Compression Ratios

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Compression Ratios

Post by tompocko » Wed Apr 26, 2017 4:33 am

Hi there,
I am doing some research into video over Internet Protocol for my dissertation. However, I am very confused about how compression ratios work. If I have a data stream of 100MBps and I use a 5:1 compression I am not sure what my output would be.
I have been thinking of it in two different ways:
1. 5:1 ratio means the compressed data rate is 1/5 of the original file e.g. (100 * 1/5) = 20

2. 5:1 means I half the data rate five times e.g. ((((100/2)/2)/2)/2) = 6.25.

If anyone could shed any light on my confusion I would be extremely grateful.

Thank you in advance!

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Re: Compression Ratios

Post by PID_Stop » Wed Apr 26, 2017 8:09 am

It's your first answer: the data compression ratio is the proportion of uncompressed size or rate to the compressed size or rate. Wikipedia's article goes into more detail.

Where many people get confused is on the difference between data compression versus audio level compression -- the two are entirely unrelated. The first reduces how much data is required to represent a signal (video, audio, or any other type of file or stream); the latter controls the dynamic range -- that is, the range of variation from softest to loudest levels -- of an audio signal.

Here's a practical example of data compression from my world: our facility mainly uses video in 720p format (most of the stations we run from this facility are ABC affiliates). The uncompressed signal (video and audio together) has a 1,485 Mb/s data rate, but under the ATSC system, the over-the-air signal has a maximum rate of 19.39 Mb/s. The MPEG-2 encoder compresses the signal down to a data rate that averages somewhere around 10 Mb/s (it varies, and our equipment is smart about trading off the actual rate with the other program streams we transmit, in order to maximize quality on every stream). Thus, our average compression ratio is on the order of 1,485:10, or 148:1.

Best of luck on your dissertation!

-- Jeff

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