Confused About Data Rates

Posted on by Larry

[ This article was first published in the September, 2010, issue of
Larry’s Monthly Final Cut Studio Newsletter. Click here to subscribe. ]


Robert Guzy writes:

I recently purchased your 1 hr Webisode on Compression using Compressor. It was excellent but left me with a few questions. I am confused by the Data rate info.


I have been given the following spec: video= 500 kbps-1 Mbps.


When using Compressor it seems that the only control I have is in the video settings>Data Rate restrict which is given as …kbits/sec. When I create an H.264 file of a 4:47 sec DV QT file and set above Data Rate Restrict at 900 kbits/sec I end up with a file of 36.6MB (36,593,794bytes)


My confusion is how all these numbers relate as (MB,kbps, kbits) all seem to be different info, but nowhere is there a constant that is common between all.


How do I know that my 36.6MB file fulfills required spec. of 500kbps-1Mbps

Larry replies: Bob, data rates, too, are very confusing because of a single letter.

The constant is “bps” — which stands for “bits per second.”

Thus, “kbps” is “kilo bits per second” — 1,000’s of bits

And, “mbps” is “mega bits per second” — 1,000,000’s of bits.

So, their requirement of 500 kbps to 1 mbps translates to “500 – 1,000 kbps.”

Your compression setting of 900 kbps falls perfectly into that range.

36 MB for a 5 minute files seems to me to be reasonable in size — assuming your frame size is something like 640 x 360 or so.

If you CAPITALIZE the “Bps” — now you are talking “Bytes per second”. This is the normal way of measuring hard disk speed, but not the Internet.

“bps” / 8 = “Bps” — confusing, I know, but that’s the way it is…

As I tried to stress during the webinar, Compressor doesn’t care what format your media starts with. SD or HD, is all the same. What you are doing is specifying how you want it to end up. Thus, you change the setting in the Geometry tab to determine the final image size of your compressed file.

Hope this helps.


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