Video Capture Guide

Part 1: The Basics

Hardware

Obviously my experience of video capture hardware is quite limited because I don't have unlimited money and I have only dealt with a couple of capture cards. Also keep in mind that hardware is improving all the time so when you search there may be more options than when this article was written. You will also be limited by how much you are willing to spend on your hardware and what you need it to do with it. In light of these facts I cannot recommend any particular card or TV tuner to use. I suggest you search around and read some reviews. Toms Hardware Guide is where I usually go to check out the latest info on hardware so check him out at: http://www.tomshardware.com/.

The expensive dedicated video capture cards designed for professional video editing are obviously the best quality but are usually out of most peoples price ranges. If you are very serious about it, there are some good quality and yet nicely priced cards sold for both home and proffesional use by Pinnacle Systems such as the DC10plus or perhaps even the DC50.

The less expensive Graphics Cards like the Nvidia GeForce, Matrox and ATI All In Wonder, and more recently the Radeon are good cheap alternatives. Just remember, despite claims of professional video quality, these all-singing-all-dancing graphics cards almost always produce worse results than a dedicated capture card. Of these graphics cards I think the current opinion is that the Matrox is the best quality, then the ATI (almost as good), then the GeForce and Voodoo ranges respectively. Many of these cards give the option of either Mpeg-2 or MJPEG hardware capture. Mpeg-2 gives best compression and MJPEG best quality especially if it is hardware based like the Matrox cards. If it doesn't offer any hardware capture option then its probably no better than a normal graphic card with a cheap TV Tuner stuck to it!

Basic TV Tuners cards are the cheapest option if you don't want a big upgrade and come with many low priced graphics cards. Because they are so cheap people assume they are not much good. Truth is, if your computer is 'man enough' then you will often be able to get equal or superior capture to those all-in graphics cards! In fact, I'd go as far as to say that if you have good enough hardware then you may be able to improve the quality of many graphic card captures by turning off hardware compression and using it like a TV Tuner instead!

What do I mean by 'man enough'?

The most perfect graphics format for video capture would be 'uncompressed AVI' with 'uncompressed PCM audio'. More specifically that is one uncompressed bitmap image for every frame along with a Wave file played in time to it. Many TV Tuners and Capture Cards will offer such an option or an equivalent lossless compression. But, especially at high resolutions most computer hardware will not be able to handle such an influx of data and you will get bad quality!

In light of this, most graphics cards use video compression as a standard part of capture to avoid problems. The ATI graphics cards may say something like 'AVI 1.0' but don't let the name AVI confuse you, it is actually a highly compressed format! Your hard disk can only save so much information per second. If you are capturing more information than your hard disk can take, you will loose frames and get bad, jerky video. To illustrate this effect, just try copying a 500MB file from one folder on your hard drive to another (don't cut and paste, just copy and paste). How long does it say it will take to copy? 3 minutes? 5 minutes? longer? So what happens if you are capturing 30 seconds of video at 640 x 480 in uncompressed AVI? Only 30 seconds of this format will take over 500MB of hard disk space! Your hard disk will just not be able to keep up! And this also highlights the other huge downside of uncompressed AVI - it takes a huge amount of space to store! For all these reason almost all video capture cards will offer to compress the video file directly to MJPEG, Mpeg-1 or Mpeg-2. But, if set correctly, any of these formats can give us VHS quality video.

A TV Tuner must use your computers Hard Disk and CPU power to compress or store the AVI in real time, so you will need to know that your machine is able to handle it. You certainly need a fast hard drive if you are capturing at 640 x 480. About 7200 RPM should be enough or 5400 RPM for 352 x 288 capture. I think Seagate do the best Hard Disks. If money is not a problem get SCSI HDD's because you can buy ones that reach speeds of 10000 RPM which will give you unlimited power in the storage department! On the other hand some of the newer IDE's are able to handle 25MB/s or more without trouble and are much cheaper. You will always need lots HD space, you can never have enough with video capture and 20 GB or more is probably a conservative estamate. I also suggest at least a 300Mhz CPU and an absolute minimum of 64MB RAM - the more the better of course.

