mpeg2 vs. mpeg4

[solidsnakeiv]

this is really probably a noob question, but i have noticed that on some other posts that some of the hd channels on d* below channel 100 are mpeg4 format.....now is that technically a higher quality picture than the mpeg2 on channel 206? I'm a little confused...thanks

 

[Ronnie-]

All of the upper number HD channels on D* or in mpeg 4. The old legacy channels in the 70s are in mpeg2 there, and in mpeg 4 on their higher numbers. The old ones are scheduled to be shut off soon.


Mpeg 4 is another form of compression that is more efficient than mpeg 2 and generally gives a better picture at the same bitrate. it allows them to put more channels out on the same amount of space if they so chose.

 

[cfb]

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.

 

[PMKS]

I've been watching HD on DirecTV for a while.....

In my opinion the Mpeg-4 picture seems to be better on my H21 than the Mpeg-2 picture I used to watch on my old RCA DTC-210.

This may have been due to the fact they were cramming the HD content on the limited transponder space with the 110/119 sats..... before the 99/103 sats were launched.

 

[broadcasttheworld]

this is really probably a noob question, but i have noticed that on some other posts that some of the hd channels on d* below channel 100 are mpeg4 format.....now is that technically a higher quality picture than the mpeg2 on channel 206? I'm a little confused...thanks

Modern video compression schemes are very complex because there are numerous variables that interact and influence the overall performance. I’ll try to keep the following to the point.

Remember that each set of processes has a sweet spot. After MPEG-2 became well accepted, it was noticed that the very low rate streaming video application was not well served. MPEG-4, which is known by several different names but which all work similarly, was developed to improve the efficiency of very low rate coding. Microsoft called it WM9 and later VC-1. Since they were not willing to share the toolset with the world, the ITU developed their own variant known as AVC. AVC has proven to be generally better overall than WM9 so outside of MS apps, AVC has taken over.

MPEG-4 takes advantage of Moore’s law where the math processes available now are much more powerful per dollar than they were when MPEG-2 was built.

The basic improvement results from the usage of variable size macro blocks compared to MP2 which uses fixed size 8x8 DCT samples. The result of this process is a reduction in the visibility of sampling errors.

MPEG-4 quantization was kept at 8 bits for luma and chroma because these plans were meant for end-user delivery and not contribution transport. Broadcasters have been vociferous about using 10-bit in-plant video products for years; this is what killed the D1 tape format.

When the Motion Picture industry was first considering electronic delivery (DCI), they quickly threw out 8-bit compression including MPEG-2 and 4. JPEG-2000 was the product of a University research project funded by the DCI.

MPEG-4 is most efficient around 1 Mbps, and gradually loses efficiency such that at 12-15 Mbps with HD, there is little improvement over MPEG-2.

Early on there were predictions of 100% improvement over MP2. Actual hardware proved these claims to be incorrect. Later we learned that the claims were based on very stupid or deliberately surreptitious parameters such as 720P/24 with a 10 frame GOP – these might find their way into a PC but never into a broadcast signal.

There are ongoing arguments about actual improvement over MP2; my personal opinion based on actual product measurements for a very large satellite provider (IP-Prime) is that in the sweet spot, MP4 will give you 30% better coding efficiency than MP2. The most debated parameter is GOP length because this directly impacts settling time and recovery time following a disturbance. U-verse initially picked a 5 second GOP to get the BW down, really ugly! This is where JPG really shines – it recovers in one frame.

Until recently MP4 was limited to 4:2:0 chroma sampling – this eliminated the plan from any serious broadcast use.

There have been two recent developments: The standards have been “extended’ to include 4:2:2 sampling and hardware to do this exists now. The MP4 decoder we recommended for Dix Hills can decode 4:2:2 MP2 and MP4. There is a second extension to 10 bit sampling. The ITU call this AVC-H10. I do not know of any encoder or decoder that is purchasable today that can implement H10 but when this shows up, it will be a serious challenge to MP2 which will never be extended to 10 bit.

The problem you will have is latency. MP4 inherently is a longer interval coding process which is one tradeoff for improved efficiency. This is an interesting question to ask your customers, I’m not sure it has been raised in the past. If you could give them a cost savings for equal quality and 10 bit depth, they might be willing to accept a longer transit period.

Hope this helps, sorry for the lengthy post.

 

[broadcasttheworld]

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.

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.

 

[raoul5788]

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.

All are picked up ota? Are you sure? I thought I read somewhere that some are fed by fiber.

 

[cfb]

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.

 

[raoul5788]

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.

And despite all that, the Directv hd locals still look pretty darn good!

 

[cfb]

Sure they do. Now get an HR20 and get them OTA, or ramp up your OTA tuner capable 1080i tv and watch them side by side.

Not to say the directv mpeg2 -> mpeg4 -> reduced bitrate versions dont look better than SD. They do.

"The beatings will continue until morale improves" does not equate with quality.