Calculate highest modulating frequency for T.V. system

Question

Calculate the highest modulating frequency for following data:

Case: Alternate black and white dots for equal vertical and horizontal resolution.The no. of scanning lines are 250.Interlace ratio is 1:1 picture repetition rate is 50/second aspect ratio 12:9.Vertical retrace time 10% of frame delay.Horizontal retrace period is 20% of line period.Resolution factor is 80%.

Above question asked by my professor.I solved it.

My ans:

resolution factor=80%=o.8=kell factor

Vr=Na*k=250*0.8=200

Hr=Na*k*aspect ratio =250*0.8*(12/9) =266.66 pixels or line equivalent 267 pixels

There is 250 lines/frame there is 50 frames/second

So, 250*50=12,500 total lines

So time required is,

1/12,500=80 micro second

Retrace period is 20% of total time,

So, 80 micro second*20% =16 micro second

So,trace time = 80-16=64 micro second

1 horizontal line=267 pixel 2 pixel=1 cycle

So,

1 cycle=267/2=133.5=133 cycle (equivalent)

For 133 cycle required 64 micro second,So for 1 cycle

=64*10^-6/133 =0.48 micro second

So,

fmax=1/Tmin=10/0.48 micro second=2.08 mhz

But sir told me there is some small mistake in my solution but I didn't get whats wrong in above solution.

So is there anybody who can help me with this ?

Thank you.

Answer 1

I'm going to assume that "Interlace ratio is 1:1" means that this is a non-interlaced (i.e., progressive) system.

If the number of active lines per frame is 250, and 10% of the frame time is needed for vertical retrace, then there is a total of 250/.9 = 278 lines per frame.

If the frame rate is 50 Hz, then then line rate is 278×50 = 13900 Hz.

If the aspect ratio (active pixels per line / active lines per frame) is 12/9, then the number of active pixels is 250×12/9 = 333.

If 20% of the line time is required for horizontal retrace, then there is a total of 333/.8 = 416 pixels per line.

Since the line rate is 13900 Hz, the pixel rate is 13900×416 = 5.7824 MHz.

Since it takes two pixels to make a cycle, the highest video frequency is 5.7824 MHz/2 = 2.8912 MHz.

Note that if this were a 2:1 interlaced system, the line rate would be the same, but the number of pixels per line would need to double in order to match the 500-line vertical resolution, and so the pixel rate and highest video frequency would double as well.