What are the differences between 1 pass and 2 pass encodings?

[FallenAngelII]

What's the difference between 1 pass and 2 pass encoded files? I'm curious and want to know the full list of differences.

[thecoalman]

Far from an exhuastive technical explantion (cause I can't give you one :P ) but basically the first pass examines the clip to determine where the most bitrate is needed e.g. frames with a lot of motion. When it's encoded on the second pass the encoder will be able to distribute the bitrate more efficiently, robbing peter to pay paul but in this case peter don't need it.

[guns1inger]

The basic methods for encoding are

Constant BitRate (CBR) : Every frame is allocated the same amount of data, regardless of what it actually needs. This bitrate is user set. Slow scenes will be allocated the same amount of data as fast scenes with lots of motion. The output size is very predictable.

Constant Quality (CQ) : Bitrate is allocated by the encoder based on a quality selected by the user. Bitrate is variable, but with little or no constraints. Output size in unpredictable.

Variable BitRate : Bitrate is allocated on an as needs basis, calculated around (usually) am average bitrate (user selected) with max and min constraints. VBR encoding can be single pass, in which case it works like a constrained CQ encode, with unpredictable output size. 2 or multi-pass encoding means that 1 or more passes of the video are performed to work out how best to allocate the data, with the final pass doing the actual encoding. File size for multi-pass encoding is very predictable.

[jagabo]

When to use them:

1-pass constant bitrate: when you want a fast encode and don't care about optimizing quality for a given file size. Or you don't mind using a large file size (high bitrate) to assure quality.

1-pass constant quality (aka constant quantizer, target quantizer, etc): when you want a specific quality but don't care about file size. You know what the quality will be, you don't know what the file size will be.

1-pass variable bitrate: realtime encoding (ie video capture).

2-pass variable bitrate: when you want to optimize quality for a specific file size. You know what the file size will be , but you don't now the exact quality -- just that you'll get the best quality for that file size.

[dphirschler]

CBR is also the only realistic way to encode a live stream, something we called RTE (real-time encoding) when I worked at SA. VBR typically gives you more quality for the space allowed, so you use that when you have the time to properly master something for DVD distribution.


Darryl

[Alex_ander]

I'm curious what is CBR multi-pass for in CCE 2.70 and which encoding parameters can be optimized at CBR and fixed file size?

[zoobie]

I was told 2 pass is for larger resolutions. For example, you won't see any difference with a low resolution 320x240 video but you will with 720x480.

[Wam7]

I was told 2 pass is for larger resolutions. For example, you won't see any difference with a low resolution 320x240 video but you will with 720x480.

You were told incorrectly. The difference is very clear at 320x240 as any quick test will show.

[jagabo]

I don't see much difference between single pass CBR and multipass CBR with CCE.

[FallenAngelII]

I don't see much difference between single pass CBR and multipass CBR with CCE.

Obviously you won't be able to tell much different with only an untrained eye if your output file has a high bitrate.

Encode something from a DVD source into a 2 pass h.264 file with a low bitrate (400 ish).

Then do screenshots of the same frame if you can't tell the difference with your naked eye.

[zoobie]

My software is encoding my wmv in CBR with a 2 pass...uh...
anyway, it looks a leeetle bit better
but my source was originally hi8 analog
so mine have gone thru at least 2 encodes
plus the usual treatments...

[jagabo]

Encode something from a DVD source into a 2 pass h.264 file with a low bitrate (400 ish).

Then do screenshots of the same frame if you can't tell the difference with your naked eye.

Are you talking about multi-pass VBR vs single-pass CBR? Of course there's a difference between those two with limited bitrate.

I was responing to Alex_ander's query about CCE's multi-pass CBR mode.

[FallenAngelII]

Hmm... it could just be CCE which sucks (I've never used it). I've learned the hard way that you should stick to Guis >_>'.

[Alex_ander]

Thank you for replies on CBR multy-pass. I've just found a recent thread in doom9's forum:

http://forum.doom9.org/showthread.php?t=116640

- the things are not as simple as they seem!

[Marvingj]

2 pass CBR can produce better encoding compared to single pass. However, if bitrate is set high enough (as originally stated at 9200kbps), isn't a single pass encoding "equal enough" to a 2 pass one?