[mythtv-users] Antenna attenuation question
cbdd at fullnet.com
Tue Jul 1 07:21:21 UTC 2008
> Brian Wood wrote:
> On Monday 30 June 2008 21:21:25 Daniel wrote:
> there is no such thing as multi path interference with
> digital broadcast......I have three antennas on my tower pointed in three
> different direction, amp on each one combined in my basement......
> Say What ??? !!!
> It manifests itself somewhat differently, but you can definitely have
> multipath problems with digital broadcasts, especially in reflector strewn
> areas like Manhattan.
> Multipath is a huge problem for digital transmissions which is why they
> equalizers to remove the multipath. If you get more multipath than the
> circuits can correct, the receiver can no longer get the signal. If you
> have less than that, it isn't a problem.
> I used to build equipment to test this stuff.
> I do not think you understand how 8-vsb(VSB) is working
> see. http://www.broadcast.net/~sbe1/8vsb/8vsb.htm..
> ....it is possible to have (dynamic multi path interference) i.e.. trees
> high wind (moving objects between the transmitting and receiving
> it is impossible to receive a 8vsb-modulated HDTV signal in a moving
> but as far as multi path interference i.e.. antenna on your house with no
> trees directly in front of it......YOU WILL NOT HAVE....multi path
> interference.....unless your house is going down the road or moving....if
> is flying through the air you might be allright..
I don't really want to get into an argument, I am just trying to help
this guy out with his problem. Let me just say that I have met with the
people who invented 8-vsb. I visited them at Zenith before it was
bought by LG, while they were still inventing it and fighting with GI
over which spec would become the standard. Suffice it to say that I
believe I know what I am talking about.
Now, that said, you are correct in the the equalizers will have a
difficult time with moving multipath. This was a big problem in the
early days of 8vsb in that the signal would go out if a helicopter went
by or an airplane. So, they do handle some moving multipath but you are
correct that it is not designed to deal with a moving vehicle.
It is designed to deal with static multipath. However, all equalizers
fail at some point and they do so rather quickly and catastrophically.
If you put in enough multipath, the equalizers won't be able to deal
with it as good as they are.
This guy clearly has the potential of having the multipath problem he is
worried about as he is getting the main signal off the back of the
antenna. The reflected path may even be stronger than the main path. I
guarantee you that if the reflected path is the same strength as the
main path, the equalizer will not be able to deal with it.
The guy who said Manhattan is a problem is also correct. In the
concrete jungle, you can have very strong reflected signals. These can
exceed the ability of the equalizer to deal with it.
That said, I don't think the problem is multipath because he said that
removing the amplifier gain makes the signal come in fine. If it was a
multipath problem, that would not be the case.
So, we agree that he is probably not having multipath problems but I
would not rule it out because of the signal being behind the antenna.
I do not have a way with words like your self.....I am sure you know how the
least mean squares (LMS) algorithm work and the different tap weights etc.
I am just trying to suggest point another antenna in the other direction
with a amp .......and combine them in the house.....this will not cause a
co-channel or multi path problem: as, it would with analog TV
people try to tell me I did not know what I am talking about........I forget
people can not hear what i am thinking. HA HA
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