Analog Television Scanning Test

Answers and Explanations

Question 1: What is the primary purpose of scanning in analog television systems?

Correct Answer: A - To convert a 2D image into a 1D electrical signal

Scanning systematically converts the spatial information of a 2D image into a time-varying 1D electrical signal that can be transmitted. The electron beam moves across the screen in a specific pattern (raster) to sample the image point by point and line by line.

Question 2: In the NTSC system, what is the total number of scan lines per frame?

Correct Answer: B - 525 lines

The NTSC (National Television System Committee) standard used in North America and parts of Asia employs 525 scan lines per frame. This includes both visible lines and lines in the vertical blanking interval.

Question 3: What is the primary reason interlaced scanning was used in analog television?

Correct Answer: A - To reduce flicker without increasing bandwidth

Interlaced scanning displays alternating sets of lines (fields) at a higher frequency than the frame rate, effectively doubling the perceived refresh rate without requiring additional bandwidth. This reduces the perception of flicker while maintaining the same data rate.

Question 4: In interlaced scanning, what is the relationship between fields and frames?

Correct Answer: A - One frame consists of two fields

In interlaced scanning, each complete frame is composed of two fields: one containing the odd-numbered lines and the other containing the even-numbered lines. These fields are displayed sequentially, with the second field filling in the gaps between the lines of the first field.

Question 5: What is the field rate in the NTSC system?

Correct Answer: D - 59.94 fields per second

The NTSC color television system uses a field rate of approximately 59.94 Hz (originally 60 Hz for black and white). This slight reduction from 60 Hz was necessary to accommodate the color subcarrier frequency while minimizing interference with the audio subcarrier.

Question 6: What is the purpose of the vertical blanking interval?

Correct Answer: D - Both A and C

The vertical blanking interval serves multiple purposes: it allows time for the electron beam to return from the bottom to the top of the screen (vertical retrace), and it provides a period during which additional information like test signals, closed captioning, and other data can be transmitted without affecting the visible picture.

Question 7: In progressive scanning, how are lines displayed?

Correct Answer: A - All lines are displayed sequentially from top to bottom

In progressive scanning (non-interlaced), all lines of the image are displayed in sequence from top to bottom in a single pass. This creates a complete frame in one scanning cycle, unlike interlaced scanning which requires two fields to create a complete frame.

Question 8: What is the Kell factor in television scanning?

Correct Answer: C - A factor accounting for the reduction in effective resolution due to discrete sampling

The Kell factor (typically around 0.7) accounts for the loss of effective resolution in raster-scanned systems due to the discrete nature of sampling. It represents the ratio between the theoretical maximum resolution and the practically achievable resolution, considering factors like spot size and sampling effects.

Question 9: What is the primary disadvantage of interlaced scanning compared to progressive scanning?

Correct Answer: B - It can cause visible artifacts with moving objects

The main disadvantage of interlaced scanning is that it can produce visible artifacts, particularly with moving objects. Since the two fields of a frame are captured at slightly different times, fast-moving objects may appear to have "tearing" or "combing" effects where the object appears in different positions in each field.

Question 10: In the NTSC system, how many active lines are visible per frame after accounting for vertical blanking?

Correct Answer: A - Approximately 483 lines

While the NTSC system has 525 total lines per frame, not all are visible. Approximately 42 lines are used for the vertical blanking interval, leaving about 483 active lines that contain visible picture information.

Question 11: What is the purpose of the horizontal blanking interval?

Correct Answer: D - All of the above

The horizontal blanking interval serves multiple functions: it provides time for the electron beam to return from the right side to the left side of the screen (horizontal retrace), it contains synchronization pulses to control the horizontal deflection circuitry, and it prevents the retrace lines from being visible on the display.

Question 12: Why was 59.94 Hz chosen as the field rate for NTSC color television instead of exactly 60 Hz?

Correct Answer: C - To minimize the visibility of the color subcarrier

When color was added to the NTSC system, the field rate was slightly reduced from 60 Hz to 59.94 Hz to create a specific relationship between the color subcarrier frequency and the line frequency. This relationship was chosen to minimize the visibility of the color subcarrier pattern on the screen by making it interlace in both space and time.