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cfb said:

Sort of like asking "Car #1 has 200 horsepower, car #2 has 400 horsepower, which one is faster?".

Depends on a lot of stuff.

Msmith is right that at the same bitrate, mpeg2 and 4 should look somewhat alike while mpeg4 will take up less bandwidth. But there are some characteristics of mpeg4 compression that i'm not completely fond of.

Here's the kicker though...nobody produces mpeg4 streams from an analog source or high quality uncompressed original format as happens when a high quality mpeg4 blu-ray is made from a higher quality source.

All of directv's source material is first compressed into mpeg2 at its origination, most likely in the camera that took the shot, probably transcoded to a different bitrate for transmission at least once, and then sent by the network/source in mpeg2 where directv transcodes it into mpeg4.

Given a very high quality stock thats converted at the same bitrate to mpeg2 and mpeg4, the appearance should be quite similar although some argue that at very high bitrates mpeg2 has some advantages.

However you cant make something out of nothing. Taking the original networks mpeg2 and converting it to mpeg4 cant make the output look better than the mpeg2 original. Plus since it looks like directv produces lower bitrate mpeg4 streams on some channels at some times so as to accommodate more PPV and sports offerings, chances are the original mpeg2 source looks better than what comes out of the back of your directv receiver.

This is why the fiber optic sources like fios and uverse have such great looking SD and HD. Its probably the original mpeg2 passed on without recompression or transcoding in many cases.

So while the 'mpeg2 or mpeg4' question is academically interesting, I think the answer is that the picture thats been transcoded the most and had its bitrate reduced the most will look the worst, while the one thats closest to its original format with as few changes made along the line will generally be the best, within the limits of the quality of the original format.

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broadcasttheworld said:

FYI all of Direct tv source content (Local channels) are pulled out of the air in the local DMA using off air antenna. The national content is most likely MPEG2/4 already or perhaps even H.264 but I would be surprized if any of it is over 20 - 30Mb/s and is most likely transported over GIGe or ASI transport. Only a few channels are sending out the uncompressed HD-SDI (1080i) content except National Geographic and a few others.

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cfb said:

Some of the locals are fed by fiber, however both the fiber and OTA feeds are already in mpeg2, so my original point stands. You cant improve on an mpeg2 picture by transcoding it into mpeg4, so the mpeg4 picture sent by directv can be no better than its original mpeg2 broadcast format, regardless of whether it was 'received' by antenna or fiber. Further, it seems the boxes that directv uses to convert the stream to mpeg4 has issues that cause the 'briippp/zipper' issue that is still fairly evident.

In fact, you get the worst compression issues of both mpeg2 and mpeg4 with directvs output.

If you have a super high quality original source, you can then argue the merits of mpeg2 vs mpeg4 quality although clearly mpeg4 has a capacity advantage. Over standard cable or satellite, its not applicable.

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