Cheap Time Base Correction
for VHS Tapes
I spend more time watching movies than I should. As a corollary, I accumulate odd videocassettes and laserdiscs of films that have never been released on DVD. (I also have players which are region-free and format-transparent, so these movies must never have made it to DVD anywhere in the civilized world. If they had, they would be available on eBay.)
Naturally, I want to transfer the movies to DVD media so I can watch them more conveniently. And since I'm cheap, I try to do it with common hardware and free software. A combination of a Leadtek video capture card and VirtualDub (thanks, Avery Lee!) do the job pretty well. But there's a problem...
Frame from Killing Time (1998) captured | Frame from Killing Time (1998) captured |
VCRs are imperfect. The rotating head scans an analog magnetic signal that becomes the basis for the scan lines of a standard NTSC composite video signal. Due to tape wear, misalignment of the heads, or other mechanical problems, the individual scan lines sometimes don't synchronize properly with the remainder of the video frame. When the problem is severe enough, the picture becomes unstable and it isn't even viewable.
When the problem is slight, you will see anomalies that arise from improper horizontal synchronization. Each video frame (image) actually consists of individual odd- and even-numbered scan lines. The odd-numbered lines are scanned on the screen to make a 'field', then the even-numbered lines are scanned to fill in the image. The image below shows in detail a single scan line signal, starting with the synch signal ('synch tip') that tells the system when to begin each line. Each scan line should take exactly 63.5 microseconds. (That's about 60 fields per second...or EXACTLY 29.97 frames per second.) If the signal read from the tape is slightly irregular, the video information in a scan line might not line up perfectly with the lines above or below it. It's as if you cut a picture into horizontal strips and then tried to align them again...you may get them lined up perfectly, but chances are good that at least one or two strips will be offset slightly to the left or right.
Interlaced NTSC Video Showing | Single Scan Line of NTSC |
The result is what you see in the two images at the top. Each image was captured from an original VHS videocassette of Killing Time (1998) played on a typical consumer-grade Mitsubishi VCR. The first image is from the original uncompressed 640x480 AVI file I captured using a Leadtek Winfast PVR 2000XP Expert card. The second image is from the compressed MPEG-2 files after conversion to DVD format using TMPGenc 2.5. Notice the scan line anomalies in the lines on the road and the base of the light posts. It is obvious that some of the lines are offset relative to the lines near them. Very irritating.
So, what's the solution?
There are several relatively expensive solutions for this fairly common problem. The most common way to fix this is to use a video time base corrector (TBC). A TBC breaks the video signal down, either by a few lines or an entire frame at a time, and then rebuilds and regenerates the signal with new synch timing and a more accurate synch interval. TBCs are used in broadcast studios to synchronize the video from several different sources, so they can switch from source to source smoothly without visible transition anomalies. These broadcast quality units are usually rack-mounted units that require a separate, independent synch signal source for a reference. It's difficult to adapt these to consumer use and they cost a bunch, even used.
Since a TBC often significantly improves video quality from a VHS tape, some VCRs have TBC processing built-in. Most JVC DigiFine units and some VCR-DVD dubbing decks have this type of circuitry. But, of course, I didn't want to buy a new or used VCR to see if I could improve things.
So I did a bit of research. There is another common use of TBC processing. If you convert NTSC video to PAL video or SVGA video, or scale up a video in size, you need to break the signal down and reconstruct it completely. This is common in home theater installations, where different types of video sources are fed to a high-resolution video projection system. The consumer units are usually called 'video scalers' or 'line doublers'. These are also pretty expensive and often have neat-o features like separate IR remote controls. (As an added bonus, using a TBC also defeats the Macrovision copy protection on a VHS tape.)
