Exam Question - end-to-end delay from video capture to video playout

Question

Hi I'm preparing for my exam tomorrow and I've come across a 17 Mark question which seems trivially easy, so I think I'm missing a key point giving the rest of this exam is fairly difficult. The question is below and my attempt is below that.

My attempt:

I looked up video capture on wikipedia and it defined it as the process of converting an analog signal to digital - so I took the point at which frames were captured as being after they were digitised. We are told to ignore the time taken for compression and decompression, so the next part is just to send the data. The total amount of data in the frame sequence is 54KB (I'm not sure why the frame sequence starts and ends with an I-frame to be honest - doesn't seem to make sense). So to transfer 1s of video will take 0.27s.

To be play a second of frames smoothly the the playout buffer must contain 25 frames minimum. The sequence of frames is IBBPBBI which has a total size of 54 KB. In one second the playout is 54 x 25 = 0.432 MB/s.

I'm pretty sure that we would need to make some estimations to solve the rest of this.

As always I'm open to any suggestions! Thanks for reading :D

Answer 1

There are a few mistakes in your math.

First, you are multiplying an entire group of pictures (7 frames) by 25, that would be 7 seconds of video, not one.

Second, you are not considering that the first I frame of one set is the last of the next, so it should be assumed that the sequence that would be used is IBBPBBIBBPBBIBBPBBIBBPBB... It would make no sense at all to put two I frames together in a row, but sometimes the I is listed as an end.

Third, you are not considering the fact that a B frame is bi-directional and can not be processed until the next I or P frame is reached.

These three things significantly complicate the problem from the simple state you initially thought it was, though as this is an academic problem, I will leave the actual solving as an exercise for you.