Quality loss and bitrates
by Nick White on Apr 15, 2012 at 10:56:24 am
Until recently I was a still shooter exclusively. I dealt with lossy formats and non-lossy formats. jpg for instance was lossy, and it was not a good idea to keep editing and saving/reopening in that format, but to use it for final output to match desired quality/size.
But now I often get in bitrates when saving/rendering.
My question is: If I save in the same format (exactly in all respects: file type/codec/version, frame size etc) as I had as a source and at the same bitrate as the source, do I lose quality?
My other choice (AFAIK) is to render uncompressed, which results in huge, often unplayable, files, useful only for storage and/or sending to other renderers for processing without loss in the handover. I have sued these, but if there is a way to avoid the terabytes involved, yet not lose quality (even maybe not lose _perceivable_ quality)then that would be nice.
Re: Quality loss and bitrates by Andrew Rendell on Apr 22, 2012 at 8:49:20 pm
[Nick White]" If I save in the same format (exactly in all respects: file type/codec/version, frame size etc) as I had as a source and at the same bitrate as the source, do I lose quality?"
If the codec is an all I-frame one, like Apple's ProRes or Avid's DNxHD, then there is no noticeable loss in quality in editing and resaving the material. If the codec is a GOP one (h264, AVCHD, etc) and you edit it, the computer has to recreate the GOP structure, effectively decoding and the re-encoding the video stream, which does have the potential for losing quality.
In the UK, most broadcasters are following the BBC's precedent that video is "broadcast quality" if it is (a) 4:2:2 sampled, and (b) 50 Megabits per second or better.
In practice, there is plenty of 35Mbps XDCam in use (plus a fair amount of other formats that are lower than the above spec, e.g., DSLR) and I can't usually tell the difference between that and 50Mbps by the time it's transmitted (things like lens quality, chip size, exposure settings and grading make at least as much difference IMO).
So it's important to keep the number of decoding/encoding steps to the absolute minimum and anything that's originated in a GOP format should be converted to a high quality I-frame one, but it's not necessary to make your material uncompressed. (IMO uncompressed is only really useful as an intermediate if you're going to make copies in various other codecs).
Re: Quality loss and bitrates by Nick White on Apr 23, 2012 at 2:53:47 am
Wow! Ok. This commercial stuff is mostly above my aims or station. :) My camera is "prosumer": Panasonic HDC HS700. So yeah it's pretty good, as your consumer stuff goes, but big zoom and small chips etc. It puts out about 35mBPS AVCHD. I do have an HD -capable DSLR, but for everyday shooting it has versatility problems AFAIAC.
I do realise that it's better to keep the number of renders to a minimum. It's just that so often it seems that you need to export a rendered version to allow work that cannot be done "here". Mind you, that is where AVID's DNxHD uncompressed pops up, I guess.
I will look into all you say, make no mistake! Your first paragraph is very interesting. The idea of getting everything into the I-frame format is new to me and I am going to follow up.