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Article: How To Buy A TBC & Genlock

Oct. 6/92
Third Draft of "How to Buy a TBC and a Genlock"

1/ Time Base Correctors (TBC's) and Genlocks have become essential hardware for doing respectable video production. This article will provide general advice on how to select the proper models for your particular needs and which features to look for.

2/ Although a video picture from a VCR may look stable on most new monitors, it really has mechanically induced timing instabilities that make it difficult for a downstream device or recording VCR to lock up to. TBC's are usually necessary for timing and stabilizing the video when feeding a downstream device such as a switcher, or Genlock. Even when you are just doing straight cuts editing, you will find a TBC useful for its processing amplifier (proc amp) controls (video, black set up, chroma and hue), and its ability to straighten the flagging and repair some other problems in a picture.

3/ The noun "Genlock" is really a misnomer since it's really three or four devices in one; not to be confused with the verb, "to genlock". A Genlock is a Red/Green/Blue (RGB) to video ENCODER which GENLOCKS (locks or synchronizes) to the incoming video signal and KEYS the computer graphics over it. Technically it should be called a Genlock/Encoder/Keyer or GEK, but the name Genlock has stuck, so we'll continue using it. A Genlock is designed to take a native graphic, titling or animation from your computer and superimpose it on your video so it can be recorded. This article will not list encoder only devices that can't key over incoming video, nor will it include graphics cards that turn the video into RGB and back to video again. Computers such as IBM PC's or Mac's use a progressively scanned RGB picture, so Genlocks for these computers also need a SCAN LINE CONVERTER built into them to convert the computer image to interlaced and clock it out at video frequencies (15.75 KHz line frequency for NTSC). Commodore Amiga computers are designed to operate at video frequencies in interlace mode, so scan line conversion is not necessary.

4/ This article will not list optional or built in TBC's that plug inside your player. TBC's and Genlocks come in stand alone rack mountable boxes or circuit board cards that slip inside your IBM or Amiga computer. Genlocks are usually dedicated to a specific computer platform to keep their cost low; however, there are very high end Genlocks such as the YEM line that can work with a variety of computers. TBC's and Genlocks designed for a computer slot, often can be mounted in an optional outboard box with power supply and controls for those who don't have enough room in their computer or when the computer's power supply is heavily taxed. Some cards are less power hungry than others. The Kitchen Sync dual TBC on a card is very power efficient using only 7.8 watts, and doesn't draw much from the negative portion of the power supply. Some TBC or Genlock cards will have an external video breakout box supplying more surface area for video connectors than would otherwise be available on the back of the card so that you don't have to double up usage of a connector. These breakout boxes also make hook up changes easier since they don't have to be mounted at the rear of the computer.

5/ There are four standard video signal paths used by recording VCR's; composite, Y/C 3.58 (S-Video), Component (Y, R-Y, B-Y) and Dub 688 (Y/C 688). You need to decide which signal paths you will require when purchasing your TBC or Genlock. The "composite" signal is common to all tape formats and is usually labeled "line" or "video" at its BNC connector (RCA connector for consumer products). Occasionally, the label "NTSC" will be incorrectly and interchangeably used for the word "composite". Unlike broadcast over the air NTSC, the direct composite signal is not necessarily limited to a bandwidth of approx. 4.2 MHz (332 lines of horizontal resolution). Composite shouldn't be called NTSC because it doesn't properly describe the composite signal in a PAL or SECAM country. "Y/C 3.58" (S Video) is available on SVHS, HI-8 and, more recently, for interformat compatibility on other formats such as the new Industrial Betacam SP and MII. Y/C is a dual signal path with luma on one line and chroma on the other. "Component" is a three wire signal path used in Betacam and MII. Some new SVHS decks will have this connection for interformat compatibility, but they don't actually record the signal in a component fashion. Component carries luma (Y) on one line, and color difference information on the other two lines called R-Y (red minus Y) and B-Y (blue minus Y). Component recording comes a little closer to the pure RGB signal than Y/C because it offers higher resolution for the color signal, but at an additional cost because of three channels of processing. It is interesting to note that the luma and chroma bandwidth (resolution) of many TBC's or Genlock's in composite or Y/C, exceeds that which is needed for component recorders, and therefore would be transparent enough to do an adequate job. Each 1 MHz of bandwidth is worth roughly 80 lines of horizontal resolution. Yet another signal path type is Y/C 688 or (Dub 688). This is used on 3/4" VCR's and is similar to Y/C 3.58 except that the color signal frequency is at 688 Kilohertz (KHz) instead of 3.58 Megahertz (MHz). To make matters more confusing, there is also a Y/C 629 dub signal available on Pro series JVC SVHS & Panasonic VHS and SVHS editing decks; however, there are very few TBC's or Genlocks that bother with this signal since the standard of Y/C 3.58 has been established. Your tape format and signal compatibility with other equipment in your suite will probably dictate which types of video signal paths you will need.

