Difference Between 1 pass & 2 pass encoding

[iqbal88]

Dear friends,
Pls, discuss about what is the real difference between 1 pass encoding & 2 pass encoding

[hech54]

http://en.wikipedia.org/wiki/Variable_bitrate#Multi-pass_encoding_and_single-pass_encoding

[Vladrial]

In short, in one-pass encoding the encoding is done in the first pass itself, in case of 2-pass encoding the file is analyzed throughly in the first pass and a intermediate file is created. In the second pass the encoder looks up the intermediate file and appropriately allocates bits, the actual encoding takes place in the second pass. 2-pass encoding is usually better than single pass encoding ( Assuming we are considering the bitrate mode )

[SLK001]

Dear friends,
Pls, discuss about what is the real difference between 1 pass encoding & 2 pass encoding

In one word, the real difference between the two is "quality".

[edDV]

In one sentance "to predict file size".

[redwudz]

Maybe just my opinion:

Two pass can give you smaller files with better quality at lower bitrates.
At high bitrates, two pass or single pass can have the same quality.

Two pass takes a fair amount more time for encoding compared to single pass.
As mentioned, with two pass you can control the encoded file size.

Single pass can also be quality based. You don't have control of the filesize, but you can control the quality.

[manono]

Dear friends,
Pls, discuss about what is the real difference between 1 pass encoding & 2 pass encoding

One pass.

[AlanHK]

Dear friends,
Pls, discuss about what is the real difference between 1 pass encoding & 2 pass encoding

In one word, the real difference between the two is "quality".

No, you can do single pass with fixed quality setting (and variable bit rate).

Single pass, fixed rate, will give you lower quality (for the same size).

[dragonkeeper]

Maybe just my opinion:

Two pass can give you smaller files with better quality at lower bitrates.
At high bitrates, two pass or single pass can have the same quality.

Two pass takes a fair amount more time for encoding compared to single pass.
As mentioned, with two pass you can control the encoded file size.

Single pass can also be quality based. You don't have control of the filesize, but you can control the quality.

I agree with Redwudz on this one from what I've seen everyone tends to make the blanket statement "2 pass gives you better quality" when they should be saying something along the lines of "2 pass gives you better quality at a given file size".

When i encode I really don't care about file size, hard drive space is cheap.

[El Heggunte]

Dear friends,
Pls, discuss about what is the real difference between 1 pass encoding & 2 pass encoding

One pass.

manono wins — FLAWLESS VICTORY

When i encode I really don't care about file size, hard drive space is cheap.

Blank DVD-R's are even cheaper today.

[jagabo]

Single pass constant bitrate: you pick the bitrate, the encoder encodes the entire video at that bitrate. If the bitrate is very low the video will be of low quality. If the bitrate is very high the video may be of good quality but bitrate will probably be wasted on lots of shots that didn't really need it.

Multipass variable bitrate: you pick the bitrate the encoder delivers whatever quality it can for that bitrate. The encoder makes two (or more) passes. During the first pass the encoder examines the entire video to see what parts need more bitrate and what parts need less. During the second pass it encodes the video using that information. It allocates more bitrate to parts that need it, less to parts that don't. Subsequent passes (if requested) refine the video futher. This produces better quality than single pass CBR because bitrate isn't wasted on parts of the video that don't need it.

Single pass constant quality: you pick the quality, the encoder uses whatever bitrate is necessary to deliver that quality at each frame.

When your primary concern is the file size you use 2-pass VBR encoding. You're saying "I don't care what the quality is, I want a file of THIS size." Because file_size = bitrate * running_time.

When your primary concern is quality you use constant quality encoding. You're saying "I don't care what the file size is, I what THIS quality."

If you encode a video using constant quality encoding, then go back and make another video the same size using 2-pass VBR encoding, the two videos will be the same size and similar quality.

[Gavino]

This must be about the 100th time you've explained this, jagabo, and you do it extremely well.
And yet there still seem to be people around who just don't 'get' it.
I'm not sure why this is - maybe too many people are still hung up on getting a '700MB rip'. ???

[_Al_]

What is the difference between 1pass CBR abd 2pass(3pass) CBR or how it works? It always made my head spin, not that I ever used it but it seems to be right thread to get an answer.

[jagabo]

This must be about the 100th time you've explained this, jagabo, and you do it extremely well.
And yet there still seem to be people around who just don't 'get' it.
I'm not sure why this is - maybe too many people are still hung up on getting a '700MB rip'. ???

I think people don't bother to search first. Or maybe they just don't know what exactly to search for. Or searches turn up old information about writing movies to CDs. Or they live in parts of the world where they can only afford CDs. I think many of the release groups are still hung up on 700 MB increments.

[AlanHK]

When i encode I really don't care about file size, hard drive space is cheap.

Blank DVD-R's are even cheaper today.

Which is why I do 2 passes, so I can get the maximum quality that fits on a DVD.

[Cornucopia]

What is the difference between 1pass CBR and 2pass(3pass) CBR or how it works? It always made my head spin, not that I ever used it but it seems to be right thread to get an answer.

While some may tell you that the difference is that, with a 2nd pass, the encoder gets to tweak more things than just bitrate, and so can still benefit CBR, in REAL LIFE the the answer is: NOTHING.

@OP,
Stop expecting us to provide you with this week's media class assignment. This is like the 3rd or 4th time you've asked us to "discuss" these WIDE OPEN, VAGUE "questions". If you have a REAL question, ask it.

