Question: Matching camera data rates to NLE data rates?

D

Denniscasey

Well-known member
Still learning all this stuff so another basic question. From literature HMC40 PH 1080i recording rate is 21 to 24 Mbps, 4:0:0 and 8 bit color sampling depth. Avid Media Composer recommends using their DNxHD145 for transcoding 1080i 8 bit to use in Media Composer editing. Avid documentation says DNxHD has a data rate of 145 Mbps. Since I have read one should try to "match" data rates of camera to editing suite data rates, there must be some different measuring metrics used between camera data rates and editing suite data rates?? I suspect I am trying to measure apples and oranges at my novice stage.

I am happy with results when I use DNxHD145 but just wonder if I would get "better" look if I use DNxHD220 or much faster editing without much loss in high definition if I use a lower data rate like DNxHD36.
 
You are correct, you're looking at apples and oranges. The HMC40's PH mode is using long-GoP h.264 at an extremely efficient data rate (and it's 4:2:0, not 4:0:0; 4:0:0 would be a black & white signal). Whereas DNxHD is an older intraframe-only codec, not nearly as space-efficient as the newer MPEG4 codecs. Which isn't bad, it's fine, DNxHD is a great codec, but the point is you can't possibly compare data rates and think that they'll result in equivalent quality.

I doubt you'll exceed what DNxHD145 can do, using an AVCHD source. You may get a very slightly better look out of 220, but -- well, hey, try it. I wouldn't use 36, that looks like it's mainly for use as an offline proxy.

Hopefully we'll have some Avid users weigh in here on their experiences, I just wanted to try to put you on the right path by saying yeah, don't worry about trying to "match" bitrates; when transcoding you always want to transcode to the highest quality format you can afford (which would beam DNxHD220 where possible) but that I would suspect that DNxHD145 is probably going to extract all the image quality that an AVCHD signal is going to contain, so going to 220 may not be necessary or even produce any noticeable difference.
 
Thank you Barry and I did mean 4:2:0. I did first quickly scan your very informative "The HMC Book" before posting (HMC Book is my first "go to" source). From practical sense sounds like I should just stay with DNxHD145. However, if I ever get my dream of an HD projector to project on big screen (6 X 8 foot or so) I wonder if I would see an improvement using DNxHD220--I don't have means to test that out myself?

As an aside, and as I mentioned many months ago, I am still intrigued by fact that you can get a 4:2:2, 10-bit signal out of HDMI connection (page 229 of your book) --something of little practical consequence for work at my level, but interesting none the less. Also, you may be interested in an e-mail exchange I had with Panasonic tech help when I was trying to understand more about using HMC40 camera mode. I asked if I would get a 3X improvement in printed resolution of photo if I used camera mode resolution vs capturing a single frame in video due to 10.6 megapixels in camera mode vs 3.5 megapixels (or whatever it is) in video mode. They replied quote " A 10.6 megapixel picuture will have more detail than a 4.5 megapizel image. However, in terms of the resolution difference you will gain only about 10 - 20% by using the highest quality mode, but the file sizes will be much bigger.. " which is just what you say somewhere in your HMC Book. I then asked "why only a 10 to 20% improvement and they replied " I can't explain but that is the information we have." Regards, Dennis


 
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