LM1881 with 405 lines - UK Vintage Radio Repair and Restoration Discussion Forum

retroteck · 6th Dec 2013, 9:25 pm

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

KeithsTV · 6th Dec 2013, 10:36 pm

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

FERNSEH · 6th Dec 2013, 10:40 pm

Doesn't the LM1881 use the equalising pulses to create the odd/even fields flag?

DFWB.

retroteck · 7th Dec 2013, 12:14 am

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

Synchrodyne · 8th Dec 2013, 11:20 pm

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,

pitbuell94 · 5th Jan 2014, 3:47 pm

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).

6th Dec 2013, 9:25 pm	#1
retroteck Tetrode Join Date: Jul 2006 Location: Harrogate, North Yorkshire, UK. Posts: 68	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
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	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
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	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
retroteck Tetrode Join Date: Jul 2006 Location: Harrogate, North Yorkshire, UK. Posts: 68	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
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	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
pitbuell94 Retired Dormant Member Join Date: May 2009 Location: Fresnes, France Posts: 124	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).