Time Base Corrector
VTRs and VCRs tend to have timing problems for play you need to correct for this. This is where a TBC (Time Base Corrector) comes in. There are to types the first type is built into the play back machine. It provides a feedback to the video servos this to very the speed and timing from a common sync reference.
The second type uses a fame storing system. The incoming video is decoded into RGB (Red Green Blue), then stored digitally to be clocked out again at a synchronised rate. This same process also works for television system conversions.
Adding text or images also can be done when the video has been digitised, the mixed RGB is then encoded back into composite video. Video level that is output from the TBC will be at a fixed level. The output is now set at true one volt peak to peak into 75 ohms.
There are two processing circuits below, they are used for two different types of applications.
Circuit number one
This circuit is made for S-Video from a DVD player, S-VHS VCR or Hi8 VCR. It has been designed to replace the frame-blanking interval with black level. This is where other signals are sent within the video waveform, such as macrovision, Videocrypt and/or teletext. Using a LM1881 to supply the timing pules for the NE555, the two NE555 are used as adjustable time delays, which are set for the blanking interval of the incoming YC video. The chrominance (C) part of this circuit is a transistor adjustable amplifier. The part of the luminance (Y) section, which is after the timing circuit, is feed into the output Y amplifier that has a bandpass switchable filter. This filter with it's adjustable gain is good for reducing videotape noise.
Circuit number two
This circuit breaks the video waveform three ways, first path for the chrominance, second for the luminance, and the third is the sync section. The Y/C parts are the same up until the outputs of the NE592 video amplifiers. There is a 270 nS delay line in these sections. The delay lines are there to delay the video components to take into account the sync processing time. Sync part of this circuit has a low pass filter to roll off any unwanted video noise. After the low pass filter, there is a LM1881 sync separator. There are two signals that are used from the LM1881, one is the colour burst flag and the other is the composite sync output. The burst flag is used for the back porch clamping; this sets up a DC reference for the video waveform. This is to eliminate any DC offset problems that may have accrued caused by AC coupling. Luminance information on the output of the NE592 can be adjusted, and the white clip control sets a level to which any luminance information over is clipped to. Sync level adjustments are done where the processed sync is remixed into the luminance. Black level adjust changes the DC voltage which the video information sits upon. The chrominance section the signal at the output of the NE592 is feed in to a 4.43 MHz bandpass filter. After the chrominance filter, the colour information is remixed back into the luminance and sync signals. Then the video waveform is amplified to the one-volt peak to peak, to go into a 75 ohm load. Take note that the output stage from this processor is DC coupled.
Text / Drawings Copyright (C) 2003-2004 ZL1WTT
Television AM modulator circuit
AM modulator layout used with the old BATC frequency multiplier boards. Used for high level modulation on the BFR91 and BFR96 output stages. I have added a LM1881 sync seperator IC, for improved performance with this design.
For better sound injection level I used a external mixer and added it in before the power block PA stage. Poor sound sub-carrier level was always a problem with these types of modulator circuits.