How to digitize analog video sources for storage and manipulation on a computer.
In 1999 I purchased my first DVD player. My wife and I had a small collection of VHS tapes containing videos that we wanted to view using our new purchase. Furthermore, optical media is very convenient and stable, and the idea of storing our video collection on CD-R discs was very attractive to us. What followed was a very indepth investigation that has flourished into an interesting hobby. In this article, I cover how to digitize analog video sources for storage and manipulation on a computer, tools for editing video on a computer and some options for publishing digital videos. One publishing option I present is storage in the video CD (VCD) format, which is compatible with many DVD players. All of these steps are performed using free software.
I am a software developer, not a video producer, so please bear with me as you read this article.
The first obstacle I encountered in my work to convert my videos was how to digitize the analog VHS tapes. Because I wanted to convert standard analog video tapes, IEEE 1394 (Apple calls this interface FireWire; Sony calls it i.LINK), though extremely powerful, defines a purely digital interface and would not suffice. Instead, I decided to purchase a video capture card. Many vendors produce these cards, which take standard analog video streams and digitize them for storage or display on a computer. I bought Hauppauge WinTV PCI video capture card that works nicely with Linux for around $80 US. Incidentally, the Linux driver framework for video capture cards is named Video4Linux.
There are a few important considerations to make when purchasing a video capture card, though they are becoming less relevant as the speed of computers continues to increase. Because capturing video from most analog sources must occur in real time, writing raw video to disk requires a very fast hard drive. In my experience, even a 10,000 RPM SCSI drive has difficulty storing raw 24-bit video with a resolution of 640 x 480 and a frame rate of 23.9 frames/second. Think about it: around 30 frames per second, 640 x 480 = 307,200 pixels per frame, and each pixel is 24 bits. In order to store uncompressed video of this quality, a hard drive needs to write 2.21 x 108 bits, or around 26MB every second!
Don't run out and buy an expensive high-speed disk array quite yet—an alternative exists. Compressing the raw video before writing it to disk shifts some work away from the hard disk. Compression can be done either by a dedicated processor, shifting work to video capture card compression hardware, or in software, shifting work to the system's CPU. Since my system has two 1,000MHz CPUs, my cheap Hauppauge card, which lacks compression hardware, performs just fine. If your computer's CPU is a little slower, it may make more sense to invest in a video capture card with hardware compression capabilities and save a relatively expensive CPU upgrade for later.
Capturing raw or losslessly compressed video is ideal for editing purposes, but capturing using a carefully chosen lossy technique such as MJPEG, which stores each frame using JPEG still image compression, is a realistic compromise. JPEG compression can be performed relatively quickly in software. In addition, many hardware video compressors output MJPEG.
Even when compressing a video stream before writing it, hard disk speed is important in digitizing video. It follows that the filesystem used is a large factor in performance. I have experimented with the ext2, ReiserFS and XFS filesystems. My experience is that capturing video to an XFS filesystem generally outperforms capturing to ext2- or ReiserFS-formatted disks. XFS has the additional benefit over ext2 of being a journaling filesystem.
Andrew Morton's low-latency kernel patch also seems to help the digitization process. I find that with Andrew's patch I am able to perform minor tasks on my computer while capturing video without losing too many frames.
As I am from the United States, I am interested in using the National Television System Committee analog video format (NTSC). Many Europeans may be more interested in PAL, which has similar properties. If you live elsewhere, a little research will reveal the analog video format used in your region. My VHS tapes are encoded using NTSC. NTSC has a range of acceptable resolutions and frame rates; when capturing from a VHS source I generally capture 640 x 480 frames at a rate of 23.976 frames/second. Though VCDs, being digital, don't have a video norm such as NTSC, DVD players generally use the frame rate that a VCD contains to decide what type of analog signal they will send to the television to which they are connected. For example, if I encode VCDs at 25 frames/second, my DVD player outputs a PAL signal that looks distorted on my NTSC television. If I encode the same video stream at 23.976 frames/second, a valid NTSC frame rate, my DVD player outputs an NTSC signal to my television.
