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.
Best Answer
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.