In-car Digital Tv Reception

[luuuc]

So I just had a nice new Alpine system installed in my car, with a 7" touch screen. I also got their TUE-T150DV mobile DVBT receiver to go with it.

Very nice it is, too.

Until the car starts moving, that is.

Then the signal drops out anything from regularly to constantly, depending on the channel.

I'm assuming that it isn't the motion itself that is the problem, but the fact that while moving I'm getting variations in signal strength at different locations. Perhaps also the car's electrics might be causing some interference ... I don't know.

The antenna system provided consists of something that looks like a small section of rear window demister. There are two, both mounted near the top of the windscreen (one on the left, one on the right). They normally like to put one of them on the rear of the car, but mine being tinted made that impossible.

I'm wondering if anyone here has any experience with car reception, and could offer suggestions on relatively discreet ways to improve it.

Car is a Ford Focus hatchback, FWIW.

Edited September 24, 2008 by luuuc

[mtv]

The Australian DVB-T transmissions are not suitable for mobile reception.

There were trials of the DVB-H system, but there are no firm plans for it's release.

http://www.broadcastaustralia.com.au/innov.../mobile-tv.aspx

[luuuc]

Does that apply in this case?

What my car has is essentially just a standard STB, but powered from 12 volts DC.

[mtv]

Does that apply in this case?

What my car has is essentially just a standard STB, but powered from 12 volts DC.

Correct.

The only difference is the tuner/antenna system you have, which consists of the two antennas in a 'diversity' setup.

They are designed so that the tuner switches between the two antennas, depending on which one is receiving the better signals of the two.

Also, the antennas (I presume) are vertical, whereas in Perth, the transmissions are horizontally polarised.

DVB-T can be difficult to receive in homes at times, even with an antenna specifically designed (and aligned) for the purpose, so trying to receive it whilst mobile is a waste of time (and money).

[luuuc]

Correct.

The only difference is the tuner/antenna system you have, which consists of the two antennas in a 'diversity' setup.

They are designed so that the tuner switches between the two antennas, depending on which one is receiving the better signals of the two.

Yep, that is my understanding of it.

Also, the antennas (I presume) are vertical, whereas in Perth, the transmissions are horizontally polarised.

My antennas have been installed horizontally

DVB-T can be difficult to receive in homes at times, even with an antenna specifically designed (and aligned) for the purpose, so trying to receive it whilst mobile is a waste of time (and money).

Aww man... that's not what I wanted to hear

[wiseguy84]

Hi Luuuc

Sorry to tell you this but it is the motion itself that is the problem,

DVB-T has no inbuilt ability to handle doppler shift which is what happens to the tv signal when your car is in motion. You've probably seen the examples of sound related doppler shift, this also happens at tv frequencies and causes problems (particularly with how dvb-t handles multipath/SFN and selective fading, as it depends on the interfering signals being orthagonal to cancel them out, if there is some doppler shift it mucks this up).

I have seen DVB-T (COFDM) decoder chips specially designed to handle doppler shift but i have not yet seen a marketed product that has them installed, one of the other forum members may know more.

As for DVB - H yes it can handle doppler but I don't think that it will be around for terribly long as the takeup of that system has been poor and most mobile phone companies are moving towards the "3G" subscription model instead.

Best of luck

Wiseguy

[D3CID3R]

i was thinking of doing this setup for a family friends car...

at least you have help the DTv community by being the guinea pig...

seriously tho, bad luck dude

Edited September 25, 2008 by D3CID3R

[mtv]

If you are not happy with it's performance (and who would be) then insist on a refund.

If they didn't tell you it won't work whilst the vehicle is mobile and it was your expectation it would, then you are entitled to a refund.

[Mr.Bitey]

Its got the word 'mobile' in its name - unless they explicitely told you that you cannot watch live DTV whilst moving, it would be reasonable to assume, given its for a motor vehicle it will work when mobile!

You're entitled to a refund for so many reasons (and laws!)

Cheers,

Bitey

[Shonky*]

I believe it is illegal to have a television in the driver's view whilst a vehicle is moving.

Some European luxury cars (I've seen it in a fairly low end BMW 3 series) also have analogue and digital tuners but as soon as the car starts to move, the screen is disabled and only the audio comes through. This can be defeated fairly easily though.

[mtv]

I believe it is illegal to have a television in the driver's view whilst a vehicle is moving.

