Capture card / Resizing

[themaster1]

I was wondering something today . All SD captures cards by default capture at 320x240. if you want to capture a vhs you usually pick 720/704x480i (custom settings) ;
The problem is 320 x 2=640 (not 704 or 720) so i was thinking today do they (the devs) use something like nearest neighbor to upscale horizontally and in such case wouldn't it be better to cap at 640x480 to gain that extra quality (it's more about not losing it actually = blurriness) and use a spline36resize with avisynth instead if i want 720x480 dvd frame size.

a reminder for vhs resolutions:
ntsc vhs:
luma-333x480
chroma 40x480

pal vhs:
luma: 335x576
chroma: 40x240

[manono]

All SD captures cards by default capture at 320x240.

I think your premise is flawed.

[themaster1]

Well i've owned 3 capture cards over the years and at times i've tried them with graphedit, 320x240 came up each time as the default resolution. Sorry for bursting your bubble

[KarMa]

Well i've owned 3 capture cards over the years and at times i've tried them with graphedit, 320x240 came up each time as the default resolution. Sorry for bursting your bubble

My hauppauge has always outputed 720x480. Was not until this thread that I realized my card even supported 320x240, as I've tried changing it before and it would always say it could not support my custom capture size besides 720x480.

[jagabo]

All SD captures cards by default capture at 320x240.

No. Some may default to 320x240 but that doesn't mean they can only capture at 320x240.

if you want to capture a vhs you usually pick 720/704x480i (custom settings) ;
The problem is 320 x 2=640 (not 704 or 720) so i was thinking today do they (the devs) use something like nearest neighbor to upscale horizontally and in such case wouldn't it be better to cap at 640x480 to gain that extra quality (it's more about not losing it actually = blurriness) and use a spline36resize with avisynth instead if i want 720x480 dvd frame size.

Since your initial premise is wrong this question doesn't follow.

VHS is a continuous analog waveform. You can capture a scan line with as many or as few horizontal samples as you want. But that waveform has a limited bandwidth -- meaning you can capture all the detail of that waveform by sampling above that bandwidth frequency. If you capture far fewer samples you will miss some detail. If you capture far more samples you are just increasing the size of your cap. For VHS that bandwidth limit is around 320 samples (at best) across the width of the screen. That doesn't mean 320 samples will get you a perfect cap but 319 will be flawed. When the frequency of a source approaches the bandwidth of a circuit the amplitude of that sine wave will decrease. The bandwidth of a circuit is often specified as the frequency that gives a 50 percent reduction of amplitude. Here's a sinusoidal waveform that increases from left to right captured from a DVD:



At the bottom you can see that the amplitude of the waveform stays constant until near the right edge. The same signal, recorded onto VHS tape and captured again:



The rise of amplitude in the middle is from the sharpening filter in the VHS deck. But you can see the amplitude drops off at lower frequencies than the DVD cap. And that drop in amplitude appears as lower contrast in the image above. The brights no longer go to full white, the darks no longer to full black.

A device that's capturing at 720x480 should be sampling that waveform (at least) 720 times across the width of the picture. Many actually capture more (for example, 2x more) and then scale down to 720.

Vertically, the signal actually has 480 separate lines (actually a few more) so 480 are captured when capturing 720x480.

[KarMa]

Stunning post as usual jagabo.

[davideck]

But you can see the amplitude drops off at lower frequencies than the DVD cap. And that drop in amplitude appears as lower contrast in the image above. The brights no longer go to full white, the darks no longer to full black.

Stated another way;

Reducing the sampling rate (and analog bandwidth) also reduces the maximum rate at which an image can transition from full black to full white. Sharp vertical edges will be stretched into softer ones.