Transcoding/Re-encoding: Your thoughts ?

[jimmalenko]

I have a number of AFL DVDs which I am backing up - all around 2&1/2 hours long. I have used DVD Shrink 3.0 beta 5 but was a little disappointed when I played one of them only to find that there is a little bit of pixelation/blockiness in certain parts of the screen in certain spots. It appears to me like there isn't enough bitrate in these spots. I am watching these on a 68cm TV.

I have had success in the past with movies by re-encoding to 352X288, which gives better quality in my eyes than Full D1 at low bitrates (1500-3000kbps). The problem is that the footage is high-motion all the way through and so bitrate is always going to be a problem. I am going to try it tonight to see if I can find a good method.

My question is: Theoretically, is demuxing into elementary streams and re-encoding the video a better method than transcoding ? Should it produce better quality ?

Any thoughts ?

[aggies11]

Theorhetically, yes.

Re-encoding (that's still technically considered transcoding I believe) should give much more superior results than "on the fly" transcoding, typically found in the shrink/compress style applications.

The "transcode" found in CloneDVD, DVDShrink etc, is more of the "poor man's transcoder".

The reason we use these is because, although a true transcode(re-encode) is better, it takes MUCH longer. You will see alot of positive comments on Pinnacle Instant Copy 8, ie. it having the best quality (IC8 you often see it referred as). Thats because (as far as I can recall) it does a real re-encode. It also takes tons more time.

Re-encodes then, often are just not practical. Better quality yes. But not many are willing to spend 4hrs of pure processing time. (Heck, I think that Deep Analysis in DVDShrink is pushing it!

So yes, a true re-encode should give better quality, but it's not very popular due to the excessive amount of time it requires. If it took only the same amount of time as a Shrink Deep Analysis, I sure wouldn't mind using it!

<edit - addition>
I think the technical explanation is that with a true re-encode, you have a lot more freedom in reorganizing the information in such a way that it is optimal for the target bitrate. (Ie. You can get it alot closer to the ideal, which would have been the original encoding at the target rate). With the fast "poor man's" transcode, there is alot less freedom. You are stuck with the original organization for the higher bitrate, and simply have to "trim" as much as you can to get it to a lower bitrate. So it's definitely not optimal. So for example, if your target bitrate is 2Mb/s, then a true re-encode does alot more with that 2Mb/s than the fast transcode would.
</edit>

Hope that helps

Aggies

[adam]

Instant copy is a transcoder just like all the rest, it just performs more calculations so that's why its slower.

Transcoders (like IC, DVD2one, dvd shrink etc...) operate in the compressed domain. That means the file structure remains the same, so no reauthoring is necessary. In order to do this, they must re-use motion vector information. Your movie is made up of this, plus quantization information. Transcoders that operate in the compressed domain can do nothing more than re-quantize the coefficients. Basically, they take a formula representing your source and just change one of the numbers. It takes less information to represent the same amount of data, thus smaller filesize, and the equation becomes less accurate, thus lower quality. Requantizing coefficients has a pretty dramatic effect on quality, and it kicks in pretty quickly. Even slight transcoding can cause very particular artifacts such as blocking.

Re-encoding something completely starts from scratch and essentially finds a happy medium between the amount of motion vector information and quantization information. Its much better to compress both of these roughly equally rather then simply compress quantization information as transcoders do. Its much much more efficient, so with any decent amount of compression an encoder will always do a better job.

[jimmalenko]

Thankyou Adam, you confirmed my suspicions.

[aggies11]

I never used Instant Copy myself, so I wasn't sure if it was doing full re-encoding.

Aggies

[jimmalenko]

I never used Instant Copy myself, so I wasn't sure if it was doing full re-encoding.

Aggies

Yet you recommend it and critique it ????

[aggies11]

Heh.

IC8 generally is at the top of most peoples lists in terms of quality. Hence the statement "You will see alot of positive comments on Pinnacle Instant Copy 8". Personally, I stick with CloneDVD and DVDShrink, because on my viewing equipment there isn't really any difference.


IC8 does take alot longer than your typical transcoders, I was under the impression that it was because it's doing actual re-encoding. (That idea has been suggested before) However I wasn't 100% certain. <edit: Ugh, it looks like I was confusing some reviews of IC8 with an actual encoder The downside to the internets easy access to information, is the resulting confusion and jumbling of all the info together *doh* >

Re-encodes still offer better quality, for the technical reasons that adam mentioned.

Aggies