Encode OR transcode???

[Mr Walker]

Can someone explain the difference between those two???

[Vejita-sama]

Encoders produce MPEG2 video streams. Transcoders are based on algorithms designed to recompress an MPEG-2 stream in real-time for TV broadcast. That is MPEG2 to a smaller MPEG2, but the key is in real time. Encoders work in anything but real time

Encoding always produces superior quaility than transcoders. But time wise transcoding is much faster.

[SaSi]

I think Vejita-Sama's explanation is just a bit innacurate, to some extent.

Although, I agree, that "transcoders are based on algorithms designed to recompress an MPEG-2 stream", I would disagree with the rest of the comment.

I don't think recompressing has anything to do with TV Broadcast. TV broadcast requires transport streams, which as slightly different system streams compared to the popular MPG system stream we view on PCs (or the VOB files found on DVDs). Also, TV broadcast, doesn't really need low bitrates. HDTV is much higher than 10MBps. If I remember correctly, it's 15Mbps. Also, broadcast used to require constant bitrate, although VBR should be usable now.

I think the real difference between a transcoder and an encoder can be summarized in the following:

An encoder takes a source video, which normally should be the master copy at high quality, and encodes it processing each frame in sequence and trying to generate spatial and temporal compression by discovering the features of each picture (spatial) and the similarities between consecutive pictures (temporal). So, I would dare say, the encoder is a clever program (or should be) that manages to reduce the bitrate by applying encoding algorithms and create an MPEG-2 structured bitrate.

The transcoder is taking an already encoded MPEG-2 stream with a given average bitrate and reduces the overall bitrate by parsing each frame and macroblock and removing (chopping off) parts of the bitrate. The transcoder never "looks" at the pictures making the movie. It just chops off parts to achieve the wanted bitrate. The methods used are rather simplistic, like removing slices or whole frames every now and then (For example, removing 1 out of 5 frames reduces the bitrate by 20%).

I would also disagree with the comment that encoders work in less than real-time speed. CCE and mainconcept can easily encode MPEG-2 DVD video in much faster than realtime. If it's CBR and single pass and the bitrate is not too low and motion search is not set too deep and half pixel search is not activated and the PC is is P4/2.4GHz and more, then almost 2X real-time speeds can be achieved. Of course, at the expence of quality.