Genlocked equipment works together in synchronization with a master sync signal. It's much like the members of a marching band "locking" their steps to the beat of the bass drum. Without the steady beat of the drum -- the synchronization pulse -- the band members wouldn't have a common reference for staying in step with each other.

Similarly, video equipment that isn't genlocked has difficulty working together. Trying to dissolve from the output of one VTR to another results in picture tearing or roll until synchronization is reestablished. With genlocked equipment, the dissolve is smooth and "in step" with the master sync.

The black burst signal (Figure 4-1) is often used for genlocking equipment. It is a composite signal with a horizontal sync pulse and a small packet of 3.58 MHz color subcarrier (color burst). The term black burst arises from the fact that the active picture portion of the signal is at 7.5 IRE (black) and it contains color burst.

 

Figure 4-1. The black burst signal used in genlocking is composed of the horizontal sync pulse, subcarrier color burst, and a 7.5 IRE black signal.

Establishing a genlocked studio is simple and need not require expensive equipment. This section will show you how to genlock your equipment with the Tektronix TSG 200 NTSC Generator (Figure 4-2). This generator is an economical source of multiple black burst signal outputs for genlocking.

 

Figure 4-2. The Tektronix TSG 200 provide multiple black burst signal outputs for economically genlocking small studios. Additional features on the TSG 200 can be used for basic video testing and other odd jobs around the studio, including ID generation, setting up picture monitors and blacking tapes.

Genlock Basics

There are three basic requirements for setting up a genlocked studio. You need:

• Video equipment that can be genlocked
  
  
• A genlock signal source
  
  
• Adjustment for synchronizing equipment with the genlock source.

The first of these requirements -- video equipment that can be genlocked -- is relatively easy to meet. Most professional video equipment is genlockable. To see if your cameras, VCRs or VTRs, and other equipment meet this requirement, check the equipment for a genlock loop-through input or an external reference input (Ext. Ref.). Also, the equipment must have some provision for adjustment of timing, whether it is a single control for adjusting both H-timing and SC-phase, or separate controls for adjusting each.

The genlock or external reference input connector is necessary for applying an external synchronization signal -- typically black burst -- to the equipment. The timing controls are necessary for adjusting the equipment for synchronization with the reference's horizontal (H) sync pulse and subcarrier (SC) color burst phase.

With a genlock signal source, such as a Tektronix TSG 200, a black burst signal can be distributed to the studio equipment as shown in Figure 4-3.

 

Figure 4-3. A black burst signal is distributed from the test signal generator for genlocking studio equipment. If the signal generator has multiple black burst outputs, as is the case with the TSG 200, a distribution amplifier may not be necessary since they have five black burst outputs.

There are several important things to note about this distribution diagram. First, the signal generator used for genlocking should have multiple black burst outputs. A unique black burst signal is not needed for each piece of equipment to be genlocked. Looping-through equipment in different locations within a facility is possible, but presents one major problem -- too long a cable run will produce delays beyond the adjustment range of the equipment being locked.

Generally speaking, a single black burst feed for each cluster of equipment is sufficient, with that single feed being looped through just one group of equipment.

If multiple black burst outputs are not available, a multiple output signal distribution amplifier should be used with one output for each cluster of equipment to be genlocked.

Also notice in Figure 4-3 that the black burst signal is distributed to the Time Base Corrector (TBC) for each VTR, rather than the VTR itself. The reason for this is that the TBC is actually the final program output of the VTR. The TBC is where genlocking and timing adjustments need to occur.

Similarly, if you use camera control units (CCUs), connect the black burst output to the CCUs, not to the cameras. Camera timing adjustments, unlike TBC/VTR adjustments, are made at the camera, not at the CCU.

Finally, note that one of the black burst signal outputs from the test signal generator is loop-through connected to the external reference inputs of a waveform monitor and vectorscope. (Don't forget the 75 ohm terminator at the end of each chain of genlock or external reference loop-throughs.) The waveform monitor and vectorscope are also loop-through connected to the Program Output of the studio switcher.

These connections to a waveform monitor and vectorscope are necessary for the final requirement -- adjusting the equipment for synchronization with the genlock source.

Adjusting TBCs

TBCs have several controls, in addition to the timing controls just mentioned, that are used to compensate for gain deficiencies and color errors on tapes. Setup (or black level), video gain, chrominance gain, and hue are commonly used adjustments on TBCs. Each of these adjustments has already been covered in this application note and will not be repeated here. The purpose of this section is to link the nearly universal inclusion of color bars on tape leaders to TBC adjustments and discuss a few guidelines to follow while making these adjustments.

Video levels and hues recorded on tape sometimes differ from facility to facility, and VTR to VTR. This is a fact of life. When differences exist they must be corrected. By recording color bars on the beginning of every tape, operators can make the necessary TBC adjustments quickly and simply with just a waveform monitor and vectorscope.

The TBC's horizontal timing and subcarrier phase controls shouldn't require checking or adjustment each time you play back a tape from a different source. Since the TBC always replaces the sync and burst of the recorded signal, playing back different tapes should not affect the TBC's system timing. Many experienced operators make TBC adjustments in the following order to minimize interaction between the adjustments: 1) setup, 2) video gain, 3) hue, and 4) chrominance gain.

