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A NOVEL idea for generating many a number of colors for NTSC using only one 7404, few resistors and diodes along with a microcontroller!!

posted Jul 2, 2010, 8:07 AM by play Box   [ updated Jul 2, 2010, 11:08 PM ]
Few popular methods used to generate different colors for NTSC are
1. using AD725 IC for RGB values to NTSC color conversion
2. using a standard 3.579545MHz as the color carrier and producing different delays to get different phase and inturn different colors in NTSC. This can be done by using a number of buffers to create delays and selecting the required color among these delayed signals by the use of a multiplexer.

We think the first method takes the fun out of it as all the work is done by an IC and very little is left for us. But the second method includes a lot of hardware (and also wires if you try it on a protoboard). So we were thinking for some simple method with which we can get quite a number of colors with minimal hardware. Our aim was to get atleast 16 colors. We now have such a circuit. This article is mainly related to that.

The basic idea here is to get 180 degree phase shifted carrier by the use of a not gate (74ls04), 90 degree phase shifted carrier by passing the carrier through an RC network and 270 degree phase shifted carrier by passing the 90 degree phase shifted carrier through a not gate.

Now by adding any of the above two signals we can get a different phase angle. For example if you add the signal of phase 0 degree and of phase 90 degree, you will get a signal having a phase of 45 degrees. So like this you can get a total of 8 colors. Now by using 2bits for luminance (2^2 = 4 shades) and 6bits for color combination selection we can easily get 256 different shades of colors. Currently we have implemented this with 2bits for luminance and 4bits for color combination. Thus we got 64 shades of color with just one 74als04 IC, few resistors and diodes. The schematic of the hardware and the images of different colors produced are attached below.



The above image shows the complete circuit diagram of an NTSC module. The RC network seen at the left end of the circuit is used to create a 90 degree phase shift. Four different phases are generated with the help of 74als04 (hex inverters). Then these signals are added as needed with the help of the gating signals (Color bit 0 to Color bit 3). When the Color control signal is in High impedance state, the corresponding color is passed and added with the other colors. If the Color control signal is made LOW, then the corresponding color is blocked. So here you need to change the Data Direction Register (DDR) of the microcontroller while keeping the PORT value LOW to choose different combination of colors. Diodes are used to avoid the effect of other gate signals on a given color signal. Different color combination can be obtained by using different resistors since the each signal amplitude depends on the resistor. We have used a 2 bit R-2R DAC for getting the luminance signal and the low pass filter is optional.

Another important thing to notice in NTSC is that the color saturation depends on the relative amplitude of the chroma signal w.r.t the reference color carrier. Therefore if the amplitude of the reference color carrier is less than that of the chroma signal. higher saturation of the color is obtained. To make this possible we have used the transistor (SL100) which acts as a bypass element for the chroma signal. When the color burst is to be given we turn off the transistor by making the color gate low, therefore now color burst passes through the 2.2k resistor. Whereas when the chroma signal is to be given we turn on the transistor by making the color gate signal high. Thus the transistor bypasses the resistor and provides a low resistance path for the chroma signal. Thus the relative amplitude of the chroma signal w.r.t the color burst signal will be higher and thus higher color saturation is obtained.

One of the advantages of this circuit is that you can change the choice of the color burst signal so that a color palette can be created and much more colors can be achieved!!

The above image shows 24 different shades of colors out of the 64 shades that can be produced.

All 64 shades are present in this image and those 64 shades are repeated vertically.

Another image of a different set of 24 shades. We can change the color burst signal for each frame and create a color palette and thus very easily we get a very large number of color shades. Thus we have achieved a large number of colors with this simple circuitry. 

- Thejasvi
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play Box,
Jul 2, 2010, 8:25 AM
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Colorgenerator.sch
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play Box,
Jul 2, 2010, 8:25 AM
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play Box,
Jul 2, 2010, 8:25 AM
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play Box,
Jul 2, 2010, 8:25 AM
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play Box,
Jul 2, 2010, 8:25 AM
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play Box,
Jul 2, 2010, 8:25 AM
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