Building a Standards Converter

[Cardigan]

I've just come to this thread and it is really fascinating, albeit way over my head. How good to see that the 5:4 viewer is catered for too.

Does the design allow for converting a native 16:9 input signal or does it have to be 4:3 to start with?

[`FRANK.C]

Sawing off the branch you're sitting on

Hi Jeffrey
That's exactly it, the worst part being that the branch was invisible . 

Hi Cardigan
The input signal has to be 4:3 as it simply crops part of the beginning and end of each line to make it fit 5:4. 

Frank

[Cardigan]

Hi Cardigan
The input signal has to be 4:3 as it simply crops part of the beginning and end of each line to make it fit 5:4. 

Thanks for that. The reason I asked is because it seems to be something that is not being addressed at the moment - the ability to convert what has become the standard aspect ratio for broadcasting into what older television sets require. I suppose that as the most likely feed is a set top box, that can easily be set to give a 4:3 output anyway so the point is moot. It also saves you having to do line-rate interpolation to get 4:3 from the 16:9 input at the same time as you are carrying out interpolation to get the 405 signal.

Yours has a valuable first - 5:4, which is commendable.

[ppppenguin]

You can never get a perfect answer for displaying 16:9 material on a 4:3 screen. The available methods are:

Letterbox: Black bars top and bottom
Pillarbox: Not relevant here, it's for displaying 4:3 material on a 16:9 screen
Crop: Slice off the left and right sides, hoping that nothing important is lost
Compromise letterbox: A mixture of letterbox and crop. The commonest solution
Tall and skinny: Just display the 16:9 on a 4:3 screen. Yuk.
Dynamic expansion: Don't know if this is the correct term but it leaves the centre of the screen intact while squeezing the edges to fit. Possibly used in conjunction with a small amount of crop. Not often used but I think it looks revolting, especially during pans.

The purists will say letterbox is the only way, practical folk usually say compromise letterbox. And so do I.

All this is normally handled in the Freeview box etc so the 625>405 converter doesn't need to be involved.

[Cardigan]

I disagree with your assertion about 14:9, I have to say. It will over time leave its mark on the top and bottom of the tube, as it did on many CRT colour sets. I would say that 4:3 centre cutout - which all STBs offer, unlike the 14:9 compromise - is the correct way to display.

[ppppenguin]

I'm usually happy with taking the central 4:3 from a 16:9 picture. Sometimes you end up missing things near the edges. There are various shoot/protect safe area guidelines that should minimise this but not all directors and camera operators are scrupulous about observing them. Anyone remember the horrors of "pan and scan" when showing widescreen films on 4:3 TV systems?

It's unfortunate that we can end up with 2 lots of aspect ratio conversion. One at the sending end where the broadcaster is transmitting 4:3 material in a 16:9 system. The purist approach here is pillarbox, otherwise you can lose an unacceptable amount of information at the top and bottom. Then again at the receiver. At a 4:3 receiver a pillarboxed picture can be restored perfectly. A top/bottom crop then loses its left and right edges so the result is equivalent to zooming in to the central part of the picture.

Pillarboxed transmission tends not to be used - it may be technically correct but it's pretty hideous to the average viewer when seen on a 16:9 display.

In other words the whole aspect ratio business is a mess. There isn't a perfect solution and I doubt there ever will be. You have to put up with what the broadcasters are sending and set the receiver to your personal preferences.

[Murphyv310]

I disagree with your assertion about 14:9, I have to say. It will over time leave its mark on the top and bottom of the tube, as it did on many CRT colour sets. I would say that 4:3 centre cutout - which all STBs offer, unlike the 14:9 compromise - is the correct way to display.

Hi.
Unless you use your vintage set day in day out and every day of the week I'd hardly say that screen burning is ever going to be a problem.
Even a test card that is most likely the commonest "picture" we display has never been seen to be leaving a mark on any of my sets and I've had them for a long time now and are on two or three times a week.
I'm sure if it does I'll be long dead and gone.
14:9 is in my view a "reasonable" compromise.

[Cardigan]

Except you end up with visually unappealing black bars at the top and bottom of the screen which would have been indicative of a fault back when the set was in normal service, so I would suggest it to be incorrect for demonstration or indeed everyday purposes. The compromise format of 14:9 used to look awful when the BBC used to transmit it in the final days of analogue, I was glad to see the back of it when analogue was switched off. It also left a slight mark on the CRT of the TV I had at the time too.

Enough of the 'action' still happens in the centre of the picture to use 4:3 - I am yet to see any set up where anything would be lost totally at the edges of the picture that would be restored to any degree by using the compromise ratio. Two people in conversation do not stand at the edges of the screen in conversation, for example.

The point I was trying to make was that it was a pity that pictures had to undergo two lots of conversion, which on some STBs can be of variable quality. It seemed a pity that this converter could not do it all in one go with a 16:9 source and do a high quality job, as the designer seems to have set out to achieve with his 405-line pictures, with the all-important 5:4 ratio as well.

[ppppenguin]

The horizontal axis size change probably needs more logic than the 625 to 405 conversion. you don't need linestores but the interpolator will need more multipliers to get a visually OK result. In the STB all this is done in LSI logic made in huge quantities so it's cheap. A bigger FPGA with more multipliers would add significantly to the cost of the 625 to 405 converter.

[`FRANK.C]

To do 16:9 I bereave it would also need a different decoder. I do intend to change the decoder later on but I hope to downsize. I mentioned two possibility earlier in the thread and a third that I have come across is the Intersil TW9910.
Intersil don't make there data sheets easy to access, which is off putting.  RS, Mouser etc. only have short data sheets which are just sales blurb, to get the proper data sheet you have to register with Intersil and then request the data sheet. Then the request has to be approved by the marketing manager and if approved you then receive it. I just did a more thorough search on the web and found it there which was a lot less hassle .

Frank