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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions. |
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6th Dec 2013, 9:25 pm | #1 |
Tetrode
Join Date: Jul 2006
Location: Harrogate, North Yorkshire, UK.
Posts: 68
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LM1881 with 405 lines
Hi All
Has anyone any experience of using the LM1881 synch separator chip with a 405 line composite input and its ability to produce a correct identical frame pulse on odd and even fields, bearing in mind the lack of equalizing pulses on the input. Rod |
6th Dec 2013, 10:36 pm | #2 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,573
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Re: LM1881 with 405 lines
I've tried it and it doesn't like the lack of equalising pulses producing a double pulse for each field sync pulse.
I don't think it is possible to produce a single pulse without equalising pulses. Keith |
6th Dec 2013, 10:40 pm | #3 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Re: LM1881 with 405 lines
Doesn't the LM1881 use the equalising pulses to create the odd/even fields flag?
DFWB. |
7th Dec 2013, 12:14 am | #4 |
Tetrode
Join Date: Jul 2006
Location: Harrogate, North Yorkshire, UK.
Posts: 68
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Re: LM1881 with 405 lines
Hi again
Yes you both are probably right. Obviously it can't distinguish between the half line and full line first frame synch pulse on alternate fields, too much to expect really. Thanks Rod |
8th Dec 2013, 11:20 pm | #5 |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: LM1881 with 405 lines
Out of curiosity, I looked up the data sheet/application note for the LM1881, available at: http://www.ti.com/lit/ds/symlink/lm1881.pdf.
The application note part does cover its use with video signals that have no serration pulses and also discusses the double pulse issue and its avoidance. I suspect though that designers were thinking more of catering for non-standard video systems with higher line frequencies than those of the analogue broadcast standards, whereas lines was going in the other direction. I think that the LM1881 arrived in the early 1980s, along with the LM1822/3, LM1884, etc., so postdated the 405-line system. Cheers, |
5th Jan 2014, 3:47 pm | #6 |
Retired Dormant Member
Join Date: May 2009
Location: Fresnes, France
Posts: 124
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Re: LM1881 with 405 lines
Hi Dear Sir,
Concerning the LM1881, I meet same problem during the time I spent to built an 625 to 819 B&W converter. I think you should replace it by a better EL4583. With this EL4583, you'll have correct signal concerning FIELD, Vsync and Hsync pulse. Please, have a look at this: https://www.vintage-radio.net/forum/...=62610&page=10 Have a good end of week. Fred.(Pitbuell94). |