Re: HDTV ghosting and brightness flicker

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From: JW (anonymous_at_Nospam.Iam)
Date: 01/29/05 

Date: Sat, 29 Jan 2005 14:01:05 -0800

I stand corrected. But I found that I am not alone there are some reputable
web sites that also believed that I did that the name of the problem
"Ghosting" referred to the symptom in digital TV and not to the symptom in
analog TV caused by a multipath problem.

"Stephen Neal" <stephen.neal@nospam.as-directed.com> wrote in message
news:ctgpeo$m2s$1$8300dec7@news.demon.co.uk...
>
> "JW" <anonymous@Nospam.Iam> wrote in message
> news:eGqfIfjBFHA.2392@TK2MSFTNGP14.phx.gbl...
>> Following is a quote from recent testing using LG's proclaimed 5th
>> generation receiver
>>
>> "
>> The fifth-generation is a much more radical approach," Lewis said. "It
>> uses a 50 microsecond equalizer window to handle pre-ghost or post-ghost
>> [multipath] and was really focused on indoor reception and
>> ease-of-reception with simple antennas.
>
> Yep - this is multipath error reduction processing - used to reduce the
> impact of multipath-induced digital decoding errors. The use of the word
> "ghost" in this context is confusing to people who assume the effects of
> "ghosting" on digital signals will be visually the same as those
> introduced on the same transmission path used by an analogue signal -they
> won't.
>
>> Earlier generations of receivers could also do pre- and post-ghost
>> correction but did not have the long pre-ghost window that the latest
>> generation has.
>>
>> "
>
> Yep - this is because 8VSB suffers from digital reception errors in
> multi-path situations - but whereas these were just annoying double image
> ghost imaging artefacts on analogue transmissions, on digital
> transmissions multi-path can cause an otherwise strong signal to be
> difficult to demodulate and decode error-free when in the presence of
> multipath (meaning either no received services, unreliable received
> services, or errors in the received service - causing freezing, blocking
> or audio drop outs/chirps etc.)
>
>>
>> Note that ghosting can be a problem where signal reflections cause
>> multiple paths to the user's antenna. And since different OTA channels
>> are of course on different frequencies the multipath effect could vary by
>> channel.
>
> Yep - I wasn't saying that multi-path isn't a problem for 8VSB OTA digital
> TV (as used for HD in the US) - just that it wouldn't appear visually as a
> double-image "ghosting" effect on the received picture.
>
> Sure multipath causes digital reception problems with 8VSB (it is less of
> a problem with the DVB-T COFDM system) - but these are, as I said,
> exhibited as a reduced capacity to receive an error-free signal - so you
> get digital break-ups, frozen images, sound disturbance, once the FEC and
> MPEG error correction and concealment stop being able to "cope" with the
> number of errors in the signal.
>
> The observed "ghosting" visual effects received aren't going to be caused
> by multipath on a digital TV signal - as the visual "ghost" double-image
> effect is a function of analogue transmission (i.e. the same video signal
> received at different times via different routes superimposed on each
> other) If you imagine the same thing happening with digital TV - you get
> overlapping digital signals, which just cause major errors in the digital
> demodulator and decoder - not a double image.
>
> If the OP is SEEING double images on his received picture via 8VSB HDTV
> transmission - rather than via analogue over-the-air reception - then
> there is a problem present that isn't being introduced by the transmission
> path. Multipath conditions won't produce double images on a digital TV
> transmission system like 8VSB or COFDM - they'll produce errors instead.
>
> (COFDM, as used for DVB-T, partially reduces this by using not one carrier
> per digital TV channel space, but 2000 or 8000, with each carrier running
> at a lower data rate, with a large gap between digital symbols. This
> allows for quite large multi-path reflections before symbol interference
> becomes a problem. However there are other issues with COFDM which mean
> 8VSB also has advantages)
>
>>
>>
>>
>> "Stephen Neal" <stephen.neal@nospam.as-directed.com> wrote in message
>> news:ctgjp9$7dq$1$8302bc10@news.demon.co.uk...
>>>
>>> "Mike Z" <MikeZ@discussions.microsoft.com> wrote in message
>>> news:AA6748C6-D62A-4BA1-A15A-DF4E49AA29F4@microsoft.com...
>>>> I'm getting some pretty severe ghosting on the HDTV NBC channel that I
>>>> am not
>>>> getting on other HDTV channels that I receive. If you are looking at a
>>>> person's head or clothes (as an example), you will see the original
>>>> image,
>>>> and then a ghosted image shifted to the right. It's like looking at an
>>>> exagerrated 3D movie without the 3D glasses. This is really bad when
>>>> watching shows with lots of lights and movement, like Las Vegas.
>>>>
>>>> I was always told that you either get the HDTV signal and it comes in
>>>> perfect, or you don't get it at all. Is that not the case? Is this
>>>> just a
>>>> weak signal? If so, what will fix it?
>>>
>>> The ghosting you see is not caused by the transmission system - or the
>>> received signal strength. If you are receiving a digital TV signal then
>>> the lack of signal exhibits itself as frozen pictures, pictures breaking
>>> up into blocks, drops outs or chirrups on the received sound etc.
>>>
>>> Because the digital TV system is carrying the digital TV feed as encoded
>>> by the broadcaster, any picture related interference issues on a digital
>>> TV feed are either present on transmission (and thus seen by all
>>> viewers), or introduced by your PC after reception.
>>>
>>> What does the channel you are watching look like on an HDTV pointed at
>>> the same transmitter?
>>>
>>>>
>>>> My HDTV card is the ATI HDTV Wonder. I have a home built MCE2005
>>>> system.
>>>> My antenna is the Silver Sensor which is not a powered antenna. Would
>>>> a
>>>> powered antenna help out with the ghosting?
>>>
>>> If you aren't getting signal drop outs - i.e. freezing, loss of signal,
>>> picture disappearing into blocks, then it is unlikely to be a signal
>>> strength issue.
>>>
>>>>
>>>> I also get a brightness flicker. It will flicker between normal and
>>>> washed
>>>> out constantly on some shows.
>>>>
>>>
>>> If other HD services are displaying fine - CBS or PBS HD for example
>>> (they use the same 1080i format as NBC for their HD video) - then it is
>>> likely a problem at the station! If Fox and ABC display fine but CBS,
>>> NBC and PBS HD don't, then it sounds like a problem with your set-up
>>> de-interlacing the 1080i material (Fox and ABC used 720p - which is
>>> progressive so doesn't need de-interlacing)
>>>
>>>> Any ideas on how to resolve either of these issues?
>>>
>>> I'd check out other HD sets viewing the same channel.
>>>
>>> This is a very stupid question - but you are watching this channel IN HD
>>> aren't you? It isn't that your PC is displaying an analogue feed ? Your
>>> descriptions sound like classic analogue reception issues.
>>>
>>> The image are widescreen aren't they?
>>>
>>
>>
>
>

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