Converting MPEG-4 simple (e.g. XViD) to MPEG-2 without a DCT step?

[julesh]

It strikes me that this ought to be possible, but I've been unable to find any software that actually does it. The results ought to be roughly an order-of-magnitude faster than decompressing & recompressing as most tools do. I've seen descriptions of software that's designed to convert MPEG-2 to H.264 written using this approach, and MPEG-1 to MPEG-2 also, but never something that converts from a newer format to an older one. Which is a shame for those of us stuck with hardware that only supports older formats. :(

Anyone come across something that does this?

[FulciLives]

Ummm what the heck are you talking about?

Just use AviSynth and FitCD to create your AviSynth script then input the AviSynth script into your MPEG-2 DVD spec encoder. HCenc works well. All three of those are FREEWARE.

If the original audio is AC-3 just demux with VirtualDubMod and to be safe run it through AC3Fix and there you go.

If you have MP3 audio then it is best to load the AVI into GOLDWAVE and save it out as a PCM WAV file (16-bit 48k Stereo PCM WAV) then use ffmpeggui to convert to AC-3 format.

Only program I mentioned that is not freeware is GOLDWAVE but it is cheap and you can actually use it many times (it is trailware) before it locks up and requires you to purchase it. By that time you will realize what a great tool it is and won't mind paying for it especially since everything else I mentioned is freeware.

- John "FulciLives" Coleman

[julesh]

Ummm what the heck are you talking about?

Just use AviSynth and FitCD to create your AviSynth script then input the AviSynth script into your MPEG-2 DVD spec encoder. HCenc works well.

Well, I'm currently using ffmpeg, which works reasonably well and is reasonably fast; I can get up to about 5 frames per second, even on my meager machine. But now I need to do this in realtime, and I'd rather not have to spend cash on new hardware. The approach I'm talking about should be able to do this: basically, you don't completely decompress the stream and recompress it, which is what all of the tools I've looked at do. As an example, here's a description of how to convert MPEG-1 to MJPEG using a similar process to the one I envisage. I've just got no idea if anyone's done anything similar for MPEG-4 to MPEG-2 conversions.

[guns1inger]

Without hardware assistance you aren't going to encode anything in realtime on that machine (as per your Computer profile - celeron 450, 256mb ram).

The original DivxtoDVD did something similar, using much of the original DCT when converting from mpeg4 to mpeg2. It allowed fast transcoding and smaller output sizes than it's competition. But there is a quality cost associated.

And there is no way you would get anywhere near realtime with it on your system.

[JohnnyMalaria]

Ummm what the heck are you talking about?

Traditional transcoding requires performing IDCT to the spatial domain, recalculation of the non-I frames and then encoding to the second format - requiring more motion estimation and DCT calculation.

I think julesh is looking for an MPEG4 to MPEG2 conversion without ever having to leave the DCT domain.

It would depend upon how similar the two standards are with regard to the motion estimation. Block matching typically is done in the spatial domain but could be done in the DCT domain. However, this would only be true for block motions of exact multiples of the dimensions of the DCT block.

DCT and IDCT calculations are the single largest bottleneck. Our DV processor keeps all the calculations in the DCT domain whenever possible. With DV, it is *relatively* straightforward since there aren't the added complications of motion estimation etc. The only real challenge is dealing with 8x8 vs 2x4x8 blocks. In such as case, we use some fairly hairy matrix algebra to convert between the two without having to go via the spatial domain.

Though I'm familiar with MPEG2 encoding, I'm not au fait enough with MPEG4 to gauge whether an entirely DCT-domain based conversion would be possible.

A search of the patent literature would soon reveal if it is!

[FulciLives]

I think someone just needs to buy a faster computer.

- John "FulciLives" Coleman