Correct.. this also includes DVD screens, not just 'TV'

[Shonky*]

Correct.. this also includes DVD screens, not just 'TV'

Yeah. Should have said "video" or something.

But how do they differentiate between say a headunit or car pc with fancy visuals the jump around in time with the music and video or TV playback?

[Wing Nut]

Being DTV, I guess the sound drops out too? I have an analogue TV system that came built-in and kind of wish it was digital at times. The picture might be pretty lousy, but it's a welcome distraction when stuck in traffic or waiting to collect someone, and at least it I can keep listening to the audio when on the move.

[luuuc]

i was thinking of doing this setup for a family friends car...

at least you have help the DTv community by being the guinea pig...

Heheh... glad I could finally give something back to this community

While I'm here though, (and thanks to everyone for their comments & explanations too, btw), yes - i was told that it would work while moving. The installer said that up to 70kph should not be a problem.

The last time I rang him he actually had the Alpine rep. in the shop at the time, who said that his is still good at 110, which is what gave me some hope that I could get it going better.

The refund is probably something I could follow up on. To be honest though, I think I'm going to keep it anyway and try to improve it within reason. It's not great, but it still has its uses. (As the only driver of the car, I wasn't really planning to watch too much tv while actually driving anyway. Would have been a nice option for my passenger though)

So with that in mind ... I'm thinking that a skinny whip-type antenna could be concealed quite neatly inside my rear (plastic) spoiler. Would that get a decent signal in there? (I'm assuming that plastic won't really degrade a signal too much), and would that be an appropriate type of antenna to use?

[dig2all]

read this thread with interest, its got to the point where what has been posted should be challenged.

1/ dvb-t mobile is accepted as successful and operational everywhere dvb-t is broadcast. it might be a little better if we had heirarchical coding, but it works quite well here. compare it to analogue mobile tv!

2/ there are 2 types of dvb-t receivers, 1 tuner and diversity 2 tuner. 2 tuner are superior because they switch transparently between 2 antennas allowing reception where 1 tuner fails.

3/ some receivers work better than others - just like stb's, which ones? - you do the testing.

4/ antenna selection and positioning is critical. they must be as far apart as possible to give diversity its best chance. if someone says the antennas must be concealed or some other fancy 'must' - they will not get good pictures - simple.

having seen testing of various receivers and antennas, suggestions that the technology is somehow immature or defective is wrong.

[mtv]

While I'm here though, (and thanks to everyone for their comments & explanations too, btw), yes - i was told that it would work while moving. The installer said that up to 70kph should not be a problem.

The last time I rang him he actually had the Alpine rep. in the shop at the time, who said that his is still good at 110, which is what gave me some hope that I could get it going better.

If that's that's the case, why doesn't your installation work as well?

If they make claims like that, they need to substantiate them.... how about a practical demonstration.. or better still, making your receiver work as claimed.

[charlesc]

I've been following this thread and it's various updating posts, as it's an area of particular interest for me from some time ago. Unfortunately I haven't had time to post anything substantial back here.

A lot of work has been carried out in diversity tuner applications by companies such as Dibcom, and there are many articles on this technology available. Dibcom have produced ever newer generations of sophisticated chipsets to allow diversity tuners to be implemented. And much of it is using existing DVB-T technology. Some of the parameters allowed by DVB-T favour reasonably successful mobile reception, so it is certainly possible. And Australia uses some of those parameters.

Of course the real solution for this is DVB-H, but that is a real change to what we have now.

I believe DVB-T with diversity tuners in a proven implementation (such as ones like Dibcom, maybe others) could provide domestic reception for end users in severely blocked line-of-sight antenna installations, as well as for those that must use indoor antennas.

There are USB diversity tuner devices that could perhaps form the input to a PC implementation of a PVR. And the antennas I am thinking of here could be dual roof mounted ones, or better performing indoor antennas.

Like most things with digital reception, a lot depends on the antennas. And very much the implementation of the diversity tuners, and the method of combining their outputs to further correct the errors that a single tuner system would deliver.

Edited September 27, 2008 by charlesc

[dig2all]

If that's that's the case, why doesn't your installation work as well?

If they make claims like that, they need to substantiate them.... how about a practical demonstration.. or better still, making your receiver work as claimed.

this sounds like a repeat of one spec. many foreign suppliers quote about dvb-t mobile. the faster you can drive with it the better they think it is. ridiculous. one point that seems factual the slower speeds refer to 1 tuner receivers, faster speeds diversity.

