Icic.
So how does the bitrate actually work? In relation to mpeg 2/4.
I try copying the dvd, mpeg 2, high quality, 1920 x 1080, 10000 bitrate.
File size of around 10/11gd. Seems to be as gd as the dvd itself.
And another, mpeg4/h264 acc3. Same resolution. File size was of course much smaller.
However, the images appeared pretty pixelated. And a bit jerky.
I am copying the dvd to store on an external hdd for connecting to a mac, then mirroring to apple tv. Just for, you know. Occassions when theres gatherings, and u wanna watch some movies. As ur know, dvds tend to get scratched. Especially when there are much younger children around.
Encoding is an art, you don't just merely use the same bitrate between codecs and then compare the quality. The fact that MPEG4 can bring more savings in space is first the algorithm employed are more sophisticated, next you need more encoding time using these more sophisticated techniques. Also the decoding complexity also increased, that's why for good quality MPEG4 encoded video, normally hardware accelerated or assisted decoding is required and for encoding, the time required is a lot more. The priority here is quality per unit size that we are trying to achieved here.
Bitrate across the video is often averaged in a VBR video that is employed in MPEG4. MPEG2 also be VBR but there are some older versions of hardware player that may not be able to decode such MPEG2 DVD and hence they are often encoded in CBR.
For VBR, there is either a single pass encoding or multi pass encoding. For multi pass encoding, normally 2 passes are used but there is option for 3 passes encoding.
On first pass, encoder analyze the video and assigned allocated birtate to each frame or block of frames by analyzing the complexity of the scene. On second pass, the encoder perform the actual encoding adjusting the quantisation matrix to fit the allocated bitrate.
At the end, the actual bitrate displayed to a video is an average bitrate. It does not mean the whole video is using that bitrate thru out. Based on the actual profile constraint, encoder respect by ensuring max bitrate is not exceeded. Players that comply to these profiles will ensure they can still continue to play the video when bitrate spikes and also other features like the maximum number of B and P frames between I frames etc.
In MPEG4 there are numerous features like motion estimation distance and different searching patterns to help make more accurate comparison between frames to ensure better compression.
Like all encoding, use a good source. Never use another encoded source like MPEG2 and encode into MPEG4 and perform comparison. It just get worse all the time due to the lossy nature of video encoding.
