Possible to adjust front/back porch on SAA713x card?

[s0ren]

I am using an SAA7131 based PCI capture card to capture PAL VHS and other PAL analog video sources with Virtual Dub in WinXP.
I think the front/back porches are a bit too wide and the image is squeezed a bit. Not much, but noticeable.

The screenshot attached to this post shows the PM5544 test card (screenshot adjusted for aspect ratio) and the circle is not quite wide enough to be a perfect circle.

Is there a way to configure the capture card, perhaps the driver, to include less back/front porch, or is it only possible with post processing filters (with subsequent fidelity loss)?

Is it "normal" and should I just ignore it, or are there better capture cards (preferably PCI)?

Overall I am pleased with the capture quality, only the geometry bothers me a little.

PS. thanks to everyone who have contributed to this forum over the years. I have read and learned a lot by coming here, but never before participated.

[dpalomaki]

The SAA7131 is listed as a "Full MPEG1 Video and Audio Decoder" not a capture/encoding device.
What is the actual capture card you are using?
What does MediaInfo report for your captured video file?

Front porch/back porch terms apply to the analog signal sync/blanking intervals and are not part of the active image/picture area of the scan line. Without seeing the actual waveform it is not possible to tell whether or not the front/back porch are of standard duration.

The captured image will be based on the sample rate, number of samples per scan line, and the start of sampling which is normally based on a set time interval after the sync pulse. The usual standard for SD video is 13.5 mHz sample rate and 720 pixels per line.

The displayed image shape (aspect ratio) is based on how the display device interprets the digital file.
FWIW computer graphics were based on square pixels, but digital SD video is based on non-square pixels. Digital SD video is typically 720 pixels per line, but many capture devices limit the image to 704 active image pixels, the lost pixels being notionally in the over scan area (legacy from CRT-based TV sets). Pixel aspect ratio is as required to preserve a 4x3 image.

[latreche34]

It's because the digitized analog video active area is 704x576 not 720x576, Before you resized to 768 you should have cropped to 704 and resized to 768 to get an almost perfect circle, try it.

Dpalomaki beat me to it.

[s0ren]

The SAA7131 is a hybrid, it supports DVB, Analog TV, S-VHS and composite. It is the capture device. In virtual dub the resolution is reported as 720x576. The reason i resized to 768 for the screenshot was to make it easier to check the geometry.

There are no lost pixels in the black area, but after checking I can see that the image is indeed 704 pixels wide like you said latreche34, and then black bars are added to get to 720 pixels. It seems like this is what is happening, and not the card oversampling into the porch areas. It is weird that the card or driver is doing this "letterboxing" thing, instead of either upping the sample rate to get to 720 pixels wide, or using the correct resolution width of 704 (simply setting the capture resolution to 704x576 still has black bars, so the black bars seem like a deliberate "feature").

Luckily 704 × 576 pixels is a supported DVD resolution so it should be no problem just cropping 8 pixels from each side, and still get a somewhat "standard" video file - with the advantage of correct aspect ratio of course. Thanks for the help!

[RobustReviews]

I can see why you're thinking it's in the 'porches' but alas not; it's a reasonable question though if you've not dealt with analogue source conversion though.

Why it is like this I've not the foggiest, I'm sure there's some logical reason that isn't presenting itself to me at the moment.

This is one of the places a (CRT) monitor is handy for diagnosis.

[latreche34]

Quote:

Originally Posted by s0ren

There are no lost pixels in the black area, but after checking I can see that the image is indeed 704 pixels wide like you said latreche34, and then black bars are added to get to 720 pixels. It seems like this is what is happening, and not the card oversampling into the porch areas. It is weird that the card or driver is doing this "letterboxing" thing, instead of either upping the sample rate to get to 720 pixels wide, or using the correct resolution width of 704 (simply setting the capture resolution to 704x576 still has black bars, so the black bars seem like a deliberate "feature").

Before rec.601 standard was established, engineers did the math and found for analog tape formats and their frequency bandwidth a scan line can be fully produced digitally with 704 samples per line, The added 16 pixels are just a safe measure. TV camera's and telecine machines back then didn't have a precise frame, so sometimes there is useful video in the area that was designed for line and frame timing (overscan) and ignored by TV sets, so engineers allocated 8 pixels horizontally on each side to accommodate for that and recover it during the digitizing process (sampling).

Vertically the number of scan lines is defined however not all of them are video so the choice was 486 for NTSC (480 for the entire frame including the head switch at the bottom, 6 lines on top for signaling such as caption, teletext and other flags). and 576 for PAL/SECAM. The remaining lines are for frame timing and do not need to be captured.

So when capturing, depends on the tape content, you may have to crop only on one side or on both sides depends on where the edge of the active video is, the same goes for vertical cropping if you happen to have a pro capture device that can capture at 486, you can use that 6 lines to crop the head switch at the bottom.

Note that consumer capture cards only capture at 480, cropping off 6 lines on top automatically, Also some but not all consumer capture cards crop off 8 pixels on each side automatically.

[dpalomaki]

Whether or not to crop and scale can also depend on the intended viewing system. Most TV sets, including new HD sets, will by default over scan sources such as VCRs, DVD players, and cable boxes. Thus cropping and scaling may lop off wanted video. On the other hand they might not over scan PC inputs using the D-15 (AKA VGA) connector. It is equipment specific and may be configurable, but most Joe and Jane Sixpack viewers do not understand this. Do what what will best fit your intended audience.

[latreche34]

Most captures are viewed from digital files from USB or via HDMI, I don't think there are a lot of people left who watch tapes from VCR's or DVD players using analog inputs on a 4k/HD TV. Most TV's now have "just scan" feature and few years from now overscan will have no purpose. So I don't think one should worry about having clean frame edges in 2022. DV, D8 and DVD did it over 2 decades ago.

[dpalomaki]

It is equipment specific. For example, the Hitachi TV I am using currently offers 5% and 0% overscan options for HDMI, DVI, and VGA( inputs at common video resolutions (1080p/1080i/720p/480p/480i) and for VGA (analog RGB) inputs. (But not for RF, AV, or component video inputs.)

If the video was shot with "action safe" composition in mind it likely doesn't matter.