6/ Contrary to popular myth, the composite signal doesn't have to be significantly inferior to Y/C, but it often is. Since the black/white or luminance video information (luma or Y), and the color or chrominance information (chroma or C), are combined (encoded) into one signal (composite), they need to be separated (decoded) when shown back on a monitor, recorded on most VCR's, or processed in most devices. Therefore, it is the quality of the decoding filter at the input of any device that determines how well it will separate the chroma and luma information. Because most composite comb, adaptive, trap or notch filters waste some of the video information, you will often see a reduction of horizontal resolution. Color dot crawl along the edges of sharp color transitions is also caused from incomplete separation of the luma and chroma. Realtime video doesn't suffer as much from color dot crawl as computer generated titling and graphics because it doesn't usually contain highly saturated color edge transitions and because it is constantly changing. Good quality composite decoding filters (analog or digital) are expensive to build into TBC's and Genlocks, which is why many suffer from soft resolution and significant amounts of dot crawl. So what is the answer to a cleaner video picture? Simple, don't combine the luma and chroma signals until you have to. The longer you can keep them in their separated states, the less encoding and decoding the signal will have to go through and the better the picture quality will be. Y/C (S Video) parallel processing eliminates the need to keep combining and separating the Y and C signals but it also adds to the cost of a device because of the dual signal path.

7/ Many TBC's and a few Genlocks allow you to transcode between composite, Y/C 3.58, component and Y/C 688. Be careful when you buy a TBC or Genlock that transcodes, as some do not do a good job transcoding from composite to Y/C because of poor composite decoding (no comb filter). Some units have an internal signal path of composite only, which eliminates most of the advantages of Y/C or component signal paths. It is relatively easy and inexpensive to do quality transcoding of Y/C to composite, which is why many devices such as the Digital Processing Systems DPS Personal TBC II or the Magni Genlock 4004 for the Amiga have Y/C in, but only composite out. This actually makes economic sense, especially if other downstream devices only require composite. My preference, would be to purchase a TBC or Genlock with the same outputs as inputs for maximum versatility (composite and Y/C in/out). Most transcoding units will allow all outputs to be active concurrently, however some only allow composite or Y/C out because of space availability on the back of the card. With only one output, you may be forced to route (loop) the signal through a device (such as a monitor) before going to the recorder thereby potentially affecting your signal integrity. Ideally I would liked to see 2 composite & 2 Y/C outputs, all active.

8/ It's been my experience that most TBC's and Genlocks don't come very well tuned up and that you can't assume that the composite and Y/C out are at the same levels. For this reason, you would be well advised to purchase your equipment from a dealer who is competent enough to fine tune these devices using a signal generator, waveform monitor and vectorscope, preferably in your suite, if it is to interface with your computer. If you've ever had to deal directly with an uncooperative manufacturer, you will know how valuable a dealer can be for sorting out any after sale technical problems. If a manufacturer can't attract or hold on to dealers in your area, that might be a good sign to buy something else. Purchasing a Service Manual is a wise investment for professional devices, as you can have a local technician tune the device or learn about other internal settings and functions that your Operator's Manual doesn't mention. A Service Manual can be invaluable if your dealer or the manufacturer go out of business. Some manufacturers don't publish a Service Manual because they never get around to it, or they are paranoid that the competition might use it against them, or because they feel there aren't enough people capable of properly tuning up their equipment. This is especially so at the Desk Top Video Level where some purchasers will return a device to the manufacturer after severely misadjusting the settings because they didn't know what they were doing. Any service technician who is capable of tuning up a broadcast camera, should be able to follow instructions for tuning up a TBC or Genlock since the adjustments are similar. If you can't get a Service Manual, consider buying another model or brand.

