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Vertical Synch
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1) Ch 1
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500 mVolt
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250 us
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NTSC vertical synch signals.
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Horizontal Synch Front Porch Back Porch
1) Ch 1
500 mVolt
10 us
A single NTSC raster (line) of data.
The vertical blanking before the vertical synch consists of 6 or 7 half lines (at 31.8 s long) followed by 6 negative half lines (these are the vertical synch pulses) and then another 6 or 7 half lines followed by 10 or 11 full lines (at 63.5 s long). With the raster gun now pointing to the top left of the CRT, data can be output a line at a time. One of the lines from this circuit is shown in Fig. 21.52. Each line is 63.5 s with a horizontal synch pulse to indicate where the line starts followed by the data to be output on the line. The data output ranges from 0.48 V (black) to 1.20 V (white), with gray being the voltages in between. In the gure you can see the different voltage levels for the horizontal synch, the front porch, the back porch, black, and white. Note that there is approximately 100 mV of noise on each of the signals. This is largely due to the prototyping construction I used for this application and the poor ground that I have for it. In a professional application, I would expect that the noise on the line would be on the order of 10 mV or less. For this application (and the very cheap used TV set that I used), the 100 mV of noise did not cause a problem with image stability (a big consideration for video applications) or the brightness output of different parts of the signal. The front porch and back porch are 1.4 and 4.4 s in length, respectively, and are at 0.40 V. The synch pulse itself is a 0 voltage active for 4.4 s. These signals (with their voltage levels) must be present in any video signal. For the 53.3 s after the synch pulse, the voltage level must be at 0.48 to 1.20 V. If the output dips below 0.48 V, there is a chance the TV (or CRT) will interpret the signal as a new horizontal synch with terrible results (in terms of the output display).
0.1 uF Tantalum Gnd
10 K Single Turn
10 K
4 16 20 MHz 15
470 RB4 _MCLR Osc 1 Osc 2
Composite Video Output
3 220
3 330
Video generator circuit.
For the TV to accept the composite video, the quoted signal lengths must be adhered to as closely as possible. Failure to have the same number of cycles on different lines will result in a broken screen. I will discuss this at length later in this project writeup. The actual circuit I used to create the composite video is amazingly simple (Fig. 21.53), based on the bill of materials listed in Table 21.26. The composite video voltage output was produced by placing different I/O pins in output mode with an output of 0. The circuit is designed so that only one pin can be
PIC16C711 20-MHz ceramic resonator 0.1- F 10k 10-k pot 470 150 220 330 Misc.
PIC16C711-JW 20 MHz with internal capacitors 0.1 F tantalum capacitor 10 k , 1/4 W resistor 10 k , single turn potentiometer 470 150 , 1/4 W resistor , 1/4 W resistor resistors resistors
220 , 1/4 W note three 220wired in parallel 330 , 1/4 W note three 330wired in parallel
Prototype board, wiring, 5-V power supply, video modulator, video modulator power supply; 75- coax cabling, TV set
enabled as output at any one time. When a pin is in output mode, the pin that it is connected to is pulled down and allows current to pass through the resistor connected to the pin. This added resistance changes the resistance of the voltage divider output and changes the voltage output from it. The circuit works very well and provides a very fast speci c digital-to-analog conversion. In the oscilloscope pictures of the composite video signals shown in this section, I used the circuit presented in Fig. 21.53. When I built this project, I used a SimmStick prototyping card for the circuit. One of the advantages of the SimmStick and other prototyping systems in applications such as this is that the unregulated voltage in is available with 5-V regulated voltage for the PIC microcontroller. I used this feature instead of having to come up with a dual power supply circuit for the application. The video modulator was attached to the prototyping card with hot-melt glue, and the power, ground, and composite video outputs were passed from the SimmStick edge connector. When I rst was setting up the project, I used a small Tyco toy video camera as a sample composite video source. An RCA socket was glued to the video modulator using hot-melt glue, the shield soldered to the modulator s case was used for ground, and the signal line was passed to the modulator s input. I originally created this SimmStick card for an Atmel AVR composite video output for the Handbook of Microcontrollers, and I ve used it on a number of projects since. Looking at the circuit (shown in Fig. 21.53) for this project, you probably have a few questions. The rst is about my use of a ceramic resonator instead of a crystal for the PIC microcontroller clock. This is probably surprising, especially in light of the harping I ve done about the importance of an accurate clock so far in this section. When working with a stand-alone video generator like this circuit, the critical parameter is to make sure that the timing is perfectly accurate relative to the various signals and reasonably accurate to the speci cations. I will discuss this in more detail later, but most modern (which is to say, built within the last 30 years) TV sets are able to work with a relatively wide range of input timing parameters. The reason for putting in this tolerance is not to make life easy for people like us but to make the TV set insensitive to changes within itself as the components age. I am continually amazed at the reliability of TV sets (and consumer electronics in general), and one of the reasons for this reliability is the ability of the circuit s designs to continue operating even though their components have degraded. This built-in tolerance makes the lives of experimenters like us much easier. The second thing you probably will notice in Fig. 21.53 and the bill of materials that go with it is the use of resistors in parallel. The three 220- resistors in parallel result in a resistance of 73 , which is close to half the 150 built into the voltage divider circuit. The three 330- resistors in parallel have an equivalent resistance of 110 . Both these resistances were calculated as part of the composite video output voltage for the 0-, 0.40-, 0.48-, and 1.20-V outputs needed for the composite video output. The actual values needed were 75 and 107 , which are not available as standard values. By applying parallel resistance theory, I was able to approximate them quite closely instead of having to rely on standard values.
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