VIDEO CAMERA PRACTICE

A.Objective :
1. Know the video camera.
2. Measuring the composite video on a video camera.
3. Determine the parameters of composite video.

B. Used Equipment:
1 Video Camera
1 Oscilloscope 40 MHz and passive probe
1 RCA cable connector – BNC (75)W

C. Circuit diagram :

D.Introduction :

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.

Composite Video

Composite video is the format of an analog television (picture only) signal before it is combined with a sound signal and modulated onto an RF carrier.

It is usually in standard formats such as NTSC,PAL, and SECAM. It is a composite of three source signals called Y, U and V (together referred to as YUV) with sync pulses. Y represents the brightness or luminance of the picture and includes synchronizing pulses, so that by itself it could be displayed as a monochrome picture. U and V represent hue and saturation or chrominance; between them they carry the color information. They are first modulated on two orthogonal phases of a color carrier signal to form a signal called the chrominance. Y and UV are then combined. Since Y is a baseband signal and UV has been mixed with a carrier, this addition is equivalent to frequency-division multiplexing.

Interlaced versus Progressive Scans

These are two different types of scanning systems. They differ in the technique used to "paint" the picture on the screen. Television signals and compatible displays are typically interlaced, and computer signals and compatible displays are typically progressive (non-interlaced). These two formats are incompatible with each other; one would need to be converted to the other before any common processing could be done. Interlaced scanning is where each picture, referred to as a frame, is divided into two separate sub-pictures, referred to as fields. Two fields make up a frame. An interlaced picture is painted on the screen in two passes, by first scanning the horizontal lines of the first field and then retracing to the top of the screen and then scanning the horizontal lines for the second field in-between the first set. Field 1 consists of lines 1 through 262 1/2, and field 2 consists of lines 262 1/2 through 525. The interlaced principle is illustrated in Figure 2. Only a few lines at the top and the bottom of each field are shown.

A progressive, or non-interlaced, picture is painted on the screen by scanning all of the horizontal lines of the picture in one pass from the top to the bottom. This is illustrated in Figure 3.

 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 depicted in Figure 3-2 and 3-3. In Figure 3-2 the camera is aimed at scene / view so that the optical image (optical image) can be focused on the target plate tube makers (pick-up tube). If you can 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 the television camera composite video signal output channel. Here is shown the reflection and focusing the camera tube.

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

Camera signal with blanking and synchronization (sync) is called a composite video signal (composite video signal).Sometimes the term is not a composite video signal is used to identify the signal with blanking camera 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.

E. Experimental Procedure

1. Set-up devices like the picture above, connect the video camera out with input CRO.
2. Consolidate his 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 MODE switch put on the TV-H position, while to see a wave of vertical sync put the MODE switch on the TV-V position.
4. Determine the synchronization pulses, blanking pulses, front and rear porch, and imageinformation.
5. Image of the wave form and specify voltage.

F. Experiment Results and Analysis

a. When the MODE switch 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 switch 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

G. 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.