9/ Two of the reasons that some Genlocks come so poorly tuned up has little to do with the Genlock itself. If the Genlock was designed to take its RGB signals off the computer at the "analog" stage rather than the "digital" stage, then it is subject to the red, green & blue voltages being at approx. .7 volt each. Commodore Amiga computers for example, have a reputation for having inconsistent RGB analog voltages from one computer to another even within the same model type. If they are low, high or uneven then your computer graphics video will be at the wrong levels, or the white or black balance will be out. You can easily tune most Genlocks to the computer, but the levels may change if you hook your Genlock to another computer. It surprises me that Genlock manufacturers haven't seen fit to design an auto white and black balance circuit for their Genlocks that works in the same manner as a broadcast camera. That would make tune up a breeze and compatibility from one computer to another within the same platform, possible without having to retune it. Genlocks, such as the Magni 4004 for the Amiga, that tap into the computer at the digital stage, deal with discrete levels and are more likely to come tuned up correctly. Analog Genlocks often depend on the RGB monitor for proper impedance termination. If each of the R, G & B signals is not terminated with 75 ohms by the monitor, then the video level from the computer will be affected. Some monitors have a non-standard impedance of as high as 210 ohms.

10/ TBC's can't correct all video problems. If you didn't properly white balance your camera when shooting, then you won't be able to completely correct for it with the hue (tint or phase) control since hue is a linear vector rotational adjustment. The DPS Personal TBC II has a control called "color balance". This isn't really a "rewhite balance" but a (feeble) nonlinear attempt to help rebalance the color by doing a black offset. I would like to see more TBC manufacturers attempt to design a "rewhite balance" control on their TBC's. Technically, white balance is the adjustment of the peak red, green and blue channels to be all at the same level (pure white). Since TBC's don't process using RGB signals, it is difficult to simulate a white balance. TBC proc amp controls have a limit to their range of adjustments. Some are so limiting that you can't fully adjust for a hot luma signal to bring it down within legal levels. Some do not allow for 360 hue adjustment for times when your incoming color phase is out of wack. Some TBC's clip white just above 100 IRE, thereby wasting detail in the white areas that could have been useful if your tape wasn't going to air (and could exceed 100). Many broadcast cameras have an auto knee (white level compression) that attempts to compress illegal luma levels down to legal levels without clipping them. This automatic feature on TBC's would be invaluable to those who want to spend their time on creating videos rather than scoping shots, but I know of no manufacturer that offers it. TBC proc amp controls can be useful for times when your picture was underlit and you need to boost it a little, but don't expect miracles if you didn't use proper lighting while shooting. TBC's are supposed to solve problems, not introduce them. All TBC's are not created equal! Over the years, I have saved sample tapes with built-in problems that will send some TBC's into convulsions. Test several TBC's in your own suite using your material. Look for things such as "hiccuping" where the picture jumps horizontally or vertically but doesn't in bypass mode. Make sure the TBC doesn't do a short freeze every time it comes across a flawed piece of video. There are three ways of clocking your video out of a professional player, each of which may affect the TBC's ability to hiccup or not. One is with the player hooked up in advanced sync mode, the second is with a stable video source (such as a camera) hooked up to the input of the player so it forces the tape to sync to the input signal, and the third is to let the player operate in freerunning mode where there is no video present at the input and it is not in advanced sync mode. I have usually found the second method to be the most reliable. If your TBC has full frame sync capability (memory enough to buffer a whole frame and keep the output synchronous), make sure it can clean up the whip at a bad edit point. Full frame sync TBC's help solve a lot of problems but remember that they can also delay your video picture up to a full frame for each generation, which eventually starts to throw the audio out of lip sync with the video. To minimize this delay you can hook up the advanced sync feature to your player which regulates the speed of the VCR from the TBC so that not as much buffer is required. If you are feeding a downstream digital effects unit such as the NewTek Toaster in "digital transition mode", you will be delaying your video 2 more frames. Three frames out of lip sync on a talking head is certainly noticeable and may actually be objectionable to some people. There are digital audio delays available now (such as the ???), that can help bring the audio back in time with the video. If you have a tape where the picture wants to roll, it may be missing the vertical sync above the top of the picture. Run this tape through the TBC to see if it stops rolling. Make sure you can see a watchable color picture that doesn't roll when scanning your tapes in fast forward and reverse mode. Some TBC's will not be able to give a watchable picture from VCR's with super fast shuttling. On unity gain settings, a TBC should not affect any of the video levels, but only Time Base Correct the signal. Run bars from tape through the TBC and watch the vectorscope to see how much the vectors wobble.

