« Topics
1234
Inspector Sands13,971 posts since 25 Aug 2004
Aha, so the bit rate of the encoders is driven by the multiplexer? Makes sense I suppose. I think I assumed that the encoders produced a stream to the bit rate needed, up to a maximum level*. Then the multiplexer then fitted them all together, reducing the bit rates if need be


*I've only encoded stuff for when it's not being multiplexed, e.g. for a satellite uplink where you fix the transport stream rate but the video bitrate varies
Last edited by Inspector Sands on 9 July 2019 2:43pm - 2 times in total
noggin14,646 posts since 26 Jun 2001
Aha, so the bit rate of the encoders is driven by the multiplexer? Makes sense I suppose. I think I assumed that the encoders produced a stream to the bit rate needed, up to a maximum level*. Then the multiplexer then fitted them all together, reducing the bit rates if need be


*I've only encoded stuff for when it's not being multiplexed, e.g. for a satellite uplink where you fix the transport stream rate but the video bitrate varies


Yep - whilst it is technically possible to reduce the bitrate of a compressed stream, the results are usually far from great (it's how some DVD shrinking programs used to get DVD9s into DVD5s ISTR without re-encoding) and you really have to encode at the target bitrate for decent quality. I think it gets even more difficult with h.264 and h.265 which don't have fixed block sizes.

Statmuxing delivers huge benefits, but it does come at a cost of requiring more encoders.

Before BBC One English regions were statmuxed, they were CBR encoded locally at 5.8Mbs with BBC Two England, BBC Three/CBBC, BBC Four/CBeebies and the BBC News Channel (not sure if Parliament was still elsewhere at this time) statmuxed centrally - reducing the number of encoders, but at the expense of less flexibility in the statmux.
2
Inspector Sands and London Lite gave kudos
TedJrr201 posts since 11 Sep 2005
Anglia (East) Look East
Hence a set of coders statmuxing for each region is more or less the only way but at a cost. ( times number of regions )....


I'd assumed (wrongly) that at some point all the UK PSB muxes would migrate to DVB_T2/HD only.

Clearly, this isn't going to happen, but as coder capability has increased (for a variety of reasons including the capacity of the mux chain to handle more complex software and the capability of the installed base of receivers) - perhaps a swap may occur. Two PSB T2 muxes with services balanced by regionally and one legacy DVB_T mux carrying all the SD PSB content.

Yes, that's a big compromise on SD service, both in number and quality, but a bonus for HD as only one of the muxes need by sub-regional, the other universal. The challenge presumably would be to balance the national -vs- sub regional element between England and Scotland.

If there was one UK wide mux, then Scotland would need 5 (or is it 6) sub-regional muxes to support all its sub-regions and overlaps, with BBC1, BBC2, BBC Scotland, BBC Alba, C4(S), STV Angus, Durris, Craigkelly, Blackhill and Border (S).

That leaves an English mux per sub-region or overlap with BBC1, BBC2, ITV's sub-region and C4 (L, E, M, N). There's still a lot of coding of BBC2, C4 etc into BBC/ITV's sub-regions and overlaps.

Perhaps you took the view that the nature of sub-regional content always demanded a lighter code, as its heavy on simple images like talking heads, or scenes of places ...etc. The most critical content you could imagine would be sporting clips, which might be pre-treated to code more neatly? If that were so then a compromise of BBC1 dropping into a CBR at op-out times allowing down-stream MPEG splicing at GOP frame boundaries, providing the rest of the mux is protected by having some pretty swinging parameter restrictions - say 4 Mbits only for a BBC1 opt-out.

The same for ITV, except of course that most of their " opt-outs " are ad spot insertions where quality may not be compromised, and anyway happen at odd times, so it would be inconvenient to shove the whole of ITV into a restrictive CBR code, whilst some sub-regional spots happen somewhere. The HD PSB mux has already shaped itself into a mux that's driven by ITV's macro-regions (eg Anglia and Meridian are one), so possibly a compromise may be for a regional mux to be based on ITV region, with ITV taking a commercial view on the value of sub-regional spots?

How many sub-regional codes do you actually need?

Presumably, this is determined by how committed broadcasters are to their current arrangements? So C4's N macro-region would be separately coded into :

Arrow Pontop/Chatten
Arrow Bilsdale (possibly - but do ITV actually sell it separately any more?)
Arrow Caldbeck (E) (Yes I know that this is traditionally in the C4 (S) macro-region, but is this so after DSO?)
Arrow South Lakes - BBC/ITV overlap (see Isle of Man)
Arrow Winter Hill
Arrow Isle of Man (is this still BBC N West/ Border (E) as South Lakes?
Arrow Emley Moor (BBC Leeds and Calendar North
Arrow Belmont (BBC Hull and Calander South)
Arrow Crosspool/ Chesterfield (BBC Leeds and Calendar South)
- are there any relays in the Peak District that take ITV Calendar South and BBC East Midlands?
- is Scarborough a copy of Emley Moor, or does it have Belmont ITV?

