How do I capture my sd video in digital format?

1. adc capture cards

Before you even think about connecting your signal into one of these, you will probably want to make sure that you have a Time Base Corrector (TBC) at the end of your chain.  Especially if you are working with anything circuit bent or home brew video synth/fx that have potential to send your signal out of expected ranges.  Almost any sd video mixer will have one though so it's not usually necessary to get a separate rackmount TBC or a broadcast VCR to add to the end if your signal chain.

Blackmagic Intensity Shuttle

This is about the best quality and affordable capture method that does not involve literally videotaping something off of a screen.  They come in usb 3 or thunderbolt editions, both of which seem to work equally well but the usb 3 ones are cheaper for some reason.  They can take in rca composite, s-video, component, and HDMI and also can pass thru whatever signal it is they are accepting.  They are not quite plug n play, you will need to download and install the Blackmagic desktop video SDK from their website.  First plug in your intensity shuttle, then run the desktop video .app or .exe and select the source you are using from a menu.  Then you can open a program (quicktime, adobe premiere, whatever windows freeware exists for this purpose) and capture the stream.  Adobe premiere has some fairly useful high depth control of settings for captures.  On osx there is a very useful program written called Black Syphon which can take the Blackmagic stream and send it to anything that can take a syphon input; there is also a handy syphon recorder app as well that seems like it is built out of quicktime but doesn't live stream while recording.  This will work with Osx, Windows, and Linux

Grass Valley Canopus (advc 100,110, 300)

These work a little bit different from the Blackmagic, the Canopus series can take in rca composite, s-video or dv and then send it into the computer as a DV signal.  DV is a compressed video signal with chroma subsampling and interlacing.  Long story short, the results will be a bit grainier and have less dynamic range than the Blackmagic captures with potential for artefacts from the interlacing and compression.  On the other hand you don't need to download an SDK or configure any menus and they are typically less expensive than the blackmagics used.  Keep in mind that unless you have some old computer hardware around that you will need to find some cables or adapters that will take you from the old school firewire dv cables to most likely something in the thunderbolt era.  Also keep in mind that if you are using just old school firewire cables that they will not supply power to the canopus so an additional power supply will be necessary.  These work the most effortlessly with Osx, windows and linux may require a bit of effort but is far from impossible.  

Ezcap/easycap/some random 20usd dongle you find on amazon

These are generally even lower quality than the canopus in terms of dynamic ranges being squashed and will usually encode the video into a compressed format like MPEG-2 which can be a bummer for doing post editing.  There are

2.Rescan (aka just pointing a digital camera at a screen and there you go)

Well its not actually as easy as all of that.  Unless you want to rescan off an LCD screen which is usually pretty easy! However unless you have a really nice LCD that does something like upscale sd to hd its not usually going to add much in terms of capture quality compared to rescanning off of a CRT.  Plus there is the issue where LCD's have an issue with contrast due to the even backlighting.  There may be some possible issues with the refresh rate of your LCD creating artefacting but usually when an LCD is receiving an SD video signal its going to automatically be refreshing 60 htz, so this would only be a problem if say there is a computer at the end of yr chain and it is connected to the LCD via VGA and the refresh rate is set to something non sexigismal like 70 or 85 or whatever.  This can be controlled via the computer.

digital camera rescanning CRTS

(note: this was written with NTSC video in mind, speaking broadly the same calculations will apply to PAL but replace 60 fields per second with 50 and 30 frames per second with 25)

there is going to be some math in here, warning!  theres no way around it though.  A CRT works by spraying 1 electron at a time in a linear manner akin to the way that we read text from the upper left hand side of the monitor down to the lower right hand side. It does this 60 times per second.  It doesn't spray the entire screen with each pass tho, in a process called interlacing it draws alternating frames (these are actually called fields so interlaced NTSC sd video signal is 60 fields per second but only 30 frames per second) in every other alternating Y lines like the illustration below

field 1 field 2
------   ____

------   ____

------   ____

------   ____

Each electron is shot out of a gun and generates a phosphor on the screen that glows for 1/30th of a second.  All of this happens fast enough for human vision to blur all of these strobing interlaced phosphor lines into 1 single perceived

In terms of capturing CRT images on a digital camera the two main things to be concerned with are shutter speed and frame rate of capture.  Shutter speed will affect the persistence of the electrons.  If your shutter speed is faster than 1/60th of a second you will notice black banding on the screen.  This is an artefact of how many phosphors have faded from the last time the electron gun spattered them onto the screen.  Ideally you would like your shutter speed to be 1/30th of a second which would mean that all the phosphors from both interlaced frames will be picked up by the camera.  Frame rate will affect flickering and artefacts from interlacing.  Recall that while the SD signal is drawing 60 fields per second, each of these is only 1/2 of an actual frame so 30fps is the goal.  It is also recommended to turn off auto exposure, auto focus, and any other auto correcting parameters on yr camera as well.  Moire is also a familiar friend in this situation, you are trying to capture one grid into another!  But defocusing your lens slightly can potenially help with that.

If you want to have enough control over this sort of thing on your cellular phone there is an app called filmic pro which people recommend as being very useful.  There is a free evaluation tool they provide so one should download that first to make sure that you have the right fps for the resolutions you wish, shutter speed control, and control of focus and light exposure.