11/ Some TBC's have the ability to store several sets of proc amp control settings for different jobs. A nice feature on a TBC is fade to black control. Some simple "cuts only" edit suites use Time Base Correction, but they can't even do a simple fade to black without also buying a switcher. Genlock ability is an option on some TBC's, but it should be a standard feature. You will eventually want to sync your TBC to another TBC or a non-genlockable camera when feeding them into a switcher. The Kitchen Sync can do NTSC or PAL but it should not be confused with a standards translator. A horizontal and vertical Y/C adjustment is a nice feature especially for second or third generation tapes. If your TBC has a freeze, then it should also have a General Purpose Interface (GPI) connector so that the freeze can be triggered at the right time by your edit controller. Some TBC's and Genlocks even have a 9 pin RS422 (Sony Serial) control for more complete remote control from your edit controller, but software often has to be written for it to function. Those TBC's that have a separate proc amp remote control or are controllable from your computer, could offer a distinct advantage if the TBC and VCR's are in another room. Some VCR's and TBC's have a 3.58 subcarrier feedback circuit that helps to make the most of a composite signal. Freeze is a feature you will use often. You should be able to choose between frame or either of the fields. If you have a Toaster, you can grab a freeze frame there, but if you want a freeze field you have to go through a motion removal process which takes time, clips anything over and under legal levels and reduces the horizontal resolution. Vertical resolution is always dropped in half when you use field instead of frame freeze. Drop Out Compensation (DOC) is found in all VCR's and some TBC's. Although most VCR's do an adequate job of filling in drop outs, I prefer to also have that feature available on my TBC in case I have such bad drop outs that I need to use both. It is my preference to leave the VCR's DOC circuit on (if it is switchable) even when using the DOC in a TBC. A DOC circuit usually requires a connection back to the player. If you are not using a commercial grade editing player, it's unlikely it will have the proper DOC connector. In my experience, the DOC circuits in many TBC's are touchy and prone to introduce artifacts if slightly misadjusted. Some TBC's have Chroma Noise Reduction (CNR) that can improve the graininess (noise) in the color part of the picture. CNR circuits borrow color information from previous fields or the lines above and below and average or slur it with the current frame to reduce noise. If it borrows from previous fields, the chroma can actually lag behind or trail the luma image on a highly color saturated moving image. The feature is often best turned on only when making the final duplicates or submaster as artifacts can accumulate if it is on at each generation. The noisier the chroma in your picture, the more you will notice the improvement. It is not worth paying extra for CNR unless you can notice an improvement. Four years ago, I spent a small fortune buying two chroma noise reduction boards for my TBC's, only to find out that the improvement was so insignificant that it could only be measured on a noise meter. Some TBC's come with special effects such as variable speed strobing. Other features available are horizontal picture positioning and black stretch. Automatic chroma level control (ACC), adjusts the chroma saturation output level in relation to the input signals burst level.

