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Puzzled: PAL PQ vs HD PQ on an SD Panel

Discussion in 'Plasma TVs' started by Rahmorak, Jan 24, 2005.

  1. Rahmorak

    Rahmorak
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    OK,

    Although I am in the market for a Hi-Def panel I have been bitten by curiosity. I have done a quick search but couldn't see anything directly related so decided to ask and put myself out of my misery. :)

    I have seen several posters mention that they see the difference between SD material and HD material when displayed on an SD panel. Now, I am just curious as to how this can be the case if my memory serves me right, e.g.

    PAL = 720x576i@50 (-overscan) **
    NTSC = 720x480i@60(-overscan) **
    (** From what I remember from my old Amiga+Opalvision days (some 12+ years ago ;)))

    SD Panels = 852x480 (with some variation)
    HD = 1280x720 or 1920x1080 etc.

    OK, from this it seems to me that a PAL broadcast or DVD signal is already carrying more information than an SD panel can display and hence information is being discarded as the panel/scaler downsamples the image.

    If this is the case, then how can feeding a HD signal show more detail than a broadcast/dvd PAL signal as both result in the need to discard information?

    R
     
  2. Leerock

    Leerock
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    I believe it s because the bitrate of an HD signal is a lot higher.
    Downscaling to an SD panel reduces resolution, not bitrate.
     
  3. Bearded_Malc

    Bearded_Malc
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  4. gizlaroc

    gizlaroc
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    A PAL VHS player outputs at 720x576i/50 the same as dvd but you don't expect it to look as good because it can not hold as much detail as the dvd. It is exactly the same for say D-Theater over DVD, more information is there in the first place so it looks better (or should if done properly).
     
  5. Liam @ Prog AV

    Liam @ Prog AV
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    The difference seen is not brought about by the different number of lines but the amount of information stored in order to reproduce each line. This is the "bit rate" and signifies how compressed the original information has been in order to get it to "fit" into the storage space available on the DVD. HDTV has a much higher bit-rate (and so a much higher filesize hence not being able to fit a HDTV movie onto a DVD) which means that a HDTV file of similar resolution to DVD (or downscaled to a similar resolution) will still be a much more satisfying picture.
     
  6. Rahmorak

    Rahmorak
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    Thanks, it looks like I made an incorrect assumption re. how much compression would be used in the HD signals (I assumed it would be comparable to existing SD material). I was also discounting Sky due to the obvious issues with a crap bitrate (I MUCH prefer a good analogue signal to watching Sky, hair detail as an example is appalling in just about anything Sky related).

    I also assumed that a good quality DVD encoded @ or near 10Mbps was as near to lossless as the eye can distinguish.

    However:

    1) Does a progressive signal carry 2x as much information as an interlaced signal? e.g. if you have 576i@50Hz and 576p@50Hz (just talking theory!) does the 576p hold 2x the data?

    If NO then would a 720i signal not require a peak bitrate of approx. 22.2 Mbps (2.2x the pixel data) in relation to a 576i DVD encoding @10Mbps and using the same compression algorithms?

    IF YES then would a 720p signal not require a peak bitrate of approx. 44.4Mbps(2.2x2x10)?

    Either way, from what I can gather 720p tends to run a bit lower than either of these, just as SD encoded material tends to run < 10Mbps. As such the amount of compression should be comparable meaning the initial encoding loses a similar % of detail? As a result I would have thought any sense of increased detail/sharpness would be purely perceptual or an artifact of the downsampling process.

    R
     
  7. Bearded_Malc

    Bearded_Malc
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    BTW video recorders output the same line twice so they have half the resolution of broadcast TV. I believe most (old?) TVs can't resolve the full numbers of lines either but they are not as bad as half.
     
  8. Cliff

    Cliff
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    A few points to consider

    576p is possible for movie films. A film is run at 0ne frame every 25th /sec so a complete scan of 576 can be done and output twice every 50th by the internal DVD electronics.
    There is a bandwidth saving on the DVD here because the two fields are the same.

    Studio stuff from TV cameras (50th/sec) is a different kettle of fish as there is a difference between the two fields so it can't be doubled. This relates to very smooth movement compared to celluloid. Our present day TV cannot be displayed in a progressive form as it is not shot that way. For sure, all new plasma and digital sets try and convert to a progressive frame and interpolate between the two fields - some algorithms are more successful than others and hence the blur you sometimes see on movement.
    When we eventually get HD the TV cameras will be shooting 720p @50

    You are dead right about the compromise in fitting this on to a 480 display!
    These displays are great for NTSC DVDs at 480p - native res etc (sorry Panasonic people!)

    Also the post (while I am in typing mode) about video recorders....
    Actually they are 625 line devices- low bandwidth yes - but they do not show half the number of lines. Myth!
    The three or four head VHS machines can scan half the lines (one field) on still frame so you don't get judder between fields.


