General mpeq quality question.

[SiG_ZA]

Say I have an mpeg2 file with a CBR of 2050 KBps.

I then use this file as a source file in my movie making software. As this software has limited options to encode to mpeg2 I encode it at max quality and say with a CBR of 4500 KBps.

This will obviously result in a larger file than the quality of the source justifies.
If I now take this new file and use TMPGEnc Plus to re-encode back to a CBR of 2050 KBps.

Will I loose quality, and if so, how much and why?

Thanks

[yoda313]

Don't increase the bitrate. No need to as it really won't make it look better. If its not there don't bother. Whenever you mess with digital video the quality is altered. Just author with the original video and leave the setting alone (unless you need to edit it with some filters or something but try to leave the bitrate the same).

[SaSi]

In general, encoding an already encoded video file at a higher bitrate using the same codec should not decrease quality, since the source video file has already lost whatever content is to be lost at the lower bitrate.

Therefore, one could argue that re-encoding back to the same bitrate as the source would bring the bitrate down without any noticeable decrease in quality.

On the other hand, the source video may be encoded at 2050kbps but the encoding at 4500 can waste some quality. Subsequently, re-encoding back at 2050 can also waste some quality.

The most tricky parts in a video file are scene changes and high motion scenes.

Since most of the video is encoded as delta frames of some sort (i.e. the frame only contains the difference between a frame and it's previous one), a scene change, where the picture of a frame and the next one are radically different taxes the encoder/codec which must identify this scene change as a point where a normal delta frame will not be ideal so as to insert a full frame (key frame, I frame or whatever).

Some codecs are not really good at this and several encoders have settings that may disallow them from automatically inserting key frames where they are needed.

So, any encoding step may decrease quality due to this factor.

Another factor is high motion scenes.

In VBR encoding, the overall bitrate may be, say, 2050kbps, but certain scenes may be encoded at as high as 6000kbps because of their high motion content. Encoding such a file at 4500kbps CBR will cause loss of quality at these high motion scenes originally encoded at a higher, local, bitrate.

Having said all the above, if your requirement is to re-encode to an intermediate file and you don't want to loose quality, encode at a very high bitrate. For full frame DVD video, this would be around 8500kbps using MPEG-2. You could also encode at a lower bitrate using a more efficient codec, like DivX or XviD (MPEG-4 is more efficient than MPEG-2) and then re-encode the final video at the desired bitrate.

However, the final encoding, if it has to be at such a low bitrate, should be done in VBR with carefully selected parameters (like variable GOP length, carefully selected Q-Matrices and motion search length). Encoding any type of video at 2050kbps CBR at full DVD resolution will cause poor quality.

[SiG_ZA]

@SaSi, Thanks

Encoding any type of video at 2050kbps CBR at full DVD resolution will cause poor quality.

The source is at 2050 KBps CBR and is a home movie from a video camera (8mm) so the quality was never that great to start with.

...if your requirement is to re-encode to an intermediate file and you don't want to loose quality, encode at a very high bitrate. For full frame DVD video, this would be around 8500kbps using MPEG-2. You could also encode at a lower bitrate using a more efficient codec, like DivX or XviD (MPEG-4 is more efficient than MPEG-2) and then re-encode the final video at the desired bitrate.

Video editing software does not have DivX or XvidD as rendering options, only mpeg1, mpeg2, mp4, wmv. The maximum bitrate for mpeg2 is 15,000,000 and for mp4 is 768,000 bps. So do you think mpeg2 is the better option?

Though the 15,000,000 bps option produces a huge file at 15,000 KBps. Shouldn't it be 15,000,000 /8 /1000 = 1875 KBps?

VBR with carefully selected parameters (like variable GOP length, carefully selected Q-Matrices and motion search length).

I would rather leave these settings at the PAL 4:3 default template settings, as I have no clue here about changing them.

[guns1inger]

Use Virtualdubmpeg2 to convert the file to uncompressed avi, or a lossless codec like huffyuv (if you editing software will read it) or even DV avi. Do your editing, then encode it at an appropriate level. I would not go back to 2050 unless you have no other choice.

15000kbps is approx 15mbps - much higher than the DVD Spec allows for. It will produce huge files, but you still have the problem that it is mpeg, and therefore poor for editing.

[SiG_ZA]

convert the file to uncompressed avi

An option, but the file would be huge. As it is, when I rendered the movie (2:15 hours) at CBR 15000 KBps MPEG2, the file was 14.5 gig large.

or a lossless codec like huffyuv

Will try, not sure if the movie editing software will read it though.

Thanks