Remember that TBC adjustments must be made when the VTR connected to it is playing back a recorded color bars signal, not when the VTR is in the electrical-to-electrical (E-to-E) mode.

Adjusting for Synchronization

As mentioned before, the horizontal sync pulse and subcarrier burst of the black burst signal are like the drum beat to which members of a marching band synchronize their steps. Everyone's footsteps should strike the ground in time with the drum. Additionally, for the whole band to be in step, everyone's left foot should be striking the ground at the same time. Occasionally, however, a band member will be out of step, and you'll see them do a little "skip-step" as a timing adjustment to get in step with the rest of the band.

Similarly, each piece of video equipment must have some timing adjustments made in order for it to be "in step" with the rest of the video system. Adjusting system timing is one of the most fundamental -- and critical -- procedures in the studio.

System timing adjustments involve setting the H-timing and SC-phase controls on each piece of equipment. The adjustments are made so that each piece of equipment's horizontal sync pulse and color burst phase line up with the sync pulse and burst phase of the reference genlock signal (black burst).

Making sure that each piece of equipment is "in step" with the genlock signal prevents horizontal jumps and color shifts of the picture when switching between video sources. Matching the timing of the sync pulses makes sure the scanning of each picture source is in step (e.g., the images will stay in the same position, without roll, tearing or "jumps" during a cut or fade). Matching the burst phase of the various sources maintains the correct color of the images during editing transitions.

Before making timing adjustments, you should check the video gain from each piece of equipment. The SMPTE color bars signal from the TSG 200 can be used for this. If necessary, correct any video gains before going on to system timing adjustments.

System timing adjustments are made with a waveform monitor and vectorscope connected to the switcher output as shown in Figure 4-3. Make sure that both the waveform monitor and vectorscope are set to trigger on the external reference signal.

The next step is to set up zero timing references on both the waveform monitor and vectorscope. The first step in this "zeroing" process is to select the test signal generator's output as the switcher's active input. This applies the generator's test signal to the Channel A inputs of both the waveform monitor and vectorscope.

On the waveform monitor, make the necessary adjustments to expand the display on the horizontal sync pulse. Be sure the waveform monitor is in the external reference mode, i.e., displaying the program output of the switcher, but referenced to the black burst. Use the waveform monitor's horizontal positioning control to place the leading edge of sync on one of the display's major timing marks. This timing mark is now the zero-time reference for inputs from all other video equipment, and the horizontal position control should not be moved during the rest of the system timing adjustments. Figure 4-4 shows an example of the reference sync edge display with the sync edge positioned on a major timing mark.

 

Figure 4-4. Waveform monitor display of the reference sync pulse edge adjusted to correspond to a major timing mark. This timing mark can now serve as the zero reference for individually adjusting the H-phase of each piece of video equipment in the system.

The next step is to zero reference the vectorscope to the reference signal's color burst subcarrier phase. This is done by using the vectorscope's phase control to position the color burst vector to the 9 o'clock position. Here again, be sure the vectorscope is in the external reference mode -- locking to the external black burst. This is shown in Figure 4-5. Once the reference signal's color burst is positioned to 9 o'clock, do not touch the vectorscope's phase control again during the rest of the timing adjustments.

 

Figure 4-5. Vectorscope display of the reference color burst vector adjusted to the 9 o'clock position. All other pieces of equipment should be SC-phase adjusted to correspond to the 9 o'clock reference.

With sync and color burst zero referencing completed, use the switcher to select the signal output from the first piece of video equipment to be adjusted.

The waveform monitor will display the horizontal sync pulse from the selected piece of equipment. If the sync edge does not line up on the previously established zero-reference mark, adjust the H-timing control on the selected piece of equipment to place the sync edge on the zero-reference marker.

Look at the color burst display on the vectorscope. Is the color burst vector aligned with the 9 o'clock position on the vectorscope display? If it isn't, use the SC-phase control on the selected piece of equipment to set color burst to the 9 o'clock zero reference.

In the case of a single timing adjustment (where SC-phase and H-timing have a fixed relationship), simply observe the vectorscope and adjust the equipment such that the color burst vector lines up with the 9 o'clock reference line on the vectorscope's graticule. Some equipment has a wide range adjustment which will continue to move the sync timing beyond the first place where the burst phase is correct -- if so, continue to adjust the control in the appropriate direction. This should move the H sync pulse to the zero-reference marker as well. If it does not, there is either an internal H-timing adjustment that is set incorrectly or the equipment needs servicing. In either case, an experienced technician should attend to the problem.

Setting sync and color burst to the established zero references completes timing alignment for the selected piece of equipment. That piece of equipment is now in step with the black burst reference signal.

Now, using the switcher, select another piece of equipment and adjust its H-timing and SC-phase controls to zero reference its sync and color burst output. Continue this equipment selection and zero-referencing process for all other pieces of video equipment in the system. Remember that for VTRs, the timing adjustments are done at the TBC.

When these adjustments have been completed for all pieces of equipment in the system, each piece of equipment will be in synchronization and properly timed with all other pieces of equipment. You now will be able to switch smoothly between video sources and make clean edits without picture roll or horizontal jumps.