Two Pass Encoding

[whaT2k]

i use two pass option everytime

i can't tell the difference between 1 pass or 2 pass encoded video
what is 2 pass supposedly do; is it it's worth the extra time

[guns1inger]

What codec ? What encoder ?

Benifits of two-pass encoding are accuracy of output file size and better allocation of data across scenes. If output size isn't a concern, or you are using a high enough average bitrate, you may not see as many benefits. Also, if you are encoding DVDs to lower resolution Xvid/Divx then you will probably not notice many of the flaws on your PC, but they will certainly show up on a TV, or if they are converted back to DVD in the future.

[whaT2k]

dvd to WMV @ 1500kbs; using WME
for playback on a 61in SAMSUMG TV

so i guess 1 pass will work fine

[redwudz]

OK.........? What format are you encoding to and what program or codec are you encoding with?

Generally a two pass encode is to be able to use a lower bitrate encode for a smaller file size, while still preserving the quality. The first pass analyzes the amount of bitrate needed for the different scenes and makes up a 'index' file. The following passes then use this information for the optimal combination of bit rate, frame rate, buffer size, and image quality based on the scene compositions to get the best quality vs size of the encoded file.

If you use a high bitrate instead, you won't see much difference between multiple passes and a single pass, but you will be wasting bitrate where it's not needed and will end up with a bigger file.

EDIT: guns1inger, typed faster.

[energy80s]

To be honest I never use two pass encoding. It's only really necessary if you are trying to compress a file a lot. Throw some decent bitrate at the file and save time for other things. Also remember that most hardware encoders only work as single pass, and they seems to manage alright.

[David K]

Back in the old days (four years ago!) when I was doing SVCD because I didn't have a DVD burner, I'd cram 48 minutes of mpeg 2 at 2mbps average on a CD at 352x480 resolution. With two pass you could get excellent quality with such low average bitrates. And the difference between one and two pass was dramatic. If your average bitrate is at 6mbps then two pass is not as necessary, but I still use it because it ensures that there will never be a bit-rate starved section of the video.

[Rudyard]

Could I throw up a scenario and could you tell me if this is right?

Say I had a 1 hour movie of which 45 mins was action packed movie. Is this correct about the different encoding methods:

A single pass variable bit rate give me a better quality picture because it would simply use max bitrate where needed (45min) and only required bit rate where not needed (15min) as it doesnt have to worry about file size or sticking to an average but can vary the bitrate when not needed.

A 2 pass variable bit rate would have to compromise quality somewhere to create the average to control the file size if 45 min of the film required the max bitrate and only 15 min could lower the bitrate.

CBR would produce the same quality as the single pass vbr but would result in a bloated file size as it wouldnt lower the bitrate for the 15mins where it was not required.

I know each file would be different but as a technical rule would the above be correct?

[guns1inger]

A single pass wouldn't necessarily give you better quality because it will still try to meet the average, but doesn't have the benefit of the first pass for analysis of data distribution. This will ultimately result in the potential for being data starved toward the end of the film, and a file size that is not accurate to what you wanted.

CBR would give the best results only if the bitrate were high enough, but the file size would be larger than to the other two in most cases.

2-pass would give you the best results most of the time by allocating the data correctly based on need. It's output filesize would be accurate, and assuming you gave it the same min/avg/max as the single pass, a better image quality in most cases.

The method you are missing, or are perhaps confusing with single pass, is Constant Quality. This is where you don't encode based on an average bitrate, but on a level of quality you want maintained. File size is not considered important, only image quality. Encoding is done in a single pass, the bitrate varies, however as there are no min/avg/max applied, all scenes get the data they need, regardless of amount.

[Rudyard]

'The method you are missing, or are perhaps confusing with single pass, is Constant Quality. This is where you don't encode based on an average bitrate, but on a level of quality you want maintained. File size is not considered important, only image quality. Encoding is done in a single pass, the bitrate varies, however as there are no min/avg/max applied, all scenes get the data they need, regardless of amount. '

Yeah that is what I thought happened with a single pass vbr!

I have never seen this option in Vegas (unless I missed it or mistook it for CBR). Is it widely used or available in common software?

[guns1inger]

It is available for mpeg4 encoders (Divx/Xvid), and some mpeg2 encoders, such as ProCoder. It is of little use for DVD encoding because it cannot produce accurate file sizes for output.

[Rudyard]

interesting, thanks for the info!