Rectangular pixels from TV-out?

[harp11iv]

I would like to play MPEG-2 files on a PC and watch them on a big TV. Let me confine the discussion to the standard-definition NTSC, 4:3 aspect ratio, for simplicity.

Many ATI graphics boards like ATI Radeon HD 2600 XT are equipped with the feature of hardware MPEG decoding. However, their TV-out terminals cannot output rectangular pixels.

A computer display consists of square pixels. On the other hand, an NTSC TV consists of rectangular pixels. That is, each pixel on NTSC TV is vertically elongated. In other words, each pixel on NTSC TV is horizontally slim. Let the vertical resolution be fixed to 480 pixels as prescribed in the "480i" format. Then, the 4:3 display aspect ratio will be 640 x 480 on a computer display, and be 720 x 480 on a NTSC TV.

Since DVD-Video was originally designed for TV, the resolution of MPEG2 picture embedded in NTSC DVD-Video is 720 x 480. To play such DVD-Video on computer monitor, most applications convert the resolution from 720 x 480 into 640 x 480, losing some resolution. This lossy conversion is one of the reasons why the picture quality of DVD played on a computer and displayed on a computer monitor is not so good as played on a stand-alone DVD player and displayed on TV.

If a graphics board with hardware MPEG decoding capability can output rectangular pixels from TV-out, then I would probably not have to buy a separate hardware MPEG decoder board. Can any graphics board of any brand output rectangular pixels? What about NVIDIA? What about Intel SDVO-based TV-out? (SDVO is Serial Digital Video Out.)

If you know of any graphics board (or on-motherboard video) that can output rectangular pixels from TV-out, please drop a comment. When you drop a comment, please also mention whether that device is equipped with a hardware MPEG-2 decoder, if you are aware.


Note: Even if the "Display" control panel lets you select the 720 x 480 resolution for your TV-out, it does not necessarily mean that your TV-out can output rectangular pixels. For example, the TV-out on ATI Radeon 9600 XT can output only square pixels. If "720 x 480" is selected in the Display control panel for the TV-out (or S-Video) port, then ATI Radeon 9600 XT will shrink the image of the size "720 x 480" down to the size "640 x 426", add ugly black horizontal bars at the top and bottom to compensate the shortened height, and output a "640 x 480" resolution signal with square pixels to TV. As a result, not only the horizontal resolution (720) but also the vertical resolution (480) will be lost. The significant vertical resolution will drop from 480 to 426. The picture quality will be very poor.

[jagabo]

I never have problems with display aspect ratio and TV out (composite, s-video, DVI/HDMI, via Nvidia, ATI, and Matrox cards). Just use a player that supports DAR flags. Like MPC, VLC, KMPlayer. Unless you're using a 15 year old CPU there's no need for hardware MPEG decoding for standard definition video. For example, VLC playing a 16:9 NTSC MPEG file only consumes about 2 percent of my Core 2 Duo CPU.

Actually, the Nvidia and ATI cards have minor issues with aspect ratio and composite/s-video but not as much as the difference between 720x480 and 640x480. Matrox is absolutely perfect.

[harp11iv]

I never have problems with display aspect ratio

Of course, not. My original post explained why not. Media player applications convert 720x480 into 640x480 to avoid the distortion of display aspect ratio, when displaying a rectangular-pixel source on a square-pixel monitor. However, by doing such conversion, the clarity of the picture is lost. It is best to maintain the pixel aspect ratio and not to downgrade the resolution.

My questions in the original post are *not* about display aspect ratio. My questions are *not* about the symptom in which everybody looks too fat or in which everybody looks too skinny. I am talking about the shape of pixel. I am talking about the clarity of picture but not the shape of the picture.

Do you understand the distinction between "pixel aspect ratio (PAR)" and "display aspect ratio (DAR)"? If you do, then I will have to look for another reason why you misunderstood my questions. Some other readers may have misunderstood, too. Perhaps, were you confused when I said "NTSC TV consists of rectangular pixels?"

By the way, the PAR of a square pixel is 1 (i.e., 1:1). The PAR of a vertically elongated rectangular pixel is smaller than 1 (e.g., 10:11).

VLC playing a 16:9 NTSC MPEG file only consumes about 2 percent of my Core 2 Duo CPU.

Interesting. Thanks for that info.

[jagabo]

Perhaps, were you confused when I said "NTSC TV consists of rectangular pixels?

NTSC video doesn't contain any pixels. It's a continuous analog waveform. This may seem pedantic but it's an important distinction.

Media player applications convert 720x480 into 640x480 to avoid the distortion of display aspect ratio, when displaying a rectangular-pixel source on a square-pixel monitor.

