Horizontal resolution of HDTV

Nick_UK

Banned
When people discuss HDTV, much is made of the line structure (480, 575, 625, 576, 1080) which makes up the vertical definition of the picture, but not much is said about the horizontal definition (resolution).

When digital TV replaced analogue in 625 lines, the horizontal definition of the picture undoubtedly dropped. 625 line analogue TV was capable of an analogue video bandwidth of ~6.5MHz, but the analogue output signal from a digibox struggles to make 5.5MHz, and the effect is that the pictures are "softer" than on analogue. People often make the mistake of trying to sharpen digital pictures, but all they end up doing is accentuating the digital artifacts.

Now, with HDTV we're increasing the number of lines in the picture, which theoretically increases the vertical bandwidth, but I'm doubtful that with the increased compression that is necessary with HDTV that we're actually gaining as much as we think.

I've heard people say that HDTV is "stunning", but is that a subjective view after watching soft digital pictures for a few years ? I've seen "stunning" 625 line analogue pictures on TV studio monitors too. I get the feeling that HDTV would not have been anywhere as easy to sell to the public, had they not been "softened up" by sitting in front of poor-quality digital pictures for a few years first. If the Sky analogue service was still running, and we could compare PAL 625 in full bandwidth side by side with pictures from a SD digibox, I think we would all get a shock !
 

10bii

Banned
Nick_UK said:
When people discuss HDTV, much is made of the line structure (480, 575, 625, 576, 1080) which makes up the vertical definition of the picture, but not much is said about the horizontal definition (resolution).

When digital TV replaced analogue in 625 lines, the horizontal definition of the picture undoubtedly dropped. 625 line analogue TV was capable of an analogue video bandwidth of ~6.5MHz, but the analogue output signal from a digibox struggles to make 5.5MHz, and the effect is that the pictures are "softer" than on analogue. People often make the mistake of trying to sharpen digital pictures, but all they end up doing is accentuating the digital artifacts.

Now, with HDTV we're increasing the number of lines in the picture, which theoretically increases the vertical bandwidth, but I'm doubtful that with the increased compression that is necessary with HDTV that we're actually gaining as much as we think.

I've heard people say that HDTV is "stunning", but is that a subjective view after watching soft digital pictures for a few years ? I've seen "stunning" 625 line analogue pictures on TV studio monitors too. I get the feeling that HDTV would not have been anywhere as easy to sell to the public, had they not been "softened up" by sitting in front of poor-quality digital pictures for a few years first. If the Sky analogue service was still running, and we could compare PAL 625 in full bandwidth side by side with pictures from a SD digibox, I think we would all get a shock !
I can give you my opinion from the other side of the Atlantic. HDTV programming quality here, overall, is in stark contrast to analog service and SD digital. The difference is so obvious with a decent display or TV that it doesn't even approach being subjective. That includes the plenty of analog PAL programming I remember seeing years ago.

That said, as with anything digital it can be mutilated to death or mediocrity. MP3s being a good example.

Proper HD will look like moving full frame quality Photoshop imagery with your monitor running resolution at the minimum 1280 resolution width. Microsoft WMVHD demos from their site don't do HD justice. I can watch Conan or Jay Leno or any CBS drama and the detail makes their samples look silly.

Hopefully your service will look at least as good.
 

Nick_UK

Banned
I've been to the US many times (last time was about 5 years ago), and it's hard to make a subjective opinion of the technical quality of American TV, because the only TV I saw was in my hotel, and hotel TV is hardly ever the best. There are quite a few myths surrounding American TV and its use of 525 lines and NTSC, (we presume that our 625 lines and PAL is better) but modern electronics really makes the two systems almost as good as each other. It did appear to me that Americans seem to be much more forgiving of poor quality than we are, and I think that's because (in years gone by) it was quite hard to transport pictures across vast expanses without some interference or distortion.
 

10bii

Banned
Nick_UK said:
I've been to the US many times (last time was about 5 years ago), and it's hard to make a subjective opinion of the technical quality of American TV, because the only TV I saw was in my hotel, and hotel TV is hardly ever the best. There are quite a few myths surrounding American TV and its use of 525 lines and NTSC, (we presume that our 625 lines and PAL is better) but modern electronics really makes the two systems almost as good as each other. It did appear to me that Americans seem to be much more forgiving of poor quality than we are, and I think that's because (in years gone by) it was quite hard to transport pictures across vast expanses without some interference or distortion.
In all my travels through many countries and literally hundreds of hotels I have very rarely seen quality TV in a hotel room. The TVs themselves are usually bottom of the barrel anyway.

