On Sep 19, 8:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com> wrote:> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...: > > > Prolonged blacks can damage television transmitters, however (video > > is inverted for transmission, so black requires full power from the > > transmitter). > > Why is video inverted for transmission?when i took a course on TV engineering 3 decades ago, it was explained to me like this: while the sound for TV is FM, the video is AM (actually VSB-AM). that means the amplitude of the envelope of the video portion of the RF signal is proportional to the (inverted) video signal. now, when a spike of noise (like lightning or something) is added to an RF signal at some totally random time, it is added to the RF at a totally random phase. probabilistically, that spike, being uncorrelated to the RF, will increase the short-time energy of the RF and thus increases the amplitude envelope of the RF and then the inverted video signal. an instantaneous spike in time will much more likely cause the envelope to INcrease rather than cause it to DEcrease (but it *could* cause it to decrease if the polarity of the spike where opposite of the instantaneous polarity of the RF and of the correct relative amplitude). so then ask yourself: if you were to get a spike of noise added to the video signal (which increases the envelope amplitude) which would you rather see, a spurious bright dot or a spurious dark dot? that is the reason i was taught for that engineering decision made 7 decades ago. r b-j
Why is video inverted for transmission?
Started by ●September 19, 2007
Reply by ●September 19, 20072007-09-19
Reply by ●September 20, 20072007-09-20
On Sep 19, 7:58 pm, "Green Xenon [Radium]" <gluceg...@excite.com> wrote:> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote inhttp://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8e...: > > > Prolonged blacks can damage television transmitters, however (video > > is inverted for transmission, so black requires full power from the > > transmitter). > > Why is video inverted for transmission?There are exceptions. System L, still used in France, for example, has peak white at max RF power. The analog audio sub-carrier is amplitude modulated. There is a digitally modulated sub-carrier for stereo audio (NICAM)
Reply by ●September 20, 20072007-09-20
John Larkin wrote: (someone wrote)>>Why is video inverted for transmission?> So black, which is the sync pulse, which is the retrace blanking, gets > lots of transmit power, so things tend to stay in sync.That is the reason I always heard. In addition, sync pulses are blacker than black to make sure that they are not visible on retrace. > Max-black also make dc restoration work nicely. Why does it make it easier? -- glen
Reply by ●September 20, 20072007-09-20
Richard Crowley wrote:> "Charles" wrote ... > >>> Why is video inverted for transmission? >> Noise in the black portion shows up more than in the white. Black is >> transmitted at higher power, more received signal. less noise. > > Yes, that is my recollection of the original explanation by the > engineers & scientists who devised the system. > > We tend to forget how primitive things were back then (>50 > years ago) by modern standards. I can't believe what they > did with vacuum tubes ("valves") in those days. >A LOT of maintenance. -- It Came From Corry Lee Smith's Unclaimed Mysteries. http://www.unclaimedmysteries.net In a time of deception telling the truth is a revolutionary act. - George Orwell
Reply by ●September 20, 20072007-09-20
On Wed, 19 Sep 2007 19:04:31 -0700, "Richard Crowley" <rcrowley@xp7rt.net> wrote:>"Charles" wrote ... > >>>Why is video inverted for transmission? >> >> Noise in the black portion shows up more than in the white. Black is >> transmitted at higher power, more received signal. less noise. > >Yes, that is my recollection of the original explanation by the >engineers & scientists who devised the system. > >We tend to forget how primitive things were back then (>50 >years ago) by modern standards. I can't believe what they >did with vacuum tubes ("valves") in those days. >Some of us were around back then. <:-)
Reply by ●September 20, 20072007-09-20
On Wed, 19 Sep 2007 21:08:28 -0800, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>John Larkin wrote: > >(someone wrote) >>>Why is video inverted for transmission? > >> So black, which is the sync pulse, which is the retrace blanking, gets >> lots of transmit power, so things tend to stay in sync. > >That is the reason I always heard. > >In addition, sync pulses are blacker than black to make sure that >they are not visible on retrace. > > > Max-black also make dc restoration work nicely. > >Why does it make it easier? > >-- glenWith the signal inverted, the tips of the sync pulses are the biggest part of the signal. They are also always exactly the same height, regardless of whether the scene is bright or dark. This means you can always rely on the black level being 2/3 (or whatever, I forget) of the way up the signal. Anything above that is sync, anything below is picture. If the signal were broadcast "right way up", with syncs at the bottom, you would recover a signal which varied in amplitude depending on the brightness of the scene, and it would be impossible to determine the exact location of either black or the sync front and back porches. d -- Pearce Consulting http://www.pearce.uk.com
Reply by ●September 20, 20072007-09-20
John Larkin wrote:> On Wed, 19 Sep 2007 17:58:23 -0700, "Green Xenon [Radium]" > <glucegen1@excite.com> wrote: > > >>On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in >>http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 : >> >> >>>Prolonged blacks can damage television transmitters, however (video >>>is inverted for transmission, so black requires full power from the >>>transmitter). >> >>Why is video inverted for transmission? > > > So black, which is the sync pulse, which is the retrace blanking, gets > lots of transmit power, so things tend to stay in sync. Max-black also > make dc restoration work nicely. >Other than brief portions of the evening news the question arises: What's the whole point in restoration these days? If OTA-TV really goes digital some distant day we might not even bother buying a new set. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●September 20, 20072007-09-20
John Larkin wrote:> On Wed, 19 Sep 2007 17:58:23 -0700, "Green Xenon [Radium]" > <glucegen1@excite.com> wrote: > > >>On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in >>http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 : >> >> >>>Prolonged blacks can damage television transmitters, however (video >>>is inverted for transmission, so black requires full power from the >>>transmitter). >> >>Why is video inverted for transmission? > > > So black, which is the sync pulse, which is the retrace blanking, gets > lots of transmit power, so things tend to stay in sync. Max-black also > make dc restoration work nicely. >Other than brief portions of the evening news the question arises: What's the whole point in restoration these days? If OTA-TV really goes digital some distant day we might not even bother buying a new set. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●September 20, 20072007-09-20
John Larkin wrote:> On Wed, 19 Sep 2007 17:58:23 -0700, "Green Xenon [Radium]" > <glucegen1@excite.com> wrote: > > >>On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in >>http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 : >> >> >>>Prolonged blacks can damage television transmitters, however (video >>>is inverted for transmission, so black requires full power from the >>>transmitter). >> >>Why is video inverted for transmission? > > > So black, which is the sync pulse, which is the retrace blanking, gets > lots of transmit power, so things tend to stay in sync. Max-black also > make dc restoration work nicely. >Other than brief portions of the evening news the question arises: What's the whole point in restoration these days? If OTA-TV really goes digital some distant day we might not even bother buying a new set. -- Regards, Joerg http://www.analogconsultants.com
Reply by ●September 20, 20072007-09-20
"Green Xenon [Radium]" <glucegen1@excite.com> wrote in message news:46f1c33d$0$32547$4c368faf@roadrunner.com> On Sep 3, 8:27 pm, isw <i...@witzend.com> wrote in > http://groups.google.com/group/sci.electronics.basics/msg/6689f0d1e8eff770 > : >> Prolonged blacks can damage television transmitters, >> however (video is inverted for transmission, so black >> requires full power from the transmitter). > > Why is video inverted for transmission?White noise is less visible than black noise.
