Joined: 04 Feb 2003 Posts: 587 Location: Lisboa, Portugal
Posted: Tue 23 Jan 2007, 18:00 Post subject:
The type of connection used to transfer the video makes no difference. What matters is the codec and the encoding settings (that incudes the bitrate). For those same settings, it doesn't matter if the file is going over a TCP connection or over a SATA or SCSI or SDI connection; the data remains the same.
Of course, some encoders that compress video "on-the-fly" will monitor the speed of the connection and adjust the compression in (near) real-time. But, in that situation, the quality will depend on the line conditions anyway, so you can't judge what it will be like under different conditions (e.x., if you do your testing while the line is almost free, it might look great, but when the line gest busy it'll look much worse, or vice-versa).
Either way, quality is a very subjective concept, it's not something that you can define with a number. For example, is it better to have 15 very sharp frames per second or 30 slightly blurry ones? Is it better to have great contrast but lots of noise, or no noise at all but poor contrast? Different codecs make different tradeoffs, and some people might prefer one over another.
You can try compositing the original and compressed streams (using an editing or animation program) in "subtract" mode; that will give you an idea of how different they are (pixels that are identical will appear black, pixels that changed will appear in colour). Of course, then it's still a matter of opinion which "differences" are more important (ex., is it more important to have faithful gradients or is it more important to have faithful outlines, etc.).
All things considered, the best "system" out there to judge image quality is a human brain (preferably one with some experience in video compression, but judging quality is simpler than achieving it). Just look at the original image, then at the compressed image (if possible flip between the two), and see what each codec or compression setting does.
Specifically, look at things such as contrast (some codecs turn white into light gray and black into dark gray), edge noise (blocks or "mosquitos" around the edges of moving objects), grain, banding (non-continuous gradients), overall sharpenss, resolution, and frame rate.
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum