100Mbps question

M

Mark-P

Well-known member
What does it actually mean by it records in 100Mbps DVCPRO HD? What is this really a measure of?

Thanks.
 
100Mbps = 100 Megabits per second.

Digital Video is data - so you have 100 megabits of information per second of DVCProHD footage.

There are 8 bits in a Byte.
So, 100 megabits per second = 12.5MB (MegaBytes) per second

Of course, it's never as clear cut as it seems - DVCProHD is 100mbps for a full 720 60fps signal, or 1080 60i signal.
Since it is an intraframe codec, it always compresses each frame equally (unlike HDV which is an interframe codec and compresses uses GOPs across frames). In interframe codecs the bitrate is variable - which means if you lower one factor (for example the frame rate) it will increase another factor (like the bitrate per frame). In intraframe codecs, each frame is always compressed exactly the same - regardless of framerate or anything else.

SO, what does that mean?
It means that each frame in DVCProHD is ALWAYS compressed at the same rate -

So, if it's 100megabits per second - and there are 60 frames per second of 720 footage (or 60 fields in 1080), then we have:

100/60 = 1.667megabits per frame


SO - if you're recording variable frame rates in 720 mode, you won't actually be recording 100mbps.
If, for example, you are recording 24fps you'll be recording a 40mbps signal.
Because 24 is 40% of 60, so you are getting 40% out of 100mbps (because you are only recording 40% of the frames, which are all compressed equally).

Does that make sense?
 
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