INFO. POSTING No 8: HOW REALITIME IS CHROME HD

INTRODUCTION: 
There have been a number of webboard posts requesting more information regarding Liquid ChromeHD’s actual realtime capabilities. This document attempts to clarify and summarise some of the key issues:

QUESTIONS:
1) Is Liquid Chrome HD a realtime system?
2) What do Yellow Slices Indicate?
3) The difference between Firewire versus BOB output
4) How does Liquid Chrome HD realtime features compare with Liquid and Liquid PRO

EXPLANATION:
Liquid ChromeHD's is a realtime system, meaning it can output multiple video/graphic layers with realtime effects without the need to render.  It should be noted that yellow slices on the timeline does not necessary indicate that a sequence needs to be rendered. A Liquid Yellow Slice simply implies that this slice consists of a ‘realtime software effect’.

This means Yellow Slices are typically realtime (up to a level of the systems performance!). Let me explain this further. Different users can be using different computers with varying configurations in terms of RAM, graphics cards, CPU's etc etc it's therefore not possible for Avid Liquid to know for sure if a complex compositing timeline is realtime or not; because this depends not only on project complexity but also on the workstation being used!  Therefore export to tape has the 'Render yellow slices' feature in order to give 100% reliable output with all certified systems and all certified configurations. This is by deliberate design because for some users; they simply just want to go to Record to Tape and be sure everything will be 100% O.K. without checking the realtime capabilities of the system.  With Liquid the choice to render has also been simplified by allowing background render to occur seamlessly in the background for GPU effects when the system is not being used for actual editing. 

Liquid ChromeHD Firewire Output:   For record to tape Liquids yellow slices can be turned ‘On’ or ‘Off’ for BOB output but is always required to be 'ON' for firewire output because this is essentially a Microsoft limitation (e.g. Microsoft 1394-firewire allows for only one realtime firewire stream only) and it's important to note that Liquid uses the Microsoft 1394 software code for all of it's firewire input/output activities. This is why firewire must render to one stream for Liquids DV and HDV firewire output. This however, does not apply to ChromeHD's BOB output.

Liquid ChromeHD BOB Output:  For BOB output yellow slices can be turned ‘On’ or ‘Off’ for BOB output giving the user more options by allowing Liquid ChromeHD to bypass the need for complete project render of all yellow slices. This requires that the user fully understands the realtime performance limits of their system. It's for this reason that Avid have also made provision not only for render of all yellow slices to tape or from the timeline; but also in Liquid ChromeHD version 7 and above; the user can selectively semi-automatically rendered only for 'selected slices' or 'slices within a range' (by right mouse clicking on the slice(s) on the timeline).  This sort of selective rendering will allow playback from the timeline or even record to tape without drop frames and without rendering the entire project from the BOB’s outputs.

For ChromeHD these rules apply to all codecs when outputting via the BOB. Therefore, if you play a complex project (with multiple layers) to tape while outputting to BOB (e.g. using SD-SDI or HD-SDI or BOB analog connections) then there will be no need to render the entire sequence. You may only need to render those slices where it's beyond your systems capabilities (e.g. <= 8 layers in most case for DV). However, this can depend not only on the number of layers but also on the codec used and the type of effects. These realtime capabilities apply to all DV, HDV, MPEG2-I frame and 2YUV ChromeHD codecs being output via the BOB. Finally, it should be noted nevertheless that due to higher data rates (compared to DV) one can expect significantly less realtime layers for MPEG2-I frame, 2YUV and HDV/HD codecs via the BOB’s output.

Liquid & Liquid PRO realtime performance versus Liquid ChromeHD
For a suitable system with a high end PCIe graphics card; if we use DV only as the benchmark we can observe up to 10 realtime layers for Liquid and Liquid PRO systems compared to 8 realtime layers for Liquid ChromeHD on the same system.

The reason for seeing 8 realtime layers with ChromeHD compared to 10 layers with Liquid is because ChromeHD is a much more complex system which has multiple codecs for capture and play (including 4:2:2 codecs) and particularly because it needs to input/output via the CineWave card and the BOB’s more extensive input and output facilities. This requires additional resources from the system and therefore this becomes a slight overhead on the system compared to Liquid which does not have all of these features and capabilities.

DISCLAMER: This information (and the other Information/Knowledge postings on this Avid Liquid Chrome HD Webboard) are the opinion of Dean Athan and not official Avid documents.

Dean Athan - Jan. 2007