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| Why does NTSC use a strange frequency for the chroma carrier? |
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| EPAIII:
PS: The broadcast video cameras did and still do have have their own, internal sync generator circuits. This allows them to be used on a stand-alone basis. But when they are in a studio environment or as part of a remote pickup with more than one camera, they are always synchronized with a sync generator or a single camera used as the master. Broadcast level cameras always have an input jack for importing that synchronizing signal. |
| SeanB:
Yes broadcast cameras can both run external sync or run free, and generally they now all are free running, because the control van normally has a frame buffer and resynchroniser for every analogue camera, and for digital inputs that is provided by the input side as it receives full frames of data and sends them back out to the control. Having a sync though does mean all the cameras will run at the exact frame rate, so there is no chance of a frame being repeated or dropped, as the clocks drift with long broadcasts. modern digital stores are great, even on things as mundane as the broadcast VTR, which has it both at input and output, which allows for a perfect freeze frame, with seamless resume, as it simply records the freeze frame off the live video, then stops the tape, and reverses back a few frames, so that it can resume at exactly that frame. All due to cheap memory, which was a game changer. IIRC the BBC still broadcast 405 line TV till 1985, starting in 1937. When the transmitters were turned off they reckoned that there were possibly 10 viewers who still had a working 405 line TV set. 48 years of the same standard, which is pretty much the longest TV standard to be in continual use. |
| rsjsouza:
I loved this information, EPAIII! Quite interesting to see how things are much more complex than my then young mind would simply take for granted. |
| Ben321:
--- Quote from: EPAIII on January 03, 2023, 10:20:43 am ---And I don't know where this power line business came from. The sync generators in the individual stations had their own master oscillators. The only nation wide clock was the network that the station belonged to and they only synchronized to the network's signal when necessary. The power lines then, and now drifted far too much to ever use even for just a black and white facility. At least from the 1960s to the present day the power line frequency was NEVER used by TV stations. Or the networks. --- End quote --- The power line business came from the fact that in whatever country a TV standard was designed in, the field rate is always the same as the power line frequency (at least for the black&white version of the standard). In the US, the TV field rate is 60Hz. In Europe it's 50Hz. If the field rate was arbitrary, and not based on the power line frequency, then why is it that TV signal standards always use the same frequency as the power line frequency in that country for the video signal field rate? |
| BrianHG:
--- Quote from: Ben321 on January 14, 2023, 05:18:46 am --- --- Quote from: EPAIII on January 03, 2023, 10:20:43 am ---And I don't know where this power line business came from. The sync generators in the individual stations had their own master oscillators. The only nation wide clock was the network that the station belonged to and they only synchronized to the network's signal when necessary. The power lines then, and now drifted far too much to ever use even for just a black and white facility. At least from the 1960s to the present day the power line frequency was NEVER used by TV stations. Or the networks. --- End quote --- The power line business came from the fact that in whatever country a TV standard was designed in, the field rate is always the same as the power line frequency (at least for the black&white version of the standard). In the US, the TV field rate is 60Hz. In Europe it's 50Hz. If the field rate was arbitrary, and not based on the power line frequency, then why is it that TV signal standards always use the same frequency as the power line frequency in that country for the video signal field rate? --- End quote --- In the day of vacuum tubes and not well regulated power supplies for CRT tubes, whose beam location is positioned by 'magnetic' field generated by the electronics, if in Europe, if you created a 55hz video standard where the power grid is 50hz, your picture would visibly 'wobble' 5 times a second because of both the internal circuitry precision and external 50hz magnetic fields coming from stray power lines or transformers. This was before they day of feedback and regulated drive circuitry for the magnetic 'yoke' which positions the electron beam forming the picture's raster. Having a 50hz picture with a source interference of 50hz from your source power, this ever so slight bend in the picture would not be visible as it would be stationary or scrolling vertically too slow to be noticed. (If this bend were to scroll vertically fast enough, IE: my above mentioned 5hz for a 55hz field rate, that bend will appear as a wobble in the image.) |
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