Please bare in mind that all this is a rough estimate. Although most PC's exceed these requirements today, there are so many factors including slow graphics cards or motherboards that it is difficult to give exact requirements. If you are unsure I suggest you just invest in an ATI All in Wonder card or something like that rather then just waste money in the hope you can upgrade your system and capture better quality without one - you have been warned!

Video Capture Formats

MJPEG: will usually give the highest quality video especially if its hardware based capture. Instead of using bitmaps for each frame it uses compressed JPEG's images. As you know the quality of JPEG is almost as good as Bitmaps but very highly compressed. If you are capturing with a TV Tuner this is just about the only choice. The Picvideo MJPEG codec is the best quality I know of for real time capture.

Mpeg-1: Mpeg-1 capture is not fast enough to do in real time unless it is hardware based capture. I know there are utilities that will capture in Mpeg-1 in "real time" but the quality is crud even on a machine with a very high CPU. Lets face it, why does it take many hours to compress to Mpeg-1 or Mpeg-2 from an AVI with Panasonic Encoder or TMPGEnc etc., if you could do it in real time with the same quality?! Most of these real time captures use the ligos codec to compress, but the Ligos Mpeg Encoder cannot capture real time, so I think that says it all!

Mpeg-2: is the next highest quality to MJPEG But there are many problems with this format. Again it must be done by hardware compression because software compression is far too slow to produce any good quality. It is also a format that is very hard to edit. VirtualDub will not open an Mpeg-2 file unless we use Flask Mpeg or AVISynth to frameserve it. This makes editing very slow indeed.

Note: As you probably know by now, Mpeg-1, 2 & 4 compress like JPEG, but to increase effectiveness they also compare two or more frames in a sequence and only save the differences. This means they contain frames with only parts of the full picture in them. These half frames are called B frames and P frames. The full JPEG-like frames are called I-Frames. Many Mpeg capture cards such as the ATI All In Wonder will offer to capture as I-Frame only - in other words, as 'full frames' only. Capturing this way will increase the quality a lot but it will take up almost as much space as an MJPEG. This is because, with all the technical difference aside, I-Frame is basically the same compression as MJPEG. Other advantages are it takes less CPU power to compress and I-Frame only Mpeg-1 is fully editable in VirtualDub! You will also have the advantage of Mpeg-2 audio compression so you can capture for a little longer than MJPEG. If you are going to capture this way I suggest you turn off all motion compensation settings too.

Mpeg-4: is the best compressed format and you can use VirtualDub to capture directly to Divx or MS Mpeg-4. But again this will not produce good quality results because, like the other Mpeg formats, it takes too long to compress. It would only be useful if you intended on very low bitrates where quality was not a high factor.

Framerates & Resolution

If you are going to edit and/or compress a video using VirtualDub or any video editing package then the better the original video the better the final result will be. Try to use the highest settings you can without going overboard. For example, there is no need to struggle to capture a video at 640 x 480 pixels if you are going to resize it to 352 x 240 because not only will this take a lot of time to resize, but it will take twice the hard drive space! The difference in quality between the two will be very small so why not just keep to 352 x 240 instead.

VHS Resolution

Concerning resolution, you should always keep in mind that VHS video is roughly 320 x 240 pixels. Even so it may be best to keep to 352 x 288 PAL or 352 x 240 NTSC if you wish to follow VCD standards. This fact sounds really weird because a TV's active picture has 480 lines and a DVD resolution is 720 x 480 pixels. Some, in a desperate attempt to capture full quality video, will make DVD resolutions of 720 x 480. Let me tell you now that these captures are not even close to DVD quality no matter how good the capture! Neither have I just picked the size 320 x 240 out of thin air either. The reason for this is based on a value of clarity that mathematicians call the Kell factor. The Kell refers to the amount of detail displayed, in this case, on an analogue TV. To simplify a rather complex subject down to the bare bones we could say that a TV picture is very fuzzy and a TV "pixel" works out to less than half the clarity of a computer pixel! This means a computer resolution of less than half that of a TV is needed to store the same information. What is worse is the fact that, if the quality of your VCR or recording is not very good, or if you are using a camcorder, you are probably looking at little more than 280 x 210 pixels or something like that!