[dig2all]

Heheh... glad I could finally give something back to this community

i was told that it would work while moving. The installer said that up to 70kph should not be a problem.

The last time I rang him he actually had the Alpine rep. in the shop at the time, who said that his is still good at 110, which is what gave me some hope that I could get it going better.

apologies - it challenged - above is the quote replied to.

this sounds like a repeat of one spec. many foreign suppliers quote about dvb-t mobile. the faster you can drive with it the better they think it is. ridiculous. one point that seems factual the slower speeds refer to 1 tuner receivers, faster speeds diversity.

[luuuc]

Thanks again for the continued interest and input.

Just to confirm - the Alpine unit is definitely a twin-input diversity system. It therefore makes "logical" sense to me that I can get better performance from the system for exactly the reasons some people have mentioned, which is that the whole point of diversity is to allow a better signal to be used if detected. In my case that's not even really possible, since both antennas are at the front of the car, both are aligned almost exactly the same (differing only due to the slight curvature of the windscreen), and are separated by only a metre or so.

I think the obvious first candidate for position #2 would be one set at 90 degrees to the front one - i.e. aligned with the side of the car instead of the windscreen.

The other thing that gives me continued hope is the fact that some channels are better than others. I was doing 70kph down the highway with the channel 9 news on, and the picture/audio would momentarily drop out maybe every 10 seconds, then recover. Switching to channel 10 and the picture was dead for as long as it was alive. No difference in conditions or driving direction. Once I got home - not far from that highway - sitting still in the driveway, which has my car facing 180 degrees to when it was driving and the windscreen now facing the hills where the tv transmitters are - I get almost no dropout on any channel.

The other point made about the doppler effect of vehicle motion left me a little unconvinced as well. I'd like to know more about the validity of that one.

For an electromagnetic wave, which travels at the speed of light, an added 70kph has got to be negligible.

Take a 200MHz signal. If my maths are correct (granted, that's a big if), then when I'm travelling towards the transmitter at 70kph, its apparent frequency would increase to 200.000013 MHz. That surely couldn't be a factor, could it?

(Happy to be corrected there - I accept that I know only enough physics to be dangerous)

Just finally, I don't know if vehicle speed is a legitimate means of measuring the performance of the signal or not. I mentioned the speeds of 70 and 110 only because they were mentioned to me by the installer and the product rep, so I assume they have some significance. It seems reasonable to assume that there would be some influence due to things like vibration, and perhaps the faster reaction time of the diversity system required when the car is moving quickly, in addition to any potential doppler-type effects on the transmission.

Edited September 27, 2008 by luuuc

[mtv]

It's not the speed, it's the constantly changing terrain and signal path/multipath.

[I am not a duck]

I believe DVB-T with diversity tuners in a proven implementation (such as ones like Dibcom, maybe others) could provide domestic reception for end users in severely blocked line-of-sight antenna installations, as well as for those that must use indoor antennas.

Covered somewhat briefly in this thread.

I have done this once using a unit sourced from GME-Kingray. AFAIK they are not available in HD, so when this customer upgraded to a Panasonic HD panel, it didn't cut the mustard. Fortunately, by a dint of good luck & judgement, ended up getting a stable reflection that got them out of trouble.

FWIW, a new GME diversity receiver here claims to cover the New Zealand digital TV spectrum, but neither the text or the manual seem to support this.

[turbotrana]

I use a Hi-TV digital tuner specially designed for mobile applications. Its DVB-T and it can be made to work OK. There are some areas where it is pretty bad but luckely on a 30km route that I regularly make its 95+% good.

The Hi-TV is set up for two antennas. It is the orientation and positioning that makes the difference in reception. For eg I have external areal which they suggest to mount under the bumper, but it does not work that well there. I found if I stuck one on the spoiler and the other on the back window of a VT Commodore perpendicular to the spoiler one (but at the window slope angle) that it worked OK.

I have been looking for aftermarket antennas that would look good on the car but there seems to be nothing out there.

If you stick the antenna onto the sheetmetal of the car it does not work as good as sticking on glass or the rear plastic spoiler.

If you are in a good reception area you can do 150kph+ and get a perfect picture

[mtv]

If you are in a good reception area you can do 150kph+ and get a perfect picture

You drive at that speed?

[Troobador]

You drive at that speed?

Who cares if he does or doesn't?

He's trying to make a point... let him.