I can remember back to the early 70s and analog TV reception was the same as it is now in that it varies depending on where you are and the stations themselves. Most people have cable or satellite anway so it is much less of a concern today. With digital over the air, as typical with all digital, you either have a signal or not so in that way it is inferior to analog where you can at least get a lousy picture with bad reception. Come to think of it, I think digital in Japan is of a system that mimics that way of operating in that you at least get a degraded picture when reception is off.

I agree with you on there not being a very big difference between NTSC and PAL.
 

MikeK

Well-known Member
I agree with you on there not being a very big difference between NTSC and PAL.

DVD, then no there isn't a massive difference (although personally I still think PAL edges it, especially on a big screen). Not so sure the same is true for broadcast TV though - I've seen some really ropey NTSC stuff in the US (although as the TV distribution system is so radically different to ours, it's hard to say where the fault for that lies)
The HDTV stuff is quite nice though, although I haven't seen a great deal of that TBH.



As for digital TV being lower definition than analogue - depends on the equipment in use as well as the channel you are watching IMO.
The bandwidth of 5.5MHz is typical, even of some DVD players - it also happens to be the bandwidth of PAL-I, our analogue broadcast standard.
Personally I think the softness seen on some channels is put there deliberately to disguise low bitrate induced MPEG2 macroblocking, rather than because of inherent bandwidth limitations of the equipment - although that may well play a part too in some cases!
DTV should be rivaling DVD on the picture quality front - alas the "model" in the UK seems to be to go for quantity over quality, although you do occasionally get very good (near DVD) quality pictures from DTV. Mostly though, it's a long way short.


As to HDTV - well clearly there's plenty of ways the quality of this could be compromised too.
However, given that the best real world resolution that SD PAL can offer is around 700x400* then it's also easy to see why true progressive 1280x720 could be seen by many to be a "stunning" improvement.

* Although PAL and 576p are typically described as being 720x576, this isn't the end of it. If strict PAL timings were considered, it would actually be 702x576, and even then, 702 would only be realisable at 6.75MHz video bandwidth. At 5.5MHz, you'd be down to around 570.
The vertical resolution isn't really 576 because of interlacing - in order for the picture not to flicker and line-twitter like mad when displayed on an interlaced display, (like a common or garden CRT TV), they have to prefilter the signal in the vertical resolution, typically by around 30%, which means that although there are indeed 576 active lines per frame, they don't contain 576 lines worth of detail, more like 400 lines worth.
Also note that any effect of deinterlacing (which you must do for display on a plasma or LCD) hasn't been considered yet - a low quality deinterlacer/scaler and video input, such as might be fitted to a low end display, could easily reduce the real resolution of even a very good PAL input signal to around 570H (or less) x 200V, or so.
Ever wondered why some of the low end plasmas etc, look dreadful with broadcast PAL, but not too bad with progressive DVD?
 

Tarbat

Well-known Member
And not all 1280x720 displays are really 1280x720. Take the HD-Ready Samsumg DLP TVs. With a 50hz signal, these DLP TVs only display a 1200x673 picture. Whereas many LCD and plasma HD-Ready TVs display the full 1280x720. So not all HD-Ready TVs are equal, even for their vetrtical definition.
 

Stephen Neal

Well-known Member
For the last 10-15 years many/most analogue PAL broadcasts will have passed through at least one 720x576 (*) digital processing device (**), even if they didn't originate in a digital format themselves. As such - most broadcasts delivered in PAL via analogue have had the same digital source resolution limit as those delivered by MPEG2 - which in the case of the BBC is also 720x576 (*), though other broadcasters like ITV on DSat drop to nearer 544x576 via MPEG2 to reduce their bandwith requirements.

For most of the 90s, and until quite recently for 4:3 shows, the BBC used a PAL digital composite VTR standard called D3. This was based on 4fsc sampling - which means the video signal was sampled not at 13.5MHz but at 17.72MHz (but as a composite, not a component signal)

In theory this would allow a higher luminance resolution to be recorded than a "D1" / CCIR 601 recording - however since the early 00s all of these D3 VTR recordings have been decoded to 13.5MHz sampled component for transmission, and since most programme sources have been CCIR 601 13.5MHz sampled for quite a while before that, rather than PAL analogue (PAL 4fsc digital production gear never really happened), anyway - there won't have been much above 13.5MHz present anyway. However D3 WAS an uncompressed composite format - so the signal was very clean, and the only artefacts present were the PAL composite ones - with no compression artefacts added.

