Computer Science compression question

[overlyeducate]

1. A streaming Netflix movie is usually 720 by 480 pixels. The color for each pixel takes 3 bytes to encode, and a feature film shows 24 frames (individual still images) per second.
a.) How many kilobytes are needed to encode one frame, without compression?

b.) How many GB are there in a two-hour movie, without compression?

c.) The maximum data transfer rate used by the Netflix software is 2200 kbps. What compression ratio does are they using? (To find the compression ratio, divide the number of bits per second actually transferred by the number of bits per second for the uncompressed movie. Express the result as a percentage.)

d.) At the compressed data transfer rate, how many GB must be transmitted to stream a two-hour movie?

e.) If Netflix pays 3 cents per GB to send out data, how much does it cost to deliver a two-hour movie? Compare this to their cost of 78 cents to send a physical DVD by mail.


Could I please have some help? For part A, I have an answer of 1012.5 kilobytes, and for part b, I have an answer of .0231742859 GB. In order to solve parts c and e, I would need to know the answer for part d. In this case, would the compressed data transfer rate be 2200 kbps, and if so, how could I determine how many GB must be transferred to stream the two hour movie? I would appreciate any help I could get. Thank you so much.

 

[SteamKing]

I think you need to check your calculation for part b) again.

 

[overlyeducate]

For part b, I first calculated how many pixels there would be in a two hour movie. Since each individual frame has 720X480 pixels, or 345600 pixels, the number of pixels for a movie, which is 24 frames, would be 345,600X24, which would be 8,294,400 pixels. Since each pixel would take 3 bytes to encode, I did 8,294,400X3, which would be 24,883,200 bytes for the entire movie. This is the movie in bytes, so to convert it to gigabytes, I took the 24,883,200 and divided it by 1,073,741,824, since 1 gigabyte =1,073,741,824 bytes. My answer was .0231742859 GB. Do you agree? Please let me know.

 

[Mark44]

For part b, I first calculated how many pixels there would be in a two hour movie. Since each individual frame has 720X480 pixels, or 345600 pixels, the number of pixels for a movie, which is 24 frames, would be 345,600X24, which would be 8,294,400 pixels.

No, a movie is not 24 frames. That's only one second of a movie. For one second of a movie, 8,294,400 pixels X 3 bytes/pixel = 24,883,200 bytes need to be streamed.

Since each pixel would take 3 bytes to encode, I did 8,294,400X3, which would be 24,883,200 bytes for the entire movie. This is the movie in bytes, so to convert it to gigabytes, I took the 24,883,200 and divided it by 1,073,741,824, since 1 gigabyte =1,073,741,824 bytes. My answer was .0231742859 GB. Do you agree? Please let me know.

 

[overlyeducate]

Oh, I see what I did there. I have to take the 24,883,200 bytes and multiply it by 7200 seconds, because that's how many seconds there are in 2 hours. I would then take that answer, which is 1.7915904 x10^11, and divide that by the 1,073,741,824. That gives me an answer of 166.85, or 167 GB. I believe this is the right answer. Would anyone happen to know how to do part d?

 

[Mark44]

Did you do part c? They're asking you for the compression rate. Part b says that the Netflix software can stream up to 2200 kB/sec. How many kB are there in 1 second of the movie (24 frames)?

 

[overlyeducate]

If one second of a movie is 24883200 bytes, then in kilobytes, that would be 24,300 kilobytes, since you would have to do 24883200 divided by 1024. Can that be used to solve part c? I seem to be confused on parts c and d. Once I have those, I could solve part e.

 

[Mark44]

I didn't check your numbers, but assuming they're correct for the moment, 1 second of the movie is 24,300 KB, but Netflix can stream only 2,200 KB per second. So what's the compression ratio? That's what they're asking for in part c.

The way it works is something like this.
Raw movie bytes ---> compression SW ---> internet ---> home connection ---> decompression SW---> TV