Scott

[Dunnie]

What is the difference between 1pass CBR and 2pass(3pass) CBR or how it works? It always made my head spin, not that I ever used it but it seems to be right thread to get an answer.

While some may tell you that the difference is that, with a 2nd pass, the encoder gets to tweak more things than just bitrate, and so can still benefit CBR, in REAL LIFE the the answer is: NOTHING.

@OP,
Stop expecting us to provide you with this week's media class assignment. This is like the 3rd or 4th time you've asked us to "discuss" these WIDE OPEN, VAGUE "questions". If you have a REAL question, ask it.

Scott

Why the agitation ? It is an ambiguous topic .. he is hoping someone with more experience can give him some tips on getting great quality at a low size. This is something I struggle with to this day. I am ripping my entire blu ray collection to my plex server and I still dont have a recipe that I am completely happy with. Either the file is too large and does not stream well everywhere, its too small and I see artifacts etc.

[_Al_]

It is really simple,
1.either you choose the quality, choosing CRF number and encoding 1pass CRF, letting encoder come up with resulting size for your encoding
or
2. you choose 2pass VBR and average bitrate and encoder would compress your video into this chosen "box" , understand limited space, and then you will be speculating for days what actual average bitrate is the best for that movie, and with the other movie you will start speculating and testing all over again

[AlanHK]

Why the agitation

Why are you asking questions about the tone of a



THREE YEAR OLD POST?

[Dunnie]

Why the agitation

Why are you asking questions about the tone of a



THREE YEAR OLD POST?

Not quite three years yet ... but point taken. BTW : Nice point size.

[knightplex]

I have a question. I recently encoded The Dark Knight Trilogy using CRF encoding x265, using Staxrip software. I used the default value of CRF=22, but there was a problem. The Dark Knight was encoded with an average bitrate of 2300 kbps, while Batman Begins and The Dark Knight Rises were encoded with an average bitrate of approximately 1500, which is a very low quality. Hence I used CRF value 20 for the first and third movie and ultimately I got all the three movies around 2000 bitrate. Does this mean I will have to use different CRF values to obtain the same quality?

[manono]

Does this mean I will have to use different CRF values to obtain the same quality?

No, it means all three you originally encoded with the same CRF value already did have the same quality. The fact that they wound up with different average bitrates means nothing as different sources compress differently.

If you really want them encoded with about the same bitrate (and wind up with differing qualities), then just run 2-pass encodes for the desired bitrate. If you want them all with the same quality, then do 1-pass CRF encodes.

[knightplex]

No no no. I want them to end up with the same quality. I used CRF 22 for all the three movies (The source is untouched Bluray original disc), and ended with bitrates I mentioned. Also I could see that the two movies with low bitrates looked low quality compared to The Dark Knight which ended with a high bitrate and a better quality as perceived by the eye.

[manono]

If all three were encoded with the same CRF and all other settings were the same for all three, the final bitrates are meaningless and all three videos should appear the same visually. If you didn't like that appearance, then use a lower CRF. Making them for the same bitrate isn't the way to give them all the same quality.

I suppose it's possible the source Blu-Rays had wildly differing qualities, which might account for what you're seeing, but there's no way for someone reading your thread to know that. Maybe check out some reviews of the Blu-Rays to see if any reviewers noticed any differences in quality.

[knightplex]

Okay, thanks. I'll try encoding again.

[DragonOmnus]

knightplex, another thought: don't look at the bitrate and other file information BEFORE you check out the quality of your rips by watching them. I can't begin to tell you how many times I have seen somebody complain about the quality of something simply because of doing something like that. They look at something like the bitrate of a file, see it is much lower than another file, and then somehow "see" a difference in the video quality because of their expectation the file will be of worse quality, then I look at the files and they are virtually indistinguishable.

Another case in point, a looooong time ago when I was first ripping/transcoding videos, I tested various settings. Some of the resulting videos had 2-3x the bitrate & file size, but no noticeable increase in quality. There was a point where going too low decreased the quality, but that's a matter of trial & error with the video you're ripping/transcoding.

Maybe there really is a difference in the visual quality of your rips, maybe not. All I'm saying is eliminate your expectations as a possibility of why you are seeing a difference first, then worry about bitrate.

[noisywan]

I'm aware that this thread is 3724 days old but no one mentioned the difference of keyframe placement (my favorite) so I wanted to add it.

In 2-pass encoding, the encoder usually places the keyframes at the scene changes (points of high bitrate demand) so you can easily browse/skip scenes by keyframe navigation with players that support it (like MPC-HC). Also such videos are easy to edit/chop without re-encoding. You can even re-arrange scenes with stream-copy.

In single pass, encoder does not scout for bitrate demand before encoding so it can not optimize according to scene changes.

[Selur]

In single pass, encoder does not scout for bitrate demand before encoding so it can not optimize according to scene changes.

.. unless the encoder has a xy frame sized lookahead, which is usuall the case (nowadays).

[hello_hello]

In single pass, encoder does not scout for bitrate demand before encoding so it can not optimize according to scene changes.

x264's single pass CRF mode does a very good job of placing keyframes on scene changes.

http://www.chaneru.com/Roku/HLS/X264_Settings.htm#scenecut
scenecut
Default: 40
Sets the threshold for I/IDR frame placement (read: scene change detection).
x264 calculates a metric for every frame to estimate how different it is from the previous frame. If the value is lower than scenecut, a 'scenecut' is detected. An I-frame is placed if it has been less than --min-keyint frames since the last IDR-frame, otherwise an IDR-frame is placed. Higher values of scenecut increase the number of scenecuts detected.