concatenation error - what is it?

[markcurtisp]

Can someone explain to me what is is?

Here is a passage I found online,


Whatever your setup, it’s best to avoid (as far as possible) concatenation errors, one of the scourges of video production. Concatenation errors are produced because every codec leaves its mark on the media in the form of artifacts which the codec designers hope will be either unnoticeable or unobjectionable. If the media is rendered into a different codec, those artifacts will be rendered too, and may become more obvious as a result. Also, the new codec will have added its own characteristic artifacts, which will be quietly waiting to potentially show themselves at the next stage of compression the media goes through, perhaps when a Blu-Ray disc is made after editing is complete.

So what should i be looking for when doing an error assessment?

 

[GrahamH]

Concatenating files usually means joining them together; for example a simple DOS command can be used to join several avis into a single file. So glitches at the point where different files or clips are linked together might be the kind of error they are concerned about - particularly with codecs with inter-frame compression?

That's a guess, though.

 

[Mitch Gross]

Think of it this way:

You have the numbers 1 to 20.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


Takes up a lot of data. So you compress it, using a codec that drops every third number to store it in a smaller space.

1 2 - 4 5 - 7 8 - 10 11 - 13 14 - 16 17 - 19 20


Then it "borrows" info from the numbers around the empty spots to guesstimate what goes in the holes.

1 2 3* 4 5 6* 7 8 9* 10 11 12* 13 14 15* 16 17 18* 19 20


Works pretty well. But then you send it through another codec, which this time drops every other number.

1 - 3* - 5 - 7 - 9* - 11 - 13 - 15* - 17 - 19 -


When reconstructing, some of these new values will be guesses based on guesses, which means that they will drift farther from the original image.

1 2** 3* 4** 5 6 7 8** 9* 10** 11 12* 13 14** 15* 16** 17 18* 19 20*


Look how these guesses begin to become oddly grouped, and you can begin to see how errors can occur. This is obviously super-simplified, but that interaction between the two compression systems (the codecs, "COmpression/DECompression") is what is termed concatenation.

 

[little bobby]

Thank you for making this extremely easy to understand.

Think of it this way:

You have the numbers 1 to 20.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


Takes up a lot of data. So you compress it, using a codec that drops every third number to store it in a smaller space.

1 2 - 4 5 - 7 8 - 10 11 - 13 14 - 16 17 - 19 20


Then it "borrows" info from the numbers around the empty spots to guesstimate what goes in the holes.

1 2 3* 4 5 6* 7 8 9* 10 11 12* 13 14 15* 16 17 18* 19 20


Works pretty well. But then you send it through another codec, which this time drops every other number.

1 - 3* - 5 - 7 - 9* - 11 - 13 - 15* - 17 - 19 -


When reconstructing, some of these new values will be guesses based on guesses, which means that they will drift farther from the original image.

1 2** 3* 4** 5 6 7 8** 9* 10** 11 12* 13 14** 15* 16** 17 18* 19 20*


Look how these guesses begin to become oddly grouped, and you can begin to see how errors can occur. This is obviously super-simplified, but that interaction between the two compression systems (the codecs, "COmpression/DECompression") is what is termed concatenation.

 

[j1clark@ucsd.edu]

Concatenating files usually means joining them together; for example a simple DOS command can be used to join several avis into a single file. So glitches at the point where different files or clips are linked together might be the kind of error they are concerned about - particularly with codecs with inter-frame compression?

That's a guess, though.

The 'concatenation' term used in this context refers to the use of a chain of codecs. So rather than 'file concatenation' that is adding more data to the file... a sequence of codecs are used to process the video data.

For example, the Camera produces video with Codec C1, the editor may use code C2, special effects, may use C3, eventually that is all compressed with C4...

So one has a series of codec C1+C2+C3+C4 that are 'concatenated' together for the final distribution result.

Ways around this include... 'shoot raw'... if the camera is capable of producing 'raw'... for processing use 'raw' data... and only compress at the final output.

Since most people can't afford 'raw' workflow, the use of certain intermediate codecs that may 'lose' data, but 'not a whole lot' are often used...

When editing in Avid/Premiere/FCP/FCX and one is dealing with exclusively 'live video' there may be only one round of compression on output. I believe all of them do have the ability to output in either 'lossless' full frame, or lossless 'compressed', but still that takes far more data storage and transport than the 'lossy' codecs.

For compositing, it would be best to output 'live video' uncompressed/lossless-compressed if possible... and add in uncompressed effects.

But again an individual's ability to have an uncompressed work flow is the limiting factor.