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same bitrate, lower resolution: better compression?

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Alan Langdon
same bitrate, lower resolution: better compression?
on Sep 15, 2012 at 4:04:09 am

I am not sure if this is the correct forum to post this question, but it is the closest I could find. If anybody can suggest a better forum, please do so :)

I have a camcorder (yes, this is the DSLR forum, but read on) which records in the usual AVCHD codec, which some DSLRs now use also.
My question is: I can shoot in 1080p or 720p, both in 24p, and both in 24 mbps compression rate. So, if I shoot in 720p (enough for what I need, in terms of resolution), will I get less compression artifacts? I don't need full HD, but I can get less compression artifacts by using the same bitrate at a lesser resolution?


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Steve Crow
Re: same bitrate, lower resolution: better compression?
on Sep 15, 2012 at 3:22:43 pm

I don't think so, the video is still being compressed into the AVCHD format in either case. You could try raising the bit rate (if possible) that should do more to improve the overall quality of the clips you are capturing. I think of it this way: The AVCHD is a compression algorithm that the raw video signal is being processed by - the frame size doesn't change how the algorithm works. Maybe there will be a slight perceived difference but nothing I would bank on....why don't you just test this to find out and let us know?

Steve Crow
Crow Digital Media
http://www.CrowDigitalMedia.com


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Peter Burger
Re: same bitrate, lower resolution: better compression?
on Sep 16, 2012 at 12:38:10 pm

[Steve Crow] "I don't think so, the video is still being compressed into the AVCHD format in either case. You could try raising the bit rate (if possible) that should do more to improve the overall quality of the clips you are capturing."

Maybe I misunderstood, but as I got the question, the bitrate is fixed but you can change the framesize.

Since 720p has just about half the information than 1080p (if I calculated correctly) and if you're compressing with the same bitrate, framerate and codec, IMHO the resulting clips will almost certainly have better quality.

The question is though - just like you wrote Steve - if it is perceivable.

But even if not perceivable by eye, I'd almost bet that in post you'll have more "room" to play with 720p clips of 24mbps than with 1080p clips of 24mbps.

One uncertain factor for me is: Does the camcorder record with cbr or vbr, meaning: are these 24mbps the maximum limit or does it use the full bandwith no matter which framesize.

But Alan, just as Steve wrote, do a little testing. Record some clips under difficult conditions like low-light, fast movement and a lot of details, like "moving leaves on a tree at dawn" ;) something like that and than compare.
And yes, please let us know! Very interesting subject!

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Dave LaRonde
Re: same bitrate, lower resolution: better compression?
on Sep 16, 2012 at 7:27:49 pm

[Alan Langdon] "...I can shoot in 1080p or 720p, both in 24p, and both in 24 mbps compression rate. So, if I shoot in 720p (enough for what I need, in terms of resolution), will I get less compression artifacts?"

If you're seeing ANY compression artifacts at all, you should be using a different camera! You've progressed in your skills past the camera's capabilities.

I'd say its a wash between 1080 and 720 at 29.97.

Camcorders will shoot interlaced (59.94i), but the total screen resolution is greater. At 720, the same camcorder will shoot 59.94p. Even though the screen resolution is lower at 720, we're talking 59.94 complete frames ever second, and not 59.94 fields.

Dave LaRonde
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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