405-line flyback lines - UK Vintage Radio Repair and Restoration Discussion Forum

Ian - G4JQT · 5th Sep 2007, 2:28 pm

Can anyone explain why 405-line television flyback lines appear as they do?

On sets with no flyback blanking and the brightness turned up, why do they appear angled and mostly only at the top? There’s also often six or so in a column towards the left of the screen and only about a quarter of a screen width. And why so few considering the number of scanning lines?

I’ve been looking through an old book of TV picture faults (yes, I really should get out more…) and it seems that they trace a similar pattern regardless of the model of television.

Incidentally, was there a time when flyback lines were only blanked on the most expensive early sets, or did their suppression always rely on correct adjustment of the brightness control until perhaps some years later?

Thanks,

Ian

Ray Cooper · 5th Sep 2007, 6:51 pm

Quote:

On sets with no flyback blanking and the brightness turned up, why do they appear angled and mostly only at the top? There’s also often six or so in a column towards the left of the screen and only about a quarter of a screen width. And why so few considering the number of scanning lines?

The bright lines are caused by the most positive part (ie black or blanking level) of the vision waveform during the frame flyback period.

They appear angled upwards because the spot is being deflected upwards (the field oscillator flying-back) at the same time as it is being deflected rightwards by the normal line scanning action.

The isolated group of narrower pulses that you see at the bottom of the screen are the most positive parts of the field sync waveform - the 'broad pulses' so-called. They're called broad because their active part is wider than the normal line-sync waveform. However the most positive part (the 'brightest part', if you like) is narrower than line syncs and produces this isolated group. There should be seven of them usually (being the separators between the eight 'broad pulses' of the field-sync waveform) but you won't usually see all of them since field flyback doesn't start instantaneously.

Confused? Don't blame you. Take a look at the 405-line waveform spec and it will all become clearer.

ppppenguin · 5th Sep 2007, 6:58 pm

The 405 line system sometimes used pedestal, that is having the black level a little above blanking level. This helped sets with poor flyback blanking. The NTSC system still has pedestal to this day although it's a complete waste of transmitter power. The final version of the 405 spec didn't have pedestal, neither did the initial version but I think it made at least one appearance. Does anyone know the history of this?

Ray Cooper · 5th Sep 2007, 9:02 pm

Quote:

The final version of the 405 spec didn't have pedestal, neither did the initial version but I think it made at least one appearance. Does anyone know the history of this?

'Rope'. See earlier posting - 'Pye 405 line colour' - back in March of this year.

Ian - G4JQT · 6th Sep 2007, 9:17 am

Thanks to everyone, particularly Ray.

It all becomes pretty obvious when I look at the video waveform. I now remember my broadcast engineering courses (mostly radio!) at Wood Norton. In an early course us radio chaps were introduced to TV waveforms and I can remember being shown the line and field sync pulses and equalization pulses in the field blanking and how the 625-line system was better designed from this point of view than the 405-line system which, we were told “only just worked”!

I can see why the most positive parts of the waveform would show like this now, and it would of course explain why they look the same on all sets.

So when did flyback blanking become common? Was it a rarity in 405 sets and/or not really necessary? I can see that the more complex field sync waveform of the 625-line system might need proper blanking.

Thanks

Ian

Focus Diode · 6th Sep 2007, 1:31 pm

The attachment, from the excellent "TV Fault Finding 405/625 lines" book by J.R. Davies (Data Publications) shows syncronised flyback lines on a typical 405-line receiver (or d/s switched to 405) when the brilliance control is set too high. This occurrs on 625 lines too though the lines tend to be at the top and more closely spaced.

I've not seen this on a d/s CTV switched to 405-lines however!

Brian R

5th Sep 2007, 2:28 pm	#1
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,326	405-line flyback lines Can anyone explain why 405-line television flyback lines appear as they do? On sets with no flyback blanking and the brightness turned up, why do they appear angled and mostly only at the top? There’s also often six or so in a column towards the left of the screen and only about a quarter of a screen width. And why so few considering the number of scanning lines? I’ve been looking through an old book of TV picture faults (yes, I really should get out more…) and it seems that they trace a similar pattern regardless of the model of television. Incidentally, was there a time when flyback lines were only blanked on the most expensive early sets, or did their suppression always rely on correct adjustment of the brightness control until perhaps some years later? Thanks, Ian

5th Sep 2007, 6:58 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: 405-line flyback lines The 405 line system sometimes used pedestal, that is having the black level a little above blanking level. This helped sets with poor flyback blanking. The NTSC system still has pedestal to this day although it's a complete waste of transmitter power. The final version of the 405 spec didn't have pedestal, neither did the initial version but I think it made at least one appearance. Does anyone know the history of this?

6th Sep 2007, 9:17 am	#5
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,326	Re: 405-line flyback lines Thanks to everyone, particularly Ray. It all becomes pretty obvious when I look at the video waveform. I now remember my broadcast engineering courses (mostly radio!) at Wood Norton. In an early course us radio chaps were introduced to TV waveforms and I can remember being shown the line and field sync pulses and equalization pulses in the field blanking and how the 625-line system was better designed from this point of view than the 405-line system which, we were told “only just worked”! I can see why the most positive parts of the waveform would show like this now, and it would of course explain why they look the same on all sets. So when did flyback blanking become common? Was it a rarity in 405 sets and/or not really necessary? I can see that the more complex field sync waveform of the 625-line system might need proper blanking. Thanks Ian

6th Sep 2007, 1:31 pm	#6
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: 405-line flyback lines The attachment, from the excellent "TV Fault Finding 405/625 lines" book by J.R. Davies (Data Publications) shows syncronised flyback lines on a typical 405-line receiver (or d/s switched to 405) when the brilliance control is set too high. This occurrs on 625 lines too though the lines tend to be at the top and more closely spaced. I've not seen this on a d/s CTV switched to 405-lines however! Brian R Attached Thumbnails