If you had to record a PAL composite signal D3 (or D2) were the best you could get - and looked cracking. (For PAL)

(*) Yes - 702x576 is the digital equivalent of a 52us PAL active line.
(**) Yes - in the early 90s it is possible 4fsc digital composite rather than 13.5MHz component synchronisers were used - with the synchroniser or TBC (which in the case of CCD based variants could be considered sampled analogue) being the digital point in the chain.

** Thinking about it the distribution to the PAL analogue transmitters is based on 140Mbs 3fsc based circuits anyway AIUI - so there can't have been much above 12MHz present off-air anyway! **
 
Q

Quickbeam

Guest
Nick_UK said:
When digital TV replaced analogue in 625 lines, the horizontal definition of the picture undoubtedly dropped. 625 line analogue TV was capable of an analogue video bandwidth of ~6.5MHz, but the analogue output signal from a digibox struggles to make 5.5MHz, and the effect is that the pictures are "softer" than on analogue. People often make the mistake of trying to sharpen digital pictures, but all they end up doing is accentuating the digital artifacts.
Digital SD TV can in theory have a higher horizontal resolution than analogue: 702 pixels versus ~550 pixels for analogue PAL.* In keeping with this most European countries broadcast 4:3 channels at a resolution of 544 x 576.** Widescreen pictures demand a higher horizontal resolution because the pixels are stretched to be 33% wider, and this is where broadcasters like ITV and Channel 4 come unstuck, by stretching 544 x 576 to widescreen proportions on their digital channels.

*The overall vision bandwidth for PAL-I is 5.5MHz, but that also includes the colour carrier at 4.43 MHz. Filtering is used to ensure colour signalling at and above 4.43MHz don't make it into the luminance otherwise there would be a nasty patterning effect on the screen. Even assuming the entire 5.5MHz actually made it through to your screen undamaged, 544 can represent over 5MHz (~5.2) so in theory you shouldn't notice much of a difference at all between a full bandwidth black and white analogue signal and a 544 x 576i MPEG-2 broadcast.

**The UK is unusual in that except for the BBC, all broadcasters use the same horizontal resolution for both 4:3 and 16:9 content. When football is shown on ITV2 (a 544 broadcaster) 4:3 international feeds are pillarboxed; this leads to an effective resolution of ~400 x 576!

***Ofcom rules demand that the five terrestrial channels use 704/720 on terrestrial, but this does not appear to apply to satellite - hence the 544 x 576 resolution of ITV1 on satellite.
 

Stephen Neal

Well-known Member
Quickbeam said:
**The UK is unusual in that except for the BBC, all broadcasters use the same horizontal resolution for both 4:3 and 16:9 content.
AIUI ITV use (or used) the same system as the BBC internally (i.e. 4:3 material is pillarboxed to 540x576ish) but they expand prior to MPEG2 encoding, whereas the BBC use AFDs and trigger a set top box expansion instead - on DTT.
 

Nick_UK

Banned
Quickbeam said:
......... *The overall vision bandwidth for PAL-I is 5.5MHz, but that also includes the colour carrier at 4.43 MHz. Filtering is used to ensure colour signalling at and above 4.43MHz don't make it into the luminance otherwise there would be a nasty patterning effect on the screen......
Agreed, but the 4.43MHz carrier is only "notched" out - it's not a low-pass filter, so frequencies above 4.5MHz should still be seen if a good quality filter is used.
 

Stephen Neal

Well-known Member
Nick_UK said:
Agreed, but the 4.43MHz carrier is only "notched" out - it's not a low-pass filter, so frequencies above 4.5MHz should still be seen if a good quality filter is used.
Yep - this entirely depends on how the PAL decoder and luma processing are implemented. Significant numbers of TVs just used to low-pass below the 4.43 carrier as it was easier and cheaper.

Higher end displays often used better techniques.

The BBC R&D Transform decoder is the current state of the art AIUI in PAL decoding terms - though it requires about 18 frames of storage to work. (The BBC master their Dr Who DVDs through it - and you would be hard pressed to spot any subcarrier artefacts in them in many cases)
 

Trending threads

Top Bottom