    Cliff
     
  9. Bearded_Malc

    Bearded_Malc
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    I suggest you read the spec in the back of any video recorder. It clearly says 312.5 lines (625/2).

    What happens is the VCR minimum of two heads read the same tape strip twice. The video signal is laid down in a strip across the tape. That way the tape can move slow enough to be feasible. Repeating the same line on alternate fields saves tape and was another compromise made to get video on to tape. You still get 25 frames per sec displayed as 50 interlaced frames but since both the frames are same, unlike broadcast TV, the effective resolution is halved.

    The number of heads is only material when doing trick stuff like still frames. That way you get a more continuous signal. Two heads are marginal since the VHS tape path is only 180 degrees. The underlying tape format is still the same.

    However, it you want to be really picky then the colour signal is only half the resolution too since that's what PAL means Phase Alternate Line (or Perfection At Last compared to NTSC, Never Twice Smae Colour). Two lines are added together to get the colour which cancels errors. But the eye saves the day as the B&W signal is 625 lines and the eye defines the colour to edges same as it did for the Victorians when they coloured their photos.
     
  10. Cliff

    Cliff
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    I can't agree on either count - although I could be wrong - has been known!

    The VHS signal is recorded on to tape the same way as it is played back ie no lines repeated on playback- anyway if you can record 625 you can play them back.
    The video drum has two heads and one head records a field on a stripe on tape diagonally top to bottom and the other head records the next field.

    These are played back in the same way to give a full frame. If the two fields were the same it would be noticable on movement- like film.

    On a two head machine you see judder on still frame as two frames are shown with movement. The third head was introduced so that you do see one field (similar to what you are describing). In this case the third head plays back the same field so you get two fields to make up a frame with no movement.

    Now the PAL statement.
    Sorry to disagree again but, although low bandwidth, the colour information is sent at the start of each line - yes all 625 lines. The Phase of the colour burst is averaged between lines so colours are true with phase differences in transmission. This was the problem with NTSC
    As a footnote SECAM colour is half the number of lines as you describe!
     
  11. Bearded_Malc

    Bearded_Malc
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    Thinking about this I can't see how mechanically two heads in the same slot can cover one field twice so they must read/write both fields. By the same argument still frames must be made up of the same field twice since the tape doesn't move.

    However, search for VHS tape and lines and all sites say VHS is only 250 lines, e.g. here. S-VHS is more (400) which is why it is better. So quite how that works I don't know. Since it was half I had assumed to save tape the same field was scanned twice but that doesn't work.

    The phase burst is averaged but so is the line (either by the eye cheap TVs, probably not the case these days). See page 10 here . The same phase error that would affect the burst would affect the information so you need to average both.
     
  12. Roswell

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    VHS is quoted as having about 250 lines. What this means is that about 250 alternating black and white vertical bars can be resolved by the analog horizontal scanline of a CRT. Basically, the vertical bars are gradually compressed horizontally across the screen until they are no longer visible as separate bars. When this happens the number of bars is quoted as the resolution of the display system. As you correctly say, this is about 400 for SVHS and 500 for a good analog broadcast signal.

    It is a bit confusing as it does not relate to actual scanlines or to the pixels of a digital display. It is describing HORIZONTAL resolution in terms of vertical bars. The vertical resolution doesn't need measuring as it is obviously the fixed number of horizontal scan lines eg. 625 or 525.

    I don't think it relates in any useful way to digital systems where the display's native resolution is fixed and data compression and hardware limitations are the important factors.
     
  13. Cliff

    Cliff
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    Bearded Malc,
    I did just check my facts. Yes the VHS video is 625lines and each head scans a field, and is then switched to the other head (180deg on the drum)

    "By the same argument still frames must be made up of the same field twice since the tape doesn't move."
    Yes, that is correct and that is what the third head is for. It follows the same path as one of the other heads to give a frame of two identical fields.


    As you say, you were looking at the horizontal resolution which is 240 lines. This is the bandwidth and how good it can resolve a picture of vertical lines - nothing to do with the actual lines.

    Interesting link by the way. - and as you can see PAL colour info is sent for each line. The colour is not averaged over two lines - it is the phase of the (suppressed) colour subcarrier. This eliminates colour shifts that you see on NTSC stuff that has to be compensated for by the tint control
     
  14. Bearded_Malc

    Bearded_Malc
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    Very strange. I'm certain I remember a video manual saying 312.5 lines in the spec which seems an odd spec if they meant bandwidth - perhaps the little grey cells are playing tricks.

    In that ref, top of page 11 it says the colour over two lines IS averaged. If you think about it any phase error will affect the whole line, burst and signal. So you have to compensate both. So you shift the phase on alternate lines then average out after compensating for the shift which in effect inverts the phase error. Hence line 1 is +error and line 2 is -error. Add the two lines and you get zero error. Same for phase burst. Both phase burst and line need to correct to get the correct colour. Adding lines is either done by the eye, or electronically. Electronically removes the blinds effect phase errors can give otherwise so I suspect that is the norm these days.
     

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