Technical info on differences between encoder and transcoder?

[TheSeeker]

Where can I find this information? I need some fairly reliable sources to cite for a project Im working on. Anyone know a place that gives a good side by side comparison on the differences between the two? If this is in the wrong forum by all means move it to the correct one.

[E-Male]

well, an encoder encodes raw material
while a transcoder modifies an already encoded file
(sometimes the term transcoding is also used for decoding and encoding to another format)

[dragongodz]

sometimes the term transcoding is also used for decoding and encoding to another format
yep but the form of transcoding generally talkked about nowdays is(to give it its full name) compressed domain transcoding. just saying transcoding is easier. :)

well, an encoder encodes raw material
while a transcoder modifies an already encoded file
sort of yes. to be more exact an encoder fully decodes the input to encoder while a transcoder works in the compressed domain and reuses parts. for example things such as motion vectors etc are all reused while an encoder makes new ones based on the encoding. so a transcoder basically drops dct coefficients and/or raises quant values to lower the original stream.

i will do a bit of a search later to see if i can find more technical information. :)

[TheSeeker]

Thanks for your assistance Dragongodz, anything you can find would be most helpful.

[dragongodz]

well this should get you started
http://www.hpl.hp.com/personal/Susie_Wee/PAPERS/hpidc97/hpidc97.html

it also has a nice little bibliography at the bottom some of which can probably be found in pdf form with some searching.

if you want more let me know.

[Nic]

I'm not sure if this the standard, but in my biz we use the terms as such:
Encoding - As in Ingesting from a tape to a digital file format like MPEG-2
Transcoding - Going from one digital format to another (i.e. MPEG-2 > WMV)
Transrating - Recompression in the compressed domain

-Nic

[TheSeeker]

Originally posted by Nic
I'm not sure if this the standard, but in my biz we use the terms as such:
Encoding - As in Ingesting from a tape to a digital file format like MPEG-2
Transcoding - Going from one digital format to another (i.e. MPEG-2 > WMV)
Transrating - Recompression in the compressed domain

-Nic

This is interesting because to me I think of the terms like this:

Encoding: Decompressing, processing, and recompressing into format like MPEG2.

Transcoding: Recompression in compressed domain

I think the term transcoding has sort of evolved into this definition. I mean, it seems that is what everyone is talking about when they use the term transcoding. Although I think you are right. the term transcoding used to mean going from one digital format to another. I think the marketing hype machines just needed something that sounded similar to encoding but was different enough that there wouldnt be any mistaking the two. Something that will help them sell their "transcoders" as something to compete with the Encoders. I have never heard the term Transrating however.

@Dragongodz

That paper is pretty good, but it seems to me rather biased towards transcoding methods. It calls encoders a "Naive" method. (referring to the need to go to the pixel-domain first then doing the processing then back to the compressed domain). I noticed that it was written in 1997. Transcoding probably looked like a miracle back then before they knew about the inevitable quality hit they must take (at least on large encodes). Plus, can you even apply noise reduction filters or resizing filters in the compressed domain?

[dragongodz]

Nic - yes transcoding is format conversion aswell and early docs i have seen actually do format conversion via compressed domain (mpeg->jpeg for example). in modern terms though you will find many papers written also use the term compressed domain transcoding even when not doing a format conversion, plenty of mpeg2 examples i have seen do that.

transrating is probably the more technically correct term, being TRANSforming the bitRATE and again there are plenty of papers that use that term aswell.

so really nowdays either term seems to be used. :)

it seems to me rather biased towards transcoding methods
well in a way yes. it IS predominantly about CDT afterall. however i did not think you needed too much encoding information aswell, there is heaps of that available. infact if you look at the bibliography you will find reading for mpeg encoding aswell.

Plus, can you even apply noise reduction filters or resizing filters in the compressed domain?
noise reduction ? no(as far as i know). however resizing actually can be done via compressed domain to some degree, meaning not by pixel but halved or quartered yes. things like the motion vectors need to be replaced though, such as averaging the original vectors of MB's scaled to a new MB. again there are papers about doing that aswell. :D

[dragongodz]

ok heres one to even things out. :)

http://www.mpeg.org/MPEG/
that should be enough to get you started with mpeg1/2 encoding information. :D

and for resizing in the compressed domain
http://www.informatik.uni-trier.de/~ley/db/journals/tcsv/tcsv12.html (look in volume 12, number 7)
and
http://mpeg.korea.ac.kr/~sjsuh/publications.html

[TheSeeker]

Dragongodz. Much thanks for your continued help. Knowledge is power, and right now Im weak as a kitten. LOL. Anyways thanks again, this will definately help.