the gate, TV has been traditionally horizontally polarized while mobile receivers are usually set up with vertical antennas. But it does go way beyond that. Making mobile reception robust is one of the things corrected in ATSC 3.0. But they've added DRM and a host of new things that make 3.0 less desirable to most.