an alternative question about terminology

[compusic]

I have seen someones use the word "transcode", while I'm used to the word "encode".
It seems that the two words have difference - under certain condition, one is more suitable than the other.

but I can't figure out the difference.

who can make it clear? thanks.

[The1]

This may make it a little clearer:

http://forum.doom9.org/showthread.php?t=111906

Or maybe this:

http://www.free-codecs.com/guides/Whats_the_difference_between_decoding_encoding_recod...ranscoding.htm

[manono]

Hi-

Q. How does the transcoding process work?

A. To understand how transcoding works, we must first understand how encoding works. Mpeg-2 encoding works by exploiting redundancies in data in two domains, the spatial domain, and the temporal domain. First, the encoder performs intra-frame compression, (i.e. Divide the frame into macroblocks, perform DiscreteCosine Transform on each block, quantize (reduce data accuracy) results) this yields a set of coefficients describing the frame. Next, using inter-frame encoding we compute motion vectors to describe what macroblocks are changing over time, then depending on what sort of frame we're encoding into, we store the appropriate information in the bitstream (3 types of frames, I-Frames, meaning only intra-frame compression is done on it, it can be decoded with no other frames' help, P-frames & B-Frames, meaning predictive frames and bi-directional frames, only the change in macroblocks is stored). As you can imagine, computation of the motion vectors is the most time consuming process because of the complicated comparisons being performed. (**Please note this is a pretty simplified explanation, I encourage you to check out Mpeg-2 specs if you want many, many more details.. http://autumn.idv.tw/ppt/138182.html **)

Transcoders work by utilizing the data we already have, to reproduce a slightly less accurate bitstream. Transcoding in the compressed domain, done by DVD2One, DVDShrink, ReJig (Requant), and others, is done through requantization of coefficients. Quantization is the process, mentioned above, of discarding an amount of accuracy in order to reduce the amount of bits the data uses. For instance, the quantization of an encoder (not Mpeg-2) could be defined as sign(x) * (abs(x)/(2*quant)) where x is the original number we're quantizing, and quant is an integer specifying the amount of accuracy to discard (i.e. higher quant, lose more of x). By simply plugging in some numbers into that equation, you can see that you're losing accuracy of the plugged in number. This is how compression works in Mpeg-2. (Quantization is essential because DCT by itself is a nearly reversible process (you do lose some data because of precision errors). Quantization is what causes the 'blocking' and pixelation errors many people hate to see) So, by keeping in mind the process of Quantization, we can easily reduce a stream's size by simply re-quantizing the co-efficients of the bitstream... and by keeping the old motion vectors, we don't have to worry about the very computationally intensive portion of encoding.

http://forum.doom9.org/showthread.php?&threadid=63587

There's more at that same link.

[MOVIEGEEK]

Although some people use both terms interchangeably the doom9 explanation is correct.
Transcoding will change the size of a video file but not the format or resolution,you sacrafice quality for speed.
Encoding can change all attributes of a video file and usually produces better video quality.
Example of transcoder: DVDShrink
Example of encoder: TMPGEnc

[lordsmurf]

Transcoding alters data without a decode.
Encoding is fed a decoded video source.

Encoding yields higher quality due to the decode.
Transcoding is faster, but at the sacrifice of quality.

A high quality transcode is possible, but require expensive hardware (thousands or millions of dollars).

[compusic]

thanks. clear now.