Obviously, the legacy SD mux would need all of these as well. The number of services that it has to carry would tend to mitigate against BBC1 and ITV being permanent CBR codes; otherwise, they'd probably get squished into something like 1.2 Mbits/s
Last edited by TedJrr on 9 July 2019 4:55pm - 2 times in total
Rkolsen2,933 posts since 20 Jan 2014
BBC World News

Yep - I think people assume that making a new ITV region HD on terrestrial just needs a new 'feed' of that region, ignoring that if that is a new unique combination of channels EVERY channel in the PSB3 mux (BBC One HD, BBC Two HD, ITV HD, C4 HD, C5 HD etc.) will require a new encoder (even if its carrying the same content already being encoded elsewhere) and for redundancy, that will mean two new sets of encoders, plus a new unique combination of services will also need dual redundant fibre distribution to the transmitters taking it

Is that right? Why do they have to seperately encode each service seperately for each mux?


BBC News Channel for example is the same everywhere, so surely they can just encode it for DTT using several redundant encoders) and then use that to provide that for each Mux? Is it just a redundancy thing?


Because DTT Uses stat muxing it needs each if the services in a mux to have its own coder ... it's what enables (to use the PSB 3 HD mux as an example) a service to have 15++ Mbit/sec when it needs it to hold quality
... while most of the time taking say 3 to 4 Mbit/s
See http://en.digitalbitrate.com/dtv.php?liste=1&live=9&lang=en&mux=BBCB-PSB3

So if you split off a channel to be fed to all regional muxes what constant bit rate do you give it and thus subtract from the statmuxed pool ...
if you said say 8 Mbit/sec that would mean that the statmuxed pool would have
to have one fewer channel ...
and also some if the statmuxed gain would be lost ...
so probably not a good thing !

And if you turn it around if you stat mux the uk wide channels and the drive a encoder per region to complete the mux ... but the regional coder only gets the bit rate that's left which is not the quality that would be sought for a premier channel like a PSB .. but there are some things that you can do to improve the quality but it gets so complex ..and not predictable.
You then get into the realm Of the Transmux that I described fr Woolacombe ... but I have not seen one in AVC .... it was mind bendingly complex in MPEG 2.

Hence a set of coders statmuxing for each region is more or less the only way
but at a cost. ( times number of regions )

Just to make it more complex the DTT Hd mux uses two pass encoding so it is coding once and then seeing if it can make it better by using a different encoding toolset / bit rate ....


What do you mean two pass encoding? Is one to send to the broadcast encoders and the second is at the encoder itself?
Don’t let anyone treat you like you’re a VO/SOT when you’re a PKG.
Technologist83 posts since 10 Oct 2018
London London
@TedJrr


I would not give up hope just yet but as the idea floated by EdVaizey in 2011 for PSB to be Mandated to emit HD only does seem a long time coming ...

What also makes sense is to have a single HD PSB regionalised mux unlike the two SD regional muxes we currently have .... thus being roughly the legacy analogue plus the BBC radio services certainly the local radio stations and main channels .
It may have the national tv in place of a uk wide BBC two though .

The BBC mux would then have the two timex channels slots (CBeebies CBbc / nithing and BBC four ) and BBC news channel and BBC parliament and may be BBC two ( or national services ) on either uk or national basis with any red button or radio to suit !

The digital3&4 mux would carry no PSB services ...... which is a problem perhaps but I'm sure that the plus 1s and daughter channels of itv ch 4 and may be ch 5 or whoever can afford it !!! May be Uktv..

There are some roughnesses to,sort out ..may be itv+1 is in the regionalised mux kicking ch five to the PSB uk wide mux...

The Comm muxes one can hope that OFCOM insist on 720(ish) by 576 SD pictures ....
But again there is more or less a spare mux of capacity ....

It is many years since I did the detailed version ..and we have had two BBC channels appear in Scotland since .... and a flurry of plus 1s and the loss of BBC three !

@Rkolsen
Dual pass coding ...
How you code a picture /group of pictures can be done a number of ways
(btw the resultant data volume for the GOP is not a good measure of quality )
So if you code the GOP say twice using a different toolkits and then decode and measure the error between each one and the original then use the best quality one that is better than just allowing the coder to be on 'average ' settings
Add in stat mux thus you can see the complexity of a picture /GOP in advance ( by The code and decode/ compare )
you can then decide how to code it bearing in mind that the other channels coders are doing the same thing ...and the overall manager is controlling /allocating the bit rate of the overall mux.
So the manager can tell each coder how it should code noting the result from the first go at coding it
So the signal can be coded twice in parallel results then looked at and compared with all the other channels bit rate demand from thier complexity results
and then the original signal is coded for output as part of the stat mux at a rate/quality/ tool kit to fit ( there is also a bit of buffering in case of overshoots)
That's why the coder takes 6 seconds to do it all .....

I think I've got it more or less right ! It's all software !
2
Rkolsen and TedJrr gave kudos