Composite Video Signal Blanking: Horizontal and Vertical Mechanisms
In the world of analogue television, a Composite Video Signal (CVS) carries all the necessary information to reconstruct an image on a screen. However, the process of drawing an image requires precise timing and brief pauses to allow the electron beam of a receiver to reset. These pauses are known as blanking.
Types of Blanking in CVS
Blanking is divided into two primary categories based on the direction of the image scan: horizontal and vertical. Horizontal blanking occurs between individual lines of the image, while vertical blanking occurs between fields, which are half-frames of the complete image.
When a TV receiver is poorly tuned, these blanking intervals may become visible to the viewer. Horizontal blanking typically appears as artifacts at the left or right edges of the screen, whereas vertical blanking manifests as distortions at the top or bottom of the image.
[ไม่มีภาพประกอบ]Vertical Blanking in Detail
Vertical blanking is a critical interval that allows the scanning mechanism to return from the bottom of the screen to the top to begin a new field. The duration and characteristics of this interval vary depending on the regional analogue television standard used.
Regional Standards: System B vs. System M
In System B (the analogue system used in most of Europe, as well as Systems G and H), the vertical blanking interval lasts approximately 1612 μs, which is equivalent to the time required to scan 25 lines. In contrast, System M (the analogue system used in the USA) has a shorter vertical blanking interval of 1333 μs, equivalent to 21 lines.
Utilization of the Blanking Interval
Not all of the vertical blanking period is used for the physical reset of the scan. While 7.5 lines are dedicated to the synchronization of the image, the remaining lines are available for auxiliary data. Specifically, VIT signals (Vertical Interval Test signals) are inserted into this space. Two lines in each field are reserved for these test signals; since a full frame consists of two fields, a total of four lines per frame are dedicated to test signals.
Key Facts
- Horizontal Blanking: Occurs between lines; visible at the sides of the image if the receiver is poorly tuned.
- Vertical Blanking: Occurs between fields; visible at the top or bottom of the image if poorly tuned.
- System B (Europe): Vertical blanking is 1612 μs (25 lines).
- System M (USA): Vertical blanking is 1333 μs (21 lines).
- Synchronization: 7.5 lines are used for image synchronization.
- Test Signals: Four lines per frame (two per field) are reserved for VIT signals.
| Feature | System B (Europe/G/H) | System M (USA) |
|---|---|---|
| Blanking Duration | 1612 μs | 1333 μs |
| Equivalent Line Count | 25 lines | 21 lines |
| Sync Line Usage | 7.5 lines | 7.5 lines |
Frequently Asked Questions
What is the difference between horizontal and vertical blanking?
Horizontal blanking happens between every line of the image, while vertical blanking happens between fields (half-frames) to allow the scan to return to the top of the screen.
What are VIT signals?
VIT signals are test signals inserted into the vertical blanking interval of a composite video signal.
How many lines are reserved for test signals in a full frame?
There are four lines reserved for test signals per frame, as two lines are allocated in each of the two fields that make up a frame.
Why can blanking sometimes be seen on a TV screen?
Blanking intervals may become visible at the edges or top and bottom of the image if the TV receiver is not tuned correctly.
How much of the vertical blanking interval is used for synchronization?
Exactly 7.5 lines of the vertical blanking interval are used for the synchronization of the image.