Actually, most players do not scale the video, they rely on the graphics card's hardware video overlay to do any scaling. A 720x480 image is written to video memory as a 720x480 array. The graphics card uses hardware scaling to adjust for the aspect ratio on the display(s). When windowed on the computer monitor with a 4:3 DAR that will be digitally scaled to the equivalent of 640x480 or 720x540 resulting in a loss of sharpness and/or moire artifacts. (Yes, you will never get an artifact free display at the correct AR in a window on a digital monitor because of the monitor imposes a square pixel reqirement on the overlay.) On the composite or s-video port the video can be scaled by adjusting the horizontal clock frequency of the video DAC -- no digital scaling need be performed. The output isn't as sharp as the equivalent 720x480 (non-scaled) image on the monitor because the video is converted to YUV format and passed through a low-pass filter (especially the chroma components) to conform to NTSC requirements.

My Matrox APVE can resolve all 720x480 lines (black/white) at the s-video port (well the horizontal 720 is close to blurring into a solid gray but it's otherwise artifact free). I have used to have an ATI card in that computer and it too was able to output a nice clean signal when set up correctly (overscan, AR, flicker filter, etc). Unfortunately, the driver would occasionally change settings for no apparent reason so I'd have to change them back.

[edDV]

...
A computer display consists of square pixels. On the other hand, an NTSC TV consists of rectangular pixels. That is, each pixel on NTSC TV is vertically elongated. In other words, each pixel on NTSC TV is horizontally slim. Let the vertical resolution be fixed to 480 pixels as prescribed in the "480i" format. Then, the 4:3 display aspect ratio will be 640 x 480 on a computer display, and be 720 x 480 on a NTSC TV.

Since DVD-Video was originally designed for TV, the resolution of MPEG2 picture embedded in NTSC DVD-Video is 720 x 480. To play such DVD-Video on computer monitor, most applications convert the resolution from 720 x 480 into 640 x 480, losing some resolution. This lossy conversion is one of the reasons why the picture quality of DVD played on a computer and displayed on a computer monitor is not so good as played on a stand-alone DVD player and displayed on TV.
...

First I agree with jagabo that S-Video, composite NTSC and YPbPr output from the display card RAMDAC is not output in horizontal pixels but 704 pixels (active line) are clocked out by the DAC to fill the 51.5 microseconds active NTSC TV line area between the sync pulses (see "C" below).



704x480 originated with the CCIR-601 standard in the mid 1980's which was a unified digital component standard that used a common 13.5MHz sample rate for both NTSC and PAL. This resulted in a 704x480 pixel raster for NTSC @29.97fps and a 704x576 pixel raster for PAL @25fps. Further refinements to the standard specified two pixel aspect ratio definitions for NTSC, one for 4:3 aspect ratio (PAR = 0.9091) and one for 16:9 (PAR = 1.2121). Note that both 4:3 and 16:9 aspect ratios use the same 13.5MHz sample rate and 704x480 raster.

When the 704x480 raster is converted to square pixels for computer display 4:3 results in a 704 x 0.9091 = 640 horizontal pixel by 480 square pixel display. Likewise 16:9 results in a 704 x 1.2121 = 854 horizontal pixel by 480 square pixel display.

None of this relates to the "bandwidth" of digital to analog video which remains 704 pixels to analog 51.5 microseconds active TV line area. This is viewed from the perspective of analog to digital to analog conversion. 13.5MHz sampling results in a thoretical 6.75MHz Nyquist bandwidth for both 4:3 and 16:9 video. Actual bandwidth with realistic filters is closer to 6.25MHz (~500 horizontal lines of resolution) or less. Horizontal lines of resolution refers to a circular area in the center of the screen so that horizontal and vertical resolutions are normalized. NTSC component digital is biased with more horizontal than vertical resolution.



The CCIR-601 standard added 8 pixels to the left and right of the 704x480 raster to serve as guard bands to protect the active 704x480 raster from side clipping through transmission and repeated A/D D/A conversions. This was the origin of the 720x480 specification. Note that PAR is the same for 704x480 or 720x480 rasters.

Note also that ATSC digital broadcast uses 704x480 without the left and right pad pixels.

[jagabo]

Also note that when an analog video device is fed a square wave (alternating black and white pixels) approaching its bandwidth limit the result is a loss in contrast as the swings from black and white get shallower and shallower, eventually averaging out at a medium gray. Digital devices will develop moire artifacts.

[harp11iv]

I never have problems with display aspect ratio and TV out (..., via Nvidia, ATI, and Matrox cards).

Is your operating system Linux, jagabo? Did you manage to use Nvidia, ATI, and Matrox graphics cards in a Linux environment?

[jagabo]

I've only used TV output with Windows (and some earlier computers like the Amiga).

[edDV]

So long as you can find a display card driver for Linux, none of the above or operational modes for DV or DVD should be any different under linux. I've managed to get all my NVidia or ATI display cards working with Linux. Not so for tuners.

I've never used the Matrox dual out card.

[Video Head]

I have used an ATI AIW Radeon 9800 Pro on SDTV's through component and s-video. The s-video leaves much to be desired picture wise and has limited ability to make adjustments. Using the component out with custom timings, flicker filter, gamma settings, etc it is possible to achieve a picture that rivals stand-alone DVD players.