UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Television Standards Converters, Modulators etc


Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

Closed Thread
Thread Tools
Old 23rd Jul 2011, 9:47 am   #1
Karen O
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
Default 'Simple' standards conversion

I'm given to understand that some 'simple' 625-405 standards converters (and possibly including the early BBC ones) work with limited line buffers and convert on a three-lines-in / two-lines-out basis.

That results in output lines that are about right but not spot on. In fact it yields a line frequency of 10.417kHz which is probably within the adjustment range of most TVs. But it aint standard.

Also, a 3-to-2 conversion produces slightly too many output lines, although with the higher line rate and the fact that the active lines of 625 are a smaller proportion of the frame time than 405, the picture is probably smaller in height than should be for a full spec 405 line picture.

More surprisingly, this 3-to-2 approach results in 416.67 lines not 405 lines (this is assuming the line frequency is not varied over the frame). At 208.33 lines per frame sync, that would result in a very strange triple interlaced output!

How do these simple converters achieve anything close to standard? Do they?

Of course, the 'proper' way to do it, and the way the Aurora does it I'm sure, is to convert on a 3.06-to-2 basis, meaning that the phase of the interpolation shifts from line to line. But that can be handled by changing the FIR coefficients according to the line. I guess a table of such coefficients can be held, indexed by output line number...?
Karen O is offline  
Old 23rd Jul 2011, 10:31 am   #2
Ray Cooper
Retired Dormant Member
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
Default Re: 'Simple' standards conversion

Originally Posted by Karen O View Post
...How do these simple converters achieve anything close to standard? Do they?...
Well, for starters, they were not quite as simpla as a 3 to 2 ratio might suggest, and in fact weren't tied down to any particular ratio. Yes, their output was standard. Get one of the early reports on the subject:-

- and, if you've the stamina to wade through that lot, you'll know exactly how they did work...
Ray Cooper is offline  
Old 23rd Jul 2011, 11:35 am   #3
Retired Dormant Member
ppppenguin's Avatar
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
Default Re: 'Simple' standards conversion

I don't think any of the 625>405 standards converters that have ever been available have used this simplified 3:2 method. The BBC CO6/501 and CO6/509 certainly didn't. If you haven't done so already, my first standards conversion article (in the publications section of my website) has a summary of conversion techniques together with links to most of the relevant BBC research papers.

Sample rate conversion, for this is what is required, can be done in various ways. Remember that this is done on the vertical axis. Most (all?) 625>405 converters leave the horizontal and time axes well alone. You can regard the 125:81 conversion ratio (yes I know the ratio of active lines is slightly different) as either as a proper fraction or an irrational number. Makes little difference. Ultimately it's a polyphase filter, the number of ocefficient sets is determined by how accurate you need the answer to be. For 625>405 conversion it's actually not too critical. BBC research found that beyond a 3 line interpolator (3 tap FIR) the improvement was negligible. The actual aperture shape isn't too critical either.

You can get really quite good results by a simple 2 line interpolator that takes either the whole of one line or a 50:50 mix of the 2 nearest lines. Works remarkably well, I think the Pineapple converter did this. The improvement over simply omitting approximately every 3rd line is huge.
ppppenguin is offline  
Old 23rd Jul 2011, 1:38 pm   #4
tubesrule's Avatar
Join Date: Jul 2004
Location: Michigan USA
Posts: 325
Default Re: 'Simple' standards conversion

When doing standards conversion, only the active portion of the video is considered. All of the VBI area in the input is ignored and the VBI in the output is generated from scratch. This means the output will always have the correct number of lines and timing. So for a simple 625>405 you are converting 288 lines to 189. 2/3 of 288 is 192 so you would need to cheat and drop a few lines off.

The other problem with this simple converter is 3 lines of 625 takes 192us while 2 lines of 405 takes 197.53us. This means the data is coming in slightly faster than its going out. At the end of a field you will have over 8 lines of input data stored up. This would be the minimum amount of memory required for such a system.

Converters like my Aurora do use a table of coefficients for the filter to achieve any arbitrary scaling ratio.

Aurora video standards converters:
tubesrule is offline  
Old 23rd Jul 2011, 7:25 pm   #5
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,018
Default Re: 'Simple' standards conversion

Hi Karen, if you PM Fernseh he may be able to give you a primer on this method as he built his own converter many years ago, well prior to microprocessor control systems.
Ed_Dinning is online now  
Old 24th Jul 2011, 2:28 am   #6
Karen O
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
Default Re: 'Simple' standards conversion

I am certain that 625-to-405 has been done in this simple fashion. I recall something about a discontinuity in the line syncs prior to the frame sync so as to avoid the wierd interlacing.

I guess it worked but there are much better ways of doing it these days.

It's quite staggering when you think about it - a conversion process that took racks and racks now fits in the palm of your hand. Amazing.
Karen O is offline  
Closed Thread

Thread Tools

All times are GMT +1. The time now is 9:25 pm.

All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at
Failure to heed this warning may result in death or serious injury to yourself and/or others.

Powered by vBulletin®
Copyright ©2000 - 2023, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.