BBC Lowers HDTV Bitrate; Users Notice

aws910 writes "According to an article on the BBC website, BBC HD lowered the bitrate of their broadcasts by almost 50% and are surprised that users noticed. From the article: 'The replacement encoders work at a bitrate of 9.7Mbps (megabits per second), while their predecessors worked at 16Mbps, the standard for other broadcasters.' The BBC claims 'We did extensive testing on the new encoders which showed that they could produce pictures at the same or even better quality than the old encoders ...' I got a good laugh off of this, but is it really possible to get better quality from a lower bitrate?"

Comments

Yes

[Unoriginal Nick]

I got a good laugh off of this, but is it really possible to get better quality from a lower bitrate?

Sure, if you also switch to a better codec, such as using H.264 instead of MPEG-2. However, I don't think that's what's happening in this case.

Yes, of course

[TheRaven64]

Any lossy compression works by throwing away bits of the picture that the viewer might not notice. You can lower the bitrate with better psychovisual and psychoacoustic models. You're still throwing away more information, but you're doing it in a way that the user is less likely to notice. This takes more CPU time on the compressor, a more optimised encoder, or a better algorithm.

Re:Yes, of course

[Andy Dodd]

LAME was a pretty good example of this for MP3 - Eventually it was able to achieve (somewhat) better quality at (somewhat) lower bitrates than the reference encoders.

Vorbis, similarly, had the AoTUV tuning - This provided significant rate/distortion tradeoff improvements compared to a "vanilla" encoder, without changing the decoder.

However, 40% reduction in bitrate with an increase in quality is very difficult unless the original encoder was CRAP. (Which is actually a definite possibility for a realtime hardware encoder.) Also, it's far more likely to have such improvements with H.264 or MPEG-4 ASP, not nearly as likely with MPEG-2, which had a far less flexible encoding scheme.

Re:Yes, of course

[TheRaven64]

Yup, fractal encoding is pretty impressive. I played with it a bit on a 386, when it took about ten minutes to compress a 320x240 image. I've not heard of any newer algorithms that improve matters much. More interesting is topological compression, which has most of the same advantages as storing a vector image (resolution independent) and a raster image (can come from a sampled source). You can extend these to video by modelling the video data as a 3D volume, rather than as a sequence of frames. The topological changes in the time dimension are usually quite gradual, and it's easy to trade special and temporal resolution. The really nice thing about this approach is that it's resolution independent in three dimensions, not just two, so it's easy to generate a signal that matches the display's refresh rate.

Re:Yes, of course

[wagnerrp]

A 40% decrease in bitrate is about what I'd expect going from a single-pass to a two-pass H.264 encoder, and it's entirely possible that a newer single-pass encoder can do the same sort of thing just by using a longer window now that RAM is a lot cheaper.

No, there is no difference in compressibility between a single pass and a two pass encoder. The two pass encoder simply allows you to set the quantizer so as to very accurately hit a target average bitrate.

Summary rounding error

[w0mprat]

Nitpick: So 39% is "almost 50%"?? I would have called that "almost 40%". Then again that is a /. summary.

It is absolutely possible

[PhrostyMcByte]

Bitrate is only part of the equation -- the H.264 spec allows for a number of different ways to compress video, and it's up to the encoder to find out which is best for your video. Even in the same encoder, you can tweak dozens of settings in ways that dramatically change output quality -- usually a trade off between time and size.

x264 has beat every commercial encoder out there -- in some cases, on a level that would indeed render higher quality with half the bitrate.

Re:They suck at math too

[gandhi_2]

Technically speaking, they suck at "maths".

It depends on the material

[Locke2005]

If you're watching a soap opera, you only need to see a few frames per week to follow the story. If you are watching a live sports event with a lot of action, most people will notice every dropped frame and compression artifact (I've noticed myself while watching the Olympics via satellite feed.) Methinks they did the testing on a relatively static video. Video compression works by (among other methods) creating a key frame, then sending diffs off that key frame for several frames. If every frame is completely different, compression does not work well.

Re:Focus group...

[jasonwc]

Yes, it IS possible to get higher picture quality out of a lower bitrate, but not with all else equal. For example, you can get higher quality with CPU-intensive settings using H.264 5.1 Profile than you can with H.264 4.1 (what Blu-Ray's/HD DVDs use), at the same bitrate. You're giving up CPU cycles in decoding for lower video size. This is why x264 can produce near-transparent encodes of Blu-Ray movies at about half the size. x264 uses much more demanding settings.

x264 at 20 Mbit which high-quality settings is far more demanding than a 40 Mbit H.264 stream from a Blu-Ray.

Re:Crap HD Quality

[Brett Buck]

I think you might want to talk to you cable company on that one. I know the effect you are seeing (it's by far the worst on local Public TV since they crammed 7 sub-channels into the same carrier), but network TV coverage of football in my area is pretty pristine for the most part. OTA is even better but cable is still awfully good.

      Of course, by "talk to your cable company", I mean "do nothing" because talking to the cable company is a complete waste of time.

      Brett

Re:Focus group...

[Anonymous Coward]

1) The alleged wife in the quote is purported to have cataracts. Cataracts typically reduce visual acuity due to the cloudiness they impart to the lens of the eye. How does a reduction of visual acuity translate to "just another racist characterization of women being incompetent with technology"?

2) If the quote had been ""Even my husband can see a reduction in picture quality and he's got cataracts," wrote one." would you have bothered to make your little rant post?

P.S. The term you were looking for is "sexist" not "racist".

Re:iPlayer appears to use H.264

[TheRaven64]

BBC HD also uses H.264 for terrestrial and satellite broadcasts. It's only if you have Virgin Media cable that you get the stream transcoded to MPEG-2.

Re:Focus group...

[bkr1_2k]

Theoretically, perhaps. In reality either one could look better given other factors.

Re:Focus group...

[PIBM]

Actually, the bandwidth at 8bits per channel not including the 5.1 sound is 16,588,800 bits per FRAME not per second, so at 60 FPS you get a 950 mb/s bandwidth requirement for the video alone, and that`s why we need to use a compressed distribution method...