I started a long reply, but decided to cut this down to specifically cable.
Somewhere between 3 out of 4 HD channels (using HD stations I watch) or 4 out of 5 HD channels (from examining this Wikipedia page
) transmit 1080i (1920x1080
, interlaced), the other 1 out of 4 or 1 out of 5 are 720p (1280x720
, progressive). In both cases, the aspect ratio is precisely 16:9, or approximately 1.78:1. (Both 1920x1080 and 1280x720 formats use square pixels, which is a contrast to SD that I'll mention below.)
Your 1080p display device is probably also 1920x1080 (pixels per line x number of lines), so if you are watching a 1080i station, no scaling takes place. (Deinterlacing does take place, but if there is little movement, you are watching 1920x1080.) If you are watching a 720p station, some upscaling has to take place, expanding the incoming signal by 50% horizontally and 50% vertically. Yes, that means a lot of the image on the screen is interpolated, but there is enough data that the scaler in most TVs do a decent job of displaying 720p content on a TV with a native resolution of 1080.
A standard definition signal, such as from a DVD or from a standard definition cable box, is 480i (720x480
, interlaced), and usually has an aspect ratio of 4:3 or approximately 1.33:1. (But 720/480 is not 1.33! True. But from what I read, only 704 of the 720 pixels per line are used [the missing 16 pixels per line being an "overscan" margin to make sure part of the original analog image wasn't lost], and then each pixel represents a slightly skinny rectangle of the picture, the width being 10/11th of the height and 704/480*10/11 is the 1.333... that we were expecting in order to fill the screen of an old fashioned 4:3 old standard TV.) Ok, we should really call this 704x480
, not 720x480. (Some DVDs are anamorphic widescreen. They do this by using pixels that are "fat" or the width is 40/33rds the height, so 704/480*40/33 = 1.777... which would fill the screen of a modern 16:9 HD TV, which means both 4:3 DVDs and 16:9 DVDs use the same number of pixels so use the same amount of space on the disc.)
To boil this down and looking at the resolution, we have:
|Source|| Resolution || Scaling Factor (hor. x vert.)|
|(the 1080 HDTV)||1920 x 1080 || 1 x 1 (included for reference)|
|1080p Blu-ray||1920 x 1080|| 1 x 1 (no scaling)|
|1080i stations||1920 x 1080||1 x 1 (no scaling, Note 1)|
|720p stations||1280 x 720||1.5 x 1.5|
|SD cable box||704 x 480||2.05 x 2.25 (Note 2)|
|SD cable box zoomed||704 x 360||2.73 x 3 (Note 3)|
|fullscreen (4:3) DVD||704 x 480||2.05 x 2.25 (Note 2)|
|anamorphic (16:9) DVD||704 x 480||2.73 x 2.25|
|Note_1:||While 1080i doesn't require rescaling to 1080, it does require deinterlacing since LCD and Plasma displays and some other display technologies are progressive-only displays.|
|Note_2:||Expansion is to 1440 x 1080, not to 1920 x 1080, because the aspect ratio of the incoming signal is 4:3 and so needs only 75% of the width of the 16:9 display to preserve the 4:3 aspect ratio.|
|Note_3:||If viewing 16:9 HD channel letterboxed into a 4:3 SD channel for a SD box, and then zooming that to fill a HD screen, 120 lines of the 480 contain the letterbox bars and the picture is contained in the remaining 360 lines that then have to fill the 1080 lines of the display device|
The "Scaling Factor" indicates how much data the scaler would have to extrapolate, from not extrapolating (1x1), to generating 78% of pixels on the screen (SD cable box), to generating 88% of the pixels on the screen (SD cable box zoomed 25% so letterboxed HD picture fills the screen). As you can imagine, if every dot on the screen comes from the original source, the resulting picture is far better than if the scaler in the TV (or Receiver, or Blu-ray player) has to create lots of dots of the picture between each received dot, and the worst case I presented with the SD cable letterboxed image of a HD channel forcing the scaler to produce 88% of what you see on the screen produces the least amount of detail and does the maximum magnification of any compression artifacts.
The scaler cannot recreate the original dots that are missing in the picture. Instead, it has to interpolate the picture data from the information that it receives; the less information, the more interpolation and the less accurate the resulting image will be. This is why on a close-up head shot on TV, if the source is Blu-ray or a HD TV channel, you might be able see every individual strands in a lock of hair on the head; but for a DVD or an SD TV channel, for the same size head shot on the TV, you can see curls and locks, but not individual hairs in a lock of hair. Or when watching the Planet Earth series, on the Blu-ray you can see the fine details in the beauty of nature, but on DVD, without as much detail for the TV to work with, the fine detail is more of a slight blur.
Why, then, do standard definition cable channels or standard definition satellite channels don't look as sharp as DVD, especially in action scenes or other quickly-changing scenes? Because cable operators and satellite TV services do a lot more compression to squeeze more channels onto the cable or into the bandwidth the satellite service is allocated, and the result is that there can be compression artifacts, like macroblocking (as bright explosions happen as the Japanese army fires at Godzilla in "Godzilla 2000" during the flashes there are a bunch of bright square regions that briefly appear and quickly change into the features of bright bolts that they should have been), pixelation (sometimes a staircase or jagged edge effect on what should have been a smooth line or curve) and, on one subchannel, I have noticed at times with just one show that there will be brief flashes of purple along the bottom edge.
I have noticed compression artifacts far more often on SD channels than on HD channels. Maybe my local cable company figures that those watching SD are doing so on smaller TVs, or maybe it's that a macroblock is a smaller piece of a 1920x1080 image than a 704x480 image, so when the scaler scales up a macroblock on a 704x480 image it is 2.05 times wider and 2.25 times taller on the TV than a macroblock of a 1920x1080 image.
The larger scaling factor and the larger screen both act as a magnifying glass on visual defects, making any visual defects and loss of detail more painfully obvious. (That's why DVDs of some 1950s shows or some VHS tapes looked good on a 21-in SD screen, but look terrible on a 50-in screen.)
I guess that is a lot of writing just to say that if you are expecting to see HD cable, you cannot substitute an SD cable channel, unless you have a really small TV. And the best scaler and the best TV in the world won't make up for having an SD source.
Maybe one thing would substitute: if your area gets good over-the-air TV reception, using an actual antenna and tuning to a station will get that station's HD channel (usually the .1 subchannel) and possibly other subchannels (e.g., .2 or .3 as an SD channel for alternate programming or a digital multicast network that got affiliated with that station for broadcast purposes). (Some stations may have both the .1 and .2 subchannels be HD and the rest be SD; one of my local stations has nothing but SD subchannels, 5 of them. It is up to the station to decide how to divide up the bandwidth they had been allocated, but the more they try to transmit in their allocated bandwidth the more compression artifacts there will be, especially in action scenes or other quickly-changing scenes.)
PS: I just did some checking of SD vs. HD channels on my cable system for which there is HD content. For local channels, some apparently do center-cut or pan-and-scan (couldn't tell which with my quick checking) and some have the HD channel letterboxed down into the 4:3 image size of the SD channel. For the non-local HD channels, it appears that some just letterbox HD down to the SD channel, and some have separate feeds but it looks like where the HD channel has 16:9 program content, the program content is letterboxed down to the SD channel, with some channels having their logo and other annoying advertising overlay in the letterboxed area (e.g., Cartoon Network), and some moving the logo and part of the annoying advetising overlay into the letterbox bars (e.g., The History Channel).
In any case, SD channels just will not give the picture quality that HD channels do, and no amount of massaging the data will create the detail that was lost by downscaling the picture to an SD channel.