señor ding dong

Sorry for the multiple posts, but I've run across a server error with larger posts.

Recently, there's been a lot of discussion about the supposedly dramatic decline in the image quality of BBC HD. Not so much around here, but elsewhere, the story has made the mainstream news, prompted the BBC's Andy Quested to explain the technical aspects of HD broadcasting in great detail over a 5-day long blog, and a petition for the channel to be restored to its (supposedly) former glory has appeared on the government's Number Ten website.

To make a long story short, in August, BBC HD upgraded their MPEG4 encoders to what they claimed was a more efficient version, and dropped the channels bitrate by nearly 40%, which caused angry viewers to complain about what they perceived as a drop in quality. Apart from identifying a small problem during certain transitions, which was quickly fixed, the BBC's own extensive testing suggested that the new encoders were so much more efficient that, even at the lower bitrate, the picture quality was as good, and often better than before. This claim that was backed up by tests performed by the consumer group Which?, but not at all by angry viewers.

señor ding dong

Now, what a lot of people didn't know at the time, and which was confirmed by the aforementioned Andy Quested, was that Virgin's cable service is MPEG2 and is encoded by Virgin themselves from an HDSDI feed direct from BBC HQ, and thus would not have been affected by the change to the BBC's MPEG4 encoders. This didn't stop angry cable viewers from complaining that the quality, which supposedly used to be "like looking out of a window", now apparently looked worse than your average YouTube video or SD broadcast. So the question is, was this just internet and media-induced hysteria, a sort of Emperor's New Clothes syndrome in reverse, or had Virgin's service really suffered a drop in quality? Thanks to a BBC HD New Year's Eve repeat of Pirates of The Caribbean 2, we might be able to shed some light on this issue, since I also happened to have a recording of the original 2008 airing. You know, from back when BBC HD was "like looking out of a window". My comparison of the two recordings suggests that whomever made this claim should probably have their windows checked, rather than blaming the BBC for any wrong-doing. Not just because no HD broadcast anywhere in the world would ever be like looking out of a window, but also because the new recording actually proved to have marginally better picture quality than the original.

señor ding dong

For the sake of keeping the images uncompressed, I've cropped them a little to keep them under Imageshack's 1.5mb limit. No other processing or filtering has been applied.

Anyway, enough rambling and onto...

Exhibit A:

Here we have Keira Knightley getting up close and personal with Johnny Depp. There's very little movement in this shot, so not much risk of anything going wrong. You couldn't really say one version is better than the other, there are just tiny differences in the film grain, but the detail in Knightley's hair is intact in both versions.

OLD
http://img51.imageshack.us/img51/1982/bbchd01old.png

NEW
http://img94.imageshack.us/img94/2237/bbchd01new.png


Exhibit B:

On this scene change, we can see some significant differences, though. On the old version, you can see very clear compression artifacts to the right of Depp's hand at the bottom of the image, and on the right side of the image, the link in the chain takes a pretty bad compression hit as well, which gives it a jagged, blocky looking edge that's almost perfectly smooth in the new version.

OLD
http://img64.imageshack.us/img64/8352/bbchd02old.png

NEW
http://img38.imageshack.us/img38/5418/bbchd02new.png

señor ding dong

Exhibit C:

Depp takes on The Kraken, and again there are very few differences between the two versions. Certainly nothing you could spot from a normal viewing distance, and certainly nothing that could be described as the difference between an SD broadcast and looking out of the window. If you zoom in really close, you can find little areas, like the upper two creases on Depp's left thigh being slightly better reproduced in the new version, and there's a minor wobble on The Kraken's top tooth in the old version. You can see it as tiny difference in the two stills, and it's slightly more noticeable on the moving video. That's really quite absurd nitpicking, though, but it does give the new version a tiny edge.

OLD
http://img94.imageshack.us/img94/3971/bbchd03old.png

NEW
http://img94.imageshack.us/img94/2862/bbchd03new.png


Exhibit D

In this scene, the camera pans quickly across the floor of a ship, so everything is moving. As expected, you do see a bit more compression artifacts on such scenes, but I wouldn't exactly say there's much difference between the two, much less claim one was significantly better than the other.

