Archived from groups: alt.tv.tech.hdtv (
More info?)
Jeff Rigby wrote:
> "avproman" <avproman@comcast.net> wrote in message
> news
eZcd.205916$wV.75494@attbi_s54...
>
>>I thought that HDTV is digital. If the broadcast is digital (ones and
>>zeros), than you either get it or you don't. How can you have
>
> "multi-path"
>
>>signals which is very common for analog signals?
>>
>
> The difference between mulitipath and ghosts:
> Ghost we all know about as the second image in an analog picture usually
> very faint in the background. As long as the difference in signal strength
> between the ghost and the origional is greather than about 10db the digital
> tuner has no problem locking to the stronger signal.
>
> Multipath refers to the different paths that the signal can take to get to
> your antenna; 1) is the direct path from the antenna transmitter tower to
> your antenna. 2) Is the upper atmosphere reflection 3) is the reflection
> off differing densitys of air caused by heat on the ground. These differing
> paths can cause the signal to add or subtract signal strength, called fade.
> The signal strength can change quite rapidly causing the digital tuner to
> loose lock. The cure for this is to have the tuner be able to quickly change
> it's adaptive equalization. That requires a FAST algorithm and a fast
> processor. Fade can usualy be seen at certain times of the day as
> occasional dropouts in the picture or pixulation. This can also be caused by
> a marginal signal, we are assuming that you already have checked for that.
> If your direct signal strength (1 above) is greater my a large margin than
> the other paths then you won't have a multipath problem.
>
> The problem with very good signal strength is that you easily get ghosts off
> buildings and other structures that can cause you to be marginal (at the 10
> db margin between orig and ghost). A drop of a couple of db caused by fade
> and you have dropouts. While a weaker signal might not have a problem. A
> directional antenna helps here unless you have stations from different
> directions.
>
> As you can see in some cases it gets complicated, especially in urban areas.
> Too complicated and consumers turn off and the industry suffers. Thus the
> push for the ultimate STB tuner.
The ultimate tuner, that is one that works, is the 5th gen however it is
only minimally acceptable. We should and could have had ultimate
receivers in 1999. They existed then in the form of COFDM receivers.
COFDM receivers are being used in most of the rest of the world and cost
as little as $45. One reason for this is that IP royalty cost for COFDM
is as little as $5 while 8-VSB (US) receivers have $40 plus IP royalty
cost. The IP cost for the DVB-T COFDM modulation alone is around $.60
while the 8-VSB modulation is more than $6.
That is what we pay for our current political system based on blatant
bribery and corruption.
>
> There have been 4 generations of tuners if you look at the ability to work
> in worst case signal conditions. I've seen the difference at my shop between
> a 3rd and 4th generation tuner. In my case the difference shows up with two
> channels, one can't be received with a third gen tuner (the difference
> between ghosts is less than 10 db and the 4th generation can lock with less
> than 10 db difference) the second channel has sound dropouts at certain
> times of the day with the third gen tuner. I've also seen analog co-channel
> interference causing reception problems with the third gen tuner.
>
>
>>>Let me address the built-in tuner -
>>>
>>>If the TV has a built in ATSC tuner, it is almost certainly not the
>
> newest
>
>>>"5th generation" ATSC tuner recently developed by LG/Zenith. Recent
>
> news
>
>>>reports indicate that the new tuner is a huge improvement over all
>>
>>previous
>>
>>>ATSC tuners. The 5th generation unit requires only the simplest indoor
>>>antenna to reliably receive most nearby stations. Earlier generation
>>>receivers are more prone to reception problems, most significantly
>>
>>something
>>
>>>called "multi-path". When using one of the earlier generation
>
> receivers,
>
>>>one must invest more effort (and sometimes money) in antenna selection
>
> and
>
>>>in antenna placement.
>>>
>>>If you do purchase the 51" HDTV with its built-in tuner, you may still
>
> be
>
>>>tempted to purchase an external set-top 5th generation tuner when they
>>>become available at the end of this year and in the first quarter of
>
> next
>
>>>year. Also, expect to see HDTVs with the new tuner built in by next
>
> year.
>
>>>Personally, I intend to buy the new receiver as soon as it hits the
>>
>>stores.
>>
>
>
>
/staff.washington.edu/mrc
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