12/ Genlocks work best when fed a stable Time Base Corrected signal; however, some Genlocks have switch settings so they will accept and lock to a signal with wider timing errors. If the signal is too unstable, it can crash the computer because the computer is trying to genlock to the video signal that is feed into the Genlock. For the Amiga computer, two of the best Genlocks with composite and Y/C in and out are the SuperGen 2000S by Digital Creations and the Omni-gen 711 by Omicron Video. The Omni-gen 711 is an outboard Genlock with its own power supply and can therefore hook up to any Amiga. The Omni-gen is one of the most expensive Genlock's for the Amiga but it really is a Cadillac unit which has helped to bring some respectability to the Amiga computer for video. The Omnigen has a comb filter for composite so it will transcode properly to Y/C and it also has a total fade to black control for Amiga graphics and incoming video. A fade to black control also can assist the proc amp controls of a TBC by letting you compress a hot signal down to legal levels. Omicron also makes a 721 and 731 unit which handle component, and there are equivalent models for PAL countries. For the IBM market there are a number of Genlocks available. The Magni VGA Producer Pro is one of the favorites, and is made by a company with a solid reputation for quality. It is a second generation product with many refinements such as flicker stabilization of single pixel horizontal lines, zoom for enlarging 80 column text so it is resolvable on VHS, and comes with Director and Character Generation (C.G.) software. Some Genlocks have software control over fading via the host computer. Consider this if your Genlock will be used in an automated system such as multimedia or cable and hotel displays, otherwise this may not be an advantage. One should keep in mind that Genlocks are not 24 bit video boards but simply encoders that take the existing native resolution and color palette capabilities of a particular platform and turn it into keyable, recordable video. Since a computer can only lock up (genlock) to one source, you can't hook two genlocks up to one computer. A Newtek Toaster should be considered a genlock which is always on, even if you haven't booted into the Toaster program. If you want to hook up an outboard Genlock, you need to physically remove the Toaster from the computer or disconnect pin ??? from the DP23 connector, and feed the same TBC'd signal to both Genlocks. Toaster 2.0 can be considered a functional Genlock since it now controls the fading of native Amiga graphics. In a side by side comparison with an Omni-gen, however, the Omni-gen wins hands down.

13/ Temperature stability of TBC's or Genlocks is a very important factor. The temperature in an edit suite gets awfully hot with all that equipment on. If you have a waveform monitor and vectorscope, run bars into your TBC or Genlock and scope them as they heat up. Some units have considerable hue changes with as little as a 5 F change in temperature, or the luma levels will rise or clip as the room heats up.

14/ Many TBC's and Genlocks have an internal adjustment that lets you set your lowest black level (pedestal or set up) at your preference. Although the North American standard for black level is officially 7.5 IRE, many fade to black switchers and Genlocks will use 0 IRE as black. Even the Toaster now lets you fade to 0 IRE black by using an open channel (with 75 ohm termination turned on). I have a strong preference for 0 IRE black for the lowest black when doing graphics as the images look much cleaner. If a TBC or Genlock bumps incoming video up from 0 to 7.5 IRE, then it is compressing the signal. For maximum white level on Genlocks I prefer to set it at 92.5 IRE. This way, if you have to readjust your black level to 7.5, you won't put white over 100 IRE, or need to retune the Genlock. Some TBC's and Genlocks have a black clamp feature that clamps the black from going below zero. This could be very helpful since rebound levels too far below zero can cause a false horizontal sync and cause your picture to break up.

15/ There are a lot of factors to consider when making an intelligent buying decision on any professional product. Price, features, quality, service, repair turn around time, dealer support, service manual availability, warranty duration and connectivity with other equipment all need to be taken into consideration. With video products you are usually wisest to go with the best you can afford, so that you will get a little longer useful life from it. Make your purchase decisions a vote for good products. If you are happy with your equipment, tell your friends. If you are unhappy, tell the manufacturer and dealer.


Writer Doug Hembruff

Bio. "Doug Hembruff is the President of Impact Televideo Productions in London, Ontario, Canada. He does corporate and broadcast video production, as well as product evaluation, writing and beta testing. Doug has recently returned from 3 weeks in Israel where he and his crew shot a sales video to promote Christian group tourism."  For more info contact Doug Hembruff  

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