OLD
http://img707.imageshack.us/img707/4498/bbchd04old.png

NEW
http://img685.imageshack.us/img685/5886/bbchd04new.png

señor ding dong

The findings above were repeated throughout other scenes in the two recordings. The new version was as good, and in some cases slightly better than the old version, and judging by this example, there has been no dramatic change in the quality of BBC HD on cable. Keep in mind, though, it's still possible that there's something funky going on with the sat version, but considering how many cable users seem to have complained about a quality degradation that simply doesn't appear to be real, I wonder if some of the sat people might be imagining things, or if it's simply a case of certain set top boxes not being compatible with the profile level for the optimized MPEG4 encoders. I'd love to do some direct cable vs sat comparisons, if any of our resident sat users are interested? I wish I could have gotten a sat recording of the Pirates movie, but I only just happened to come across the repeat as I was heading out, and it wasn't able to track down any sat people on short notice.

What's interesting about these samples is that the old broadcast had an average bitrate of 17.60 mbps, and the new one 16.50 mbps. Checking some other older recordings against recent ones, it turns out that Virgin's version of BBC HD actually dropped its bitrate a little bit when they introduced the new HD channels, but all the testing they did to fine-tune the encoders at the time, which was quite visible on the first test versions of the new HD channels, obviously means they are now at least as efficient at the slightly lower rate, and in some cases even more so.

señor ding dong

Now, just for the fun of it, I wanted to see how the BBC broadcast of this movie compared to the BluRay. Unfortunately, I couldn't find anyone who had a copy on BluRay, but I did find a full resolution x264 version. This obviously means it would have been recompressed, but my past experience with x264 has shown it to be an incredibly efficient codec, and you generally don't lose all that much. If anybody has an actual BluRay version of the movie and could grab a few uncompressed stills, that would be great, though.

For the samples below, I didn't bother including the old BBC recording.


Exhibit E:

I picked this scene because rain is something that really puts a strain on MPEG encoding, and because it had some nice, fairly sharp detail. I have to say, the BBC HD version holds up quite well to the X264 version. Apart from the slightly different cropping/zoom factor you always find between broadcasts and commercial releases, I doubt anyone would be able to notice much difference from a normal viewing distance.

BBC HD
http://img190.imageshack.us/img190/9...chd01bbchd.png

X264
http://img94.imageshack.us/img94/778...bchd01x264.png

señor ding dong

Exhibit F:

This shot was chosen because a lot of people have complained about "grain" and "noise" particularly in dark scenes, which never sounded to me like an encoding issue. HD just shows such inherit imperfections in the source material, and here the film grain is equally obvious in both versions, but other than that, there's very little difference in terms of overall sharpness and clarity, though the X264 version does have a slight edge, but it's still not something you'd be likely to spot from a normal viewing position.

BBC HD
http://img686.imageshack.us/img686/8...chd02bbchd.png

X264
http://img190.imageshack.us/img190/3...bchd02x264.png

Side note: Despite BBC HD technically being 1080i, 24/25hz material is essentially broadcast as 1080p, so it's directly comparable to the 1080p X264/BluRay version.

chrispeters

very very interesting post .. rep added .

Rat

have to agree there,many thanks señor ding dong for this detailed thread

I do find however that BBCHD on freesat compared to cable is slightly better
but saying that looking at the cable /bluray comparison there isn't a lot in that at all really
so sat to cable shouldn't be that different either ?

__________________

señor ding dong

You wouldn't have thought so, but there can be quite considerable differences depending on your exact hardware and settings. Some set top boxes don't have very good display hardware, there can be internal processing like sharpening and noise reduction going on, and TV sets themselves can also apply all manners of additional processing or use different deinterlacing/IVTC methods, particularly the 100hz+ sets, so it's difficult to make an accurate like-for-like comparison that way. However, when we have recordings from both sources, we can make direct comparisons of the source material using the same "test bed" for both, with the only variation being the codec (MPEG2 vs MPEG4) which is why I hope someone recorded the movie on sat yesterday.