Note: I haven't done the exact mathematics on this yet, but a DVD resolution must work out to over double the quality of VHS video! Which brings up the subject 'why do we need high definition DVD's if they are already so close to HDTV quality, or have I just confused myself on this one?'

What does this mean? That capturing at 352 x 240 will give full VHS resolution? Possibly, but its not quite that simple! It may depend on how your capture card works. If it disregards lines then actual information is missing when it resizes. What would be needed is to construct a full image from all 480 TV lines and resize them down to the equivalent PC pixels. For example, if you want full detail you can capture at 352 x 480 and then resize precise Bicubic with VirtualDub to 352 x 240 (or 288). This should preserve most of the fine details and I personally think this is a good alternative. As you can see, assuming we need 480 pixels high for total quality, the pixels across still remain the same, about 352 pixels. In light of this problem Mpeg-2 was developed with yet another compromise which is supported by the ATI capture card. It can capture at 352 x 480 and then when played back stretch the video back to 720 x 480 to keep the correct aspect ratio. This takes much less space since it is resized only when it is played back and stops all the arguments.

Interlace

I have explained the problem of interlace in great detail Here. So I don't need to spend too much time on it. Basically there is little you can do to solve it other than using VirtualDubs deinterlace filters. How bad the problem is, or if you get it at all, will depend on how your graphics card captures. If you only capture one field (i.e. every other line making 240 or 288 pixels) then usually you won't have the problem. Sometimes you will be given the option to capture both fields (i.e. 480 or 576 pixels) and blend both fields together, check the settings of your capture device. There is a rumour going around that capturing to Mjpeg or Mpeg-2 will solve interlace problems. This is not true, at least with every set up I've tried so far.

Audio

Since PCM Wav audio is not too large and yet still the best quality I suggest that you use that. It puts a large strain on your computer to compress to Mp3 or WMA audio and there may be more problems with audio synch if you don't. I suggest you also use 44100Hz 16 bit stereo which is CD quality audio and best for music. This is also the most compatible format. You can, of course, use any other setting such as 22050Hz if you prefer and this will to save some space at the expense of a little quality. But then again if you are going to compress the audio anyway I wouldn't worry too much.

VHS Framerates

There is never any need to capture higher than 30 frames per second. I'd also recommend that you stick to 25fps for PAL TV capture and 29.970fps for NTSC TV capture. This is because most capture cards are designed for this kind of capture and you may get bad results or synch problems if you don't.

Dropped Frames

Both VirtualDub and most capture cards will tell you if your system cannot capture all the frames you need per second for smooth playback. I think that if your computer cannot take the strain and starts dropping frames it is better to capture at a lower resolution! I would always sacrifice picture size (resolution) in favor of a high framerate unless the picture was very small. If you have the framerate dropping problem with an NTSC TV which is 29.97fps then you could try capturing 25fps instead, but, as I said, this may cause problems. Also if you intend on making a VCD at 25fps then you should also capture at 352 x 288 instead of 352 x 240.

If your computer starts dropping frames you will almost always get audio synch problems and annoying jerky playback problems. Its easy for the novice to think that if they get dropped frames at 25fps then they should set it at 30fps to compensate. Then, even if some frames were dropped, we would still get smooth playback. This is a big mistake! If your machine is dropping frames at 25fps then asking it to capture more per second puts even more strain on it and you will get much worse quality. The only answer is to lower the resolution or lower the framerate or upgrade your system =oP.

Starting Capture

Now you know the basics and have the equipment, I will explain in the next article how to set up and capture using VirtualDub and your capture device. Additionally, it is always a good idea to make some test clips before you capture and see how they play before you do a long capture. You will also need to watch the 4GB limit, Windows 98 cannot save single files larger than 4GB each so that restricts what we can do even more. If you have Windows 2000 this shouldn't be a big problem. Nevertheless all these issues will be dealt with in following articles.