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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?"
They also lowered their math standards. From 16MBps to 9.7 MBps is a 40% reduction, not "almost 50%".
I've abandoned my search for truth; now I'm just looking for some useful delusions.
It's not impossible to get better results out of lower bitrates, but you have to pay the penalty elsewhere, typically in encode/decode complexity.
If your decode hardware is fixed (it's generic HDTV hardware), then there is much less room for improvement, and half the bitrate is an enormous drop. It's no surprise that the BBC viewers complained.
I read the internet for the articles.
You can also get better compression by specifying a more sophisticated compression method within the same codec, for example, since many codecs support various levels of compression.
Generally, "better" compression (fitting a higher resolution and/or framerate into a smaller size) requires a lot more power to encode and often some more power to decode. You can use less bitrate to get a quality signal there, but you need "smarter" coders and decoders at the respective ends of the transmission. So the BBC may have upgraded their compression engine to something that can do "better" compression, thereby fitting the same resolution and framerate into a 40% smaller stream. But their customers' television sets might not have the horsepower to decode it at full quality.
That could easily explain why the BBC's testing went so well but their consumers (with varying brands of TV sets probably mostly tested for British use with the old compression) can't keep up and render an inferior picture.
It's also possible that, by compressing the video stream into a denser compression method, signal loss is having a greater effect than it did with the old compression method. The viewers may be seeing artifacts that are the decoder's attempts to fill in the blanks. The old compression method might have allowed a certain amount of redundancy or error correction that the new one lacks, and the loss of part of the signal has a more visible effect on the new one.
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Lossy compression formats depend on an understanding of human perception. Nobody has a perfect model of the human brain, and nobody has a perfect algorithm for deciding what data to keep and what data to throw away.
If you have a better model of human perception than your competitors, then your encoder will yield higher quality output. If you spend 50% of your bits on things that nobody will notice, and I spend 25% of my bits on things that nobody will notice, then my 650kbps stream is going to look better than your 900kbps stream.
LAME did not win out as the MP3 encoder of choice just because it is free. It won out because its psychoacoustic model yields better-sounding MP3s at 128kbps than its competitors managed at 160kbps or even 192kbps.
In other words, lower bitrate can be better, but only if you compare to shitty and inefficient compression.
And by this you mean compression that is state of the art two minutes ago, vs. today. Seriously, this field is moving pretty fast, and what you call shitty and inefficient was not long ago the best people could do. A few years ago when I was messing with the x264-svn libraries, stuff would get tweaked daily.
Not to mention there are other factors at play with regards to compression. A well-engineered system isn't necessarily going to go for the maximum compression rate for video quality. One has to look at other limitations, such as the decoding hardware, the method by which the video is being delievered, and even the viewing devices on the receiving end.
What is disheartening about the article is that it looks like the BBC are just in denial mode, and not really taking the complaints seriously. "Standard viewing equipment"? Seriously, what exactly are they getting at with that comment? On top of that it looks like they are trying to blame the quality of the source material, which certainly muddies the picture, but certainly the customers that are complaining would be used to these variations in quality before the change and not just suddenly notice it at the same time this equipment was rolled out.
I have respect for them sticking to their guns, but not when they are doing it with such lame excuses. Then again, the BBC spokesperson and reporter may not be the most tech savvy individuals, and its likely some of the message here is lost in translation. Lossy codec indeed.
You do understand though, that the lost information in your example is lost at the capture stage not the compression stage don't you?
Lossless compression is just that - lossless. Try compressing your copy of notepad.exe with WinZip, extract it and tell me if it still works. That's lossless compression. The result of compression then decompression is bitwise identical to the original. It has nothing to do with whether the original data is an accurate representation of what it claims to be.
"Necessity is the plea for every infringement of human freedom. It is the argument of tyrants; it is the creed of slaves