Digital media streams found on a computer are generally stored as a wrapping format containing one or more audio and video tracks. Examples of wrapping formats are AVI and QuickTime. QuickTime has the advantage of being well defined by Apple, supported on Linux and able to store video streams much larger than 4GB. Within the wrapping format, different compression techniques such as MJPEG, OpenDivX, Ogg Vorbis and MPEG audio may be used. These compression/decompression techniques are often called codecs. Wrapping formats such as QuickTime also can contain storage-intensive raw digital audio and video.
I have found that streamer, part of the xawtv package, performs the digitization task nicely. Using streamer, my system can capture 640 x 480 video at a frame rate of 23.976 frames/second from my video capture card and compress it in real time to an MJPEG encoded QuickTime before writing it to disk.
Once I had digital video on my computer, editing it was the next step. Broadcast 2000 (MandrakeSoft produces an RPM called bcast-2000c) is a GPLed nonlinear video editor [see “Movie Making on a Linux Box?”, LJ January 2001 and “NLE Video Editors”, LJ February 2002]. With it, one can manipulate audio and video tracks. A screenshot of Broadcast 2000 may be seen in Figure 1. SuSE has a pretty good guide to Broadcast 2000. Cinelerra, best for users competent in compiling development-quality code, can be found on SourceForge and is the up and coming successor to Broadcast 2000. Both of these tools work with the QuickTime output from xawtv.
Figure 1. Broadcast 2000 Screenshot
Now that I was able to digitize and edit my VHS tapes, it was time to encode them as VCDs. A little research revealed that VCDs contain MPEG-1 video streams and MPEG-1 layer 2 audio streams. This data, along with some simple directory information, is encoded onto a CD. More information about the VCD format, also known as the White Book specification, can be found at www.cdpage.com/Compact_Disc_Books/whitebook.html.
A package named mjpegtools does a good job of transcoding an MJPEG encoded QuickTime file into an MPEG-1 stream. Using vcdimager, a VCD image can be created from the resulting MPEG-1 stream. Finally, the VCD image can be burned to CD-R media using cdrdao. Here is how I use these tools to create a VCD on my system:
streamer -r 23.976024 -s 640x480 -f jpeg -F stereo
-i Composite1 -n ntsc -c /dev/v4l/video0
-b 64 -o foo.mov -p 2 -t 00:45:00
You may need to use a different argument with -c, which specifies the video capture device to use. Often it is /dev/video0. The -t parameter specifies how long the program should continue capturing:
lav2yuv +n -n 2 -d 3 foo.mov | yuvscaler -n n
-O VCD | mpeg2enc -n n -f 1 -r 16
-o foo_video.mpg
lav2wav +n foo.mov | mp2enc -V -o foo_audio.mp2
mplex -f 1 foo_audio.mp2 foo_video.mpg -o foo.mpg
vcdimager foo.mpg
cdrdao write --driver generic-mmc --device 0,4,0
--speed 1 videocd.cue
Replace 0,4,0 with the numbers that reference the CD writing drive on your system.
I have simplified this process with a package called qtutils, which includes a script named vcdize.
Many DVD players can play VCDs in addition to DVDs. However, some DVD players have trouble playing CD-R discs. My Phillips Magnavox DVD 825 DVD player plays the VCDs that I have burned to CD-R media fine.
Some relatively storage-space friendly alternatives other than the VCD format exist, including MPEG-2 and QuickTime using the OpenDivX and Ogg Vorbis codecs. These formats may be played with a computer using video players that use the QuickTime for Linux library such as X-Movie.
So there you have it. You should now be able to convert VHS tapes to a more convenient format using free software exclusively. The VCDs you produce will play in DVD players that support the VCD format and CD-R media. For more information on this subject, visit the home pages of the software projects I mentioned. Two very relevant mailing lists also exist: the video4linux and mjpegtools mailing lists, hosted by Red Hat and SourceForge respectively. Enjoy!
Mike Petullo is a platoon leader in the US Army, stationed in Germany. He fights C code bugs by night, has been tinkering with Linux since early in 1997 and welcomes your comments.