During a forum discussion re- the above a poster said "I personally use 1080i output as it doesn't require the TV to change input type every time I change between a HD or SD channel which is annoying". How would I notice this change in reality as I have a Sky HD box and it is set to Auto?

Auto - The TV will go blank for a few seconds when switching from HD to SD and vice versa, this is because the sky box is changing resolution outputs, 1080i for HD and 576p for SD.

1080i - all content (HD/SD) will be output at 1080i, therefore no interruption when switching between HD and SD.

As far as I am aware 720 isn't used by any UK broadcaster. SD is 576i.

The TV has to scale whatever signal it receives to make the image fill the screen. Obviously if the input resolution matches the screen resolution the TV doesn't really do anything.

If you set the Sky box to AUTO then it throws 576i at the telly for an SD programme and 1080(i or p) at it for HD programmes and the TV scales to suit.

If you set the Sky box to 1080 then it will do the scaling of SD up to 1080 resolution instead of the TV.

Which one you use depends on which you perceive gives the best results. But somewhere SD has to be scaled up to fill the screen. So if a TV has issues with switching between SD and HD then maybe it is best to let the Sky box do it if that doesn't have the same problem.

It isn't, and never has been - apart from small scale tests (from the BBC if I remember correctly?) before HD broadcasting even started.

The scaler in most Sky boxes sucks.

The best picture you'll get out of a Sky box then is where the scaler isn't operating. The closest you can get to this is when the box is set to Automatic mode. 1080i comes out as delivered. 576i is deinterlaced (in other words: made progressive) and comes out at 576p. Your TV will then scale and and deinterlace each resolution as needed for display on the panel.

The rub with the Sky box in Auto mode is the delay when switching from SD to HD and back. For many folk who value convenience above picture quality this delay is too much of an ask. They'll prefer 1080i mode and no delay.

If you own a scaler with the ability to edit the *EDID table then it's possible to force the Sky box to output 576i over HDMI. This is the SD signal in its native format and it is the best place to start for good scaling.



* EDID: A list of resolutions that tells the source what it can output

I value convenience but the convenience of not having to switch aspect ratios manually far outweighs the very minor inconvenience (basically none) of a slightly longer switching time.

Unfortunately the bug introduced with the new EPG means that on certain Pace boxes auto is unusable so I have to slum it with 1080i.

....or if you are still lucky enough to own/still use an original Thomson box with component outputs, SD is output at 576i - apart from the convenience of HD over component it's about the only good thing going for this box now as it's not supported by Sky.

Owners could also switch to scart if they want 576i.

Sometimes my tv will go blue when switching or turning it on (not a fault I am sure) and I manage to get out of it by turning it off and on. Is this the interruption and if so should it rectify itself?

Also: Someone said that he was going to try 720p for football as it improved the picture. Does that make sense?

Only if the poster has a HD-Ready (720/768 line) TV and it has very poor scaling capabilities.

[quote=Emrys Davies;65441674]Sometimes my tv will go blue when switching or turning it on (not a fault I am sure) and I manage to get out of it by turning it off and on. Is this the interruption and if so should it rectify itself? Quote]

I now see what you mean about the interruption when switching from SD to HD and vice versa, but I accept that, although it is quite noticeable. I now know that the blue screen which I get on occasions when switching or turning on has nothing to do with scaling so I will observe more closely and try to be more specific as to the exact steps it takes, unless someone recognizes the slight fault.

Thanks everyone. What an informative site.

[quote=Emrys Davies;65442518]
The blue screen is a failure to negotiate the HDCP (High Definition Copy Protection) handshake between the box and the TV. Switching to the TV's tuner and back to the HDMI input will force the two to renegotiate the handshake (like turning the TV off and on does). Try this next time it happens.

HDCP interrogates the destination from a HD source and will refuse to send the video unless the destination says it's only a display.

My guess is they have probably heard 720p was better for sport, setting the Sky box to 720p would not be the same as receiving a 720p broadcast, in this instance the sky box will deinterlace and downscale 1080i to 720p, so apart from the various deinterlacing and scaling that is present I don't think there would be much benefit.

Less technical intervention generally gives the better quality, try and keep the incoming signal as original as possible.

Bang on. But go a stage further. Just being a 720p broadcast guarantees nothing. The camera has to record in 720p (or 1080p) for the full benefits to be realised. If the sporting event is captured in 1080i then conversion to P later on is pointless. The limitations of an interlaced image are forever stamped on the image.

It is possible to convert p to i and then successfully reconstitute the original p frame again with a scaler or TV so long as the image is correctly flagged. But that does rely on the editing and broadcasting chain knowing what they're doing.



Good advice

Anyone know if the Sky box when set to 720p frame doubles to 720P50 ? I suspect it outputs 720p25. Since the improved performance with sports assumes 50 frames progressive it would be pointless to use this setting for most users.

No it doesn't - the source is 1080i (for HD), so while theoretically 720P 'should' be slightly better than 1080i on fast moving action, it would require the source to be P.

Downscaling is EASY - and doesn't produce artefacts, so feeding an HD Ready TV 1080i signals means the TV scales it down to fit, and even a cheap crap scaler will do an extremely good job of it. it's not difficult.

However, it's worth trying ALL the different settings (that's what they are there for), a viewer might find one he thinks is the best, even if it's mainly the placebo effect.

Pointless anyway - the source is still only i - so you're not getting any advantages of P, all you're getting is an interlaced picture de-interlaced at a different point.

Some punters think 100Hz TV's are the bees knees, what's the difference ?

Is this equally pointless ?

[quote=grahamlthompson;65442921]
Well, I told my wife that I would get to the bottom of this and so you have, most explicitly. So, apparently, turning the tv off and on is the only remedy?

Well it made large screen CRT's less flickery but made the pictures look like slimey lego in many cases

Naturally as far as the general public are concerned more Hertz means better* so this trend has continued with wise punters with 600Hz TV's getting many hundreds of times improvement over the rest of us mugs with our paltry 50Hz.**


*better is a matter of opinion rather than a fact.

** said firmly tongue in cheek, please don't reply saying how very wrong my reply was, I know it was

[quote=Emrys Davies;65444933]
Maybe not - see post 11

[quote=grahamlthompson;65446050]
I misinterpreted your message at post 11. I will try your switching suggestion if/when the problem occurs again.

I always thought so - it reduced frame flicker on CRT sets (for the minuscule percentage of the population who could see flicker), but made the pictures look far inferior to 50Hz sets (sort of 'plastic' and artificial looking).

Modern sets claim to improve the frame rate, which they do by interpolating extra frames between the two existing ones - but in a shop full of sets all running the same pictures I couldn't point out which might be 100Hz or indeed 200Hz. They 'might' perhaps be a slightly better picture, but that's because they are more expensive sets, not because of their frame rate.

That'll be why 14:9 analogue TV looked so good.

Please explain because I can't see the connection between 14:9 and downscaling.


I've always understood 14:9 to be an aspect ratio which was broadcast by the TV station, nothing to do with downscaling.

Neither can I, it was an utterly meaningless post