BBC@50fps - what's that about?

fenlander

Active Member
For some time new additions to iPlayer have been at a default HD frame rate of 50fps. As if that wasn't bad enough, it seems that new programmes are being broadcast at 50fps. Case in point: Canal Walks with Julia Bradbury on BBC4 this week.

I see no noticeable difference in the picture quality but then I don't have a 60" 4K TV. For me, the only effects are bad ones: the storage required for each programme is doubled (a 30-minute episode weighs in at 1.3GB) and moving the files elsewhere takes twice as long, as does any sort of processing or recoding. Presumably, this also affects on-box operations like decrypting. Also, my network connection to the Hummy, which is via Powerline, is no longer fast enough to play the new recordings on other networked devices. I'm having to download the files to my PC (which takes twice as long) and recode them to 25fps with Handbrake (which also takes twice as long).

Can anyone tell me what benefit there is in upping the frame rate from 25 to 50fps?
 
There is basically no benefit in using 50fps progressive in this case. Most of the source material was shot at 50fps interlaced, so it simply doesn't have the source information to fill all those frames. The information is being interpolated ie. made up.
 
So the Beeb are using double the previous broadcast bandwidth to achieve little or nothing in terms of quality. At the same time, iPlayer has dropped 720p @ 25fps in favour of a sub-HD resolution @ 50fps. I'm at a loss to understand the current policy.
 
Freeview-HD boxes cannot decode 1080P50 only 1080p25. Highest resolution usable at 50 frames progressive is 1280 x 720 (720p50).
 
So the Beeb are using double the previous broadcast bandwidth to achieve little or nothing in terms of quality. At the same time, iPlayer has dropped 720p @ 25fps in favour of a sub-HD resolution @ 50fps. I'm at a loss to understand the current policy.

See the get_iplayer mailing list for many, many threads complaining about the same issue at great length. In future I will refer them to your excellent summary:) The BBC claim their tests show it looks better, but have offered no references to their test methods or results. They do seem to be focussing on mobile devices ie. smartphones and ignoring all other iPlayer users.

The annoying thing is the HDR Fox T2 iPlayer after these BBC changes gets a lower resolution stream than it used to with HD enabled in iPlayer. I can't watch it any more the picture quality is too poor, so I'm using my TV's iPlayer which seems to get the 720p50 stream based on the bit rate over my broadband.
 
I may have misunderstood the original poster. Are you saying the BBC are broadcasting over Freeview at 50Hz progressive? That isn't possible apart from at 720p and the BBC don't use that resolution nor does any other Freeview broadcaster. All the HD streams on Freeview dynamically switch between 1080i50 and 1080p25 depending on the video content, you can see it changing sometimes second by second if you have the HDR Fox T2 info banner displayed. It's a tradeoff between higher resolution 1080p25 and more motion 1080i50. Often you will see a programme end in p25 and as soon as the credits start to roll the video switches to i50.

Edit: for reference a 1 hour Freeview HD recording of something like BBC1 HD has always been close to 4GB in size. 1.3GB for 30 minutes sounds like it has been more heavily compressed than in the past. I know this because when the HD mux started in my area I had to get a new USB stick so I could format it Ext2 so that HD recordings over an hour in length could be copied to it. Max file length on FAT32 is 4GB.
 
That's because the PSB3 mux is stat muxed and uses Variable Bit Rates (VBR) with the highest bitrate allocated to the channel with the most challenging content. As a result and also due to more efficient encoders the file sizes you get now are pretty close to the same used previously for the best SD mpeg 2 satellite channels. A far cry from the original BBC-HD test transmissions from 28.2E with a fixed bitrate of 19Mbps.
 
People bandy about the term "fps" without actually specifying what the "f" means. Is it fields or is it frames?
It really is very annoying, and even more so when they start mixing the usage of the two.

Normal telly uses 25 frames i.e. 50 fields (apart from when it's been bastardised by arty-farty types to something lower to 'improve' the content).
 
Not true

Freeview-HD is mostly genuine 1080p25 (lines 1,2,3,4 -1080) delivered in order,in 1/25 sec though some GOP's (Group of Pictures) in the data stream can be interlaced.

Satellite is always interlaced, Lines 1 3 5 etc up to 1079, in 1/50 second. Followed by lines 2, 4 up to 1080. It's a hangover from CRT displays where the decay of the screen would produce flicker and a crude frame doubling effect as the the next field would replace half the picture to effectively double the perceived frame rate.

These days most of us have a bitmapped lcd/oled/plasma display.

TV's like this can only work with a progressive signal. The TV or set-top box, recovers the individual frame data from the lossy broadcast stream. Depending on the box or TV settings, the electronics (Video Processing), scales the signal to give every pixel on the display a 24 bit RGB signal (Every pixel has 3 sub pixels normally 8 bits Red, 8 bits Green an 8 bits Blue.

Simple example. Watching BBC 1-SD on say a satellite box set to original and sent to a full-HD display . The source has 720x576 pixels. The TV will guess the mixing pixels to increase the pixels sent to the screen to 1920 x 1080 pixels. The final result depends on the quality of the TV scaler.

Anyone with a Which subscription should avoid any TV identified as having a poor scaler.
 
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