VIDEO CAMERA

1. OBJECTIVE

1.1         Getting to know the video camera.

1.2         Measuring the composite video on a video camera.

1.3         Determining the parameters of composite video.

2. EQUIPMENT USED

a.    1 Video Camera

b.    1 Oscilloscope 40 MHz and passive probe

c.    1 RCA-BNC cable connector (75 W)

3. CIRCUIT DIAGRAM

4. BASIC THEORY

Video signal

The composite monochrome video signal (CVS) is composed of a video signal superimposed on an auxiliary signal of 300 mV. The levels between 0 to 300 mV are assigned for the auxiliary signal and the levels between 300 to 1000 mV are assigned to video information.

Modulation

In analogue broadcasting the composite video signal modulates the carrier by a type of amplitude modulation named VSB. The polarity of the modulation is negative, i.e., higher the level of the CVS, lower the level of the RF signal. If the level of CVS is 0 volt the level of the RF signal is % 100. The modulation index is so arranged that, the maximum level of CVS yields a RF level of  % 10 (sometimes  % 12.5). This value is known as the level of the residual carrier. If the modulation index yields more than % 10 for maximum level input (high residual carrier), the efficiency of the transmission drops, i.e., low contrast. On the other hand, if the RF level is below % 10 (low residual carrier), aural and visual signals begin to interfere each other. So it is important to keep %10 for 1000 mV input.

Measurement

Left: Input CVS (1000 mV., sawtooth) – Right: Demodulated output with zero ref pulse

To adjust the modulation index, an input of maximum level CVS (1000 mV) is applied to the modulator. The modulated RF signal is than applied to a professional TV receiver . The receiver has a facility to switch off RF for a short interval in each consecutive line. So during this interval, modulation ratio is effectively 0 %. The interruption on all lines in a frame is observed as a vertical white bar on a visual monitor. This bar is named as 0 reference pulse (or simply 0 pulse). The oscillogram of the 0 pulse is a pulse with a level more than the maximum level of the CVS. Taking the level difference between the sync tip and the 0 pulse as % 100, the maximum CVS should be 10 % or 12.5 %. The adjustment of the modulation index is simply the level adjustment of the modulating signal at the input of the modulator.

 A comprehensive idea of a TV camera function is illustrated in Figure 3-2 and 3-3. In Figure 3-2 the camera is aimed at scene / view so that the optical image can be focused on the target plate of tube makers (pick-up tube). If you look inside, you'll see the shadow-optical. The resulting video signal is shown by the oscilloscope waveform in the bottom left of the picture. Above is a monitor oscilloscope, which shows a reproduced image.

Figure 3-3. Block diagram that shows how a television camera dispense the output composit video signal. Here not shown the reflection and focusing the camera tube.

Details of the video signal waveform which is more fully shown by the block diagram in Figure 3-3. At first, the blanking pulses added to the camera signal. They cause the signal amplitude to the black levels so retrace the MRV will not be visible. Further, the alignment pulses (sync) is inserted. Alignment (synchronization) is required to set the time of MRV horizontal and vertical.

Camera signal with blanking and synchronization (sync) is called a composite video signal. Sometimes the term video signal which is not a composite (noncompoxite video signal) is used to identify the camera signal with blanking but without alignment. Standard output level of the composite video signal from the camera is 1V peak-to-peak (pp = peak to peak) with the alignment pulses in the down position for negative polarity.

5. EXPERIMENTAL PROCEDURE

1. Set-up devices such as a picture above, connect the video camera out with input CRO.
2.  ON the instrument.
3. Set the appropriate CRO to be easily observed (MODE on the TV-H position and / or TV-V). When seeing a wave of horizontal synchronization put MODE switch on the TV-H position, while to see a wave of vertical sync put the MODE switch on the TV-V position.
4. Specify the synchronization pulses, blanking pulses, front and back porch, and image information.
5. Image of the wave form and specify voltage.

6. RESULT AND ANALYSIS

a.    When the MODE swith on the TV-H position

T / div    = 20 ms x 6,4

        = 128 ms

V / div    = 0,2 mV x 4,6

             = 0,92 mV

b.    When the MODE swith on the TV-V position

T / div    = 20 ms x 3,2

= 64 ms

V / div     = 0,2 mV x 3

              = 0,6 mV

c.    Picture of information signal

T / div    = 20 ms x 6,4

= 128

V / div    = 0,2 mV x 6

             = 1,2 mV

d.    Picture of synchronization signal

e.    Horizontal Blanking Pulses

f.     Front Porch

g.    Back Porch

7. CONCLUSION

a.    Camera signal with blanking and synchronization (sync) is called a composite video signal, it combines the brightness information (luma), the color information (chroma), and the synchronizing signals on just one cable.

b.    In video camera, the blanking pulses added to the camera signal which causes the signal amplitude to the black levels so retrace the MRV will not be visible.

c.    The parameters of composite video are :

·         Information Signal (Luminance)

·         Horizontal Blanking Pulse

·         Vertical Blanking Pulse

·         Horizontal Synchronization Pulse

·         Vertical Synchronization Pulse

·         Burst Pulse

d.    In video camera, the alignment pulses (sync) is inserted that is required to set the time of MRV horizontal and vertical.

e.    Amplitude from the switch mode in TV-V = 0,6 mV  while the switch mode in TV-H = 0,92 mV.