Native Body Price Playback. This text talks a couple of novel HDMI… | by Netflix Know-how Weblog | Jun, 2023
7 min read
Maximizing immersion for our members is a crucial purpose for the Netflix product and engineering groups to maintain our members entertained and totally engaged in our content material. Leveraging a great mixture of mature and cutting-edge consumer machine applied sciences to ship a easy playback expertise with glitch-free in-app transitions is a crucial step in direction of reaching this purpose. On this article we clarify our journey in direction of productizing a greater viewing expertise for our members by using options and capabilities in client streaming gadgets.
If in case you have a streaming machine related to your TV, reminiscent of a Roku Set High Field (STB) or an Amazon FireTV Stick, you could have come throughout an choice within the machine show setting pertaining to content material body charge. Gadget producers typically name this function “Match Content material Body Price”, “Auto modify show refresh charge” or one thing related. When you’ve ever puzzled what these options are and the way they’ll enhance your viewing expertise, maintain studying — the next sections cowl the fundamentals of this function and clarify the main points of how the Netflix software makes use of it.
Netflix’s content material catalog consists of video captured and encoded in one in all numerous body charges starting from 23.97 to 60 frames per second (fps). When a member chooses to look at a film or a TV present on a supply machine (ex. Set-top field, Streaming stick, Recreation Console, and so forth…) the content material is delivered after which decoded at its native body charge, which is the body charge it was captured and encoded in. After the decode step, the supply machine converts it to the HDMI output body charge which was configured based mostly on the capabilities of the HDMI enter port of the related sink machine (TV, AVR, Monitor and so forth). Basically, the output body charge over HDMI is routinely set to 50fps for PAL areas and 60fps for NTSC areas.
Netflix provides restricted excessive body charge content material (50fps or 60fps), however the majority of our catalog and viewing hours could be attributed to members watching 23.97 to 30fps content material. This basically signifies that more often than not, our content material goes by way of a course of known as body charge conversion (aka FRC) on the supply machine which converts the content material from its native body charge to match the HDMI output body charge by replicating frames. Determine 1 illustrates a easy FRC algorithm that converts 24fps content material to 60fps.
Changing the content material and transmitting it over HDMI on the output body charge sounds logical and easy. Actually, FRC works nicely when the output body charge is an integer a number of of the native body charge ( ex. 24→48, 25→50, 30→60, 24→120, and so forth…). Alternatively, FRC introduces a visible artifact known as Judder when non-integer a number of conversion is required (ex. 24→60, 25→60, and so forth…), which manifests as uneven video playback as illustrated under:
You will need to be aware that the severity of the judder is determined by the replication sample. For that reason, judder is extra outstanding in PAL areas due to the method of changing 24fps content material to 50fps over HDMI (see Determine 2):
- Complete of fifty frames have to be transmitted over HDMI per second
- Supply machine should replicate the unique 24 frames to fill within the lacking 26 frames
- 50 output frames from 24 authentic frames are derived as follows:
- 22 frames are duplicated ( complete of 44 frames )
- 2 frames are repeated thrice ( complete of 6 frames )
As a evaluation, judder is extra pronounced when the frequency of the variety of repeated frames is inconsistent and unfold out e.g. within the situation talked about above, the body replication issue varies between 2 and three leading to a extra outstanding judder.
Now that now we have a greater understanding of the problem, let’s evaluation the options that Netflix has invested in. As a result of fragmented nature of machine capabilities within the ecosystem, we explored a number of options to handle this situation for as many gadgets as potential. Every distinctive answer leverages present or new supply machine capabilities and comes with numerous tradeoffs.
The primary answer we explored and just lately enabled leverages the aptitude of present supply & sink gadgets to vary the outgoing body charge on the HDMI hyperlink. As soon as this function is enabled within the system settings, gadgets will match the HDMI output body charge with the content material body charge, both precisely or an integer a number of, with out consumer intervention.
Whereas this seems like the right answer, gadgets that assist older HDMI applied sciences e.g. HDMI v<2.1, can’t change the body charge with out additionally altering the HDMI knowledge charge. This leads to what is commonly referred as an “HDMI bonk” which causes the TV to show a clean display screen momentarily. Not solely is that this a disruptive expertise for members, however the period of the clean display screen varies relying on how briskly the supply and sink gadgets can resynchronize. Determine 3 under is an instance of how this transition appears:
Enhancements within the latest HDMI requirements (HDMI 2.1+) now enable a supply machine to ship the video content material at its native body charge with no need an HDMI resynchronization. That is potential by way of an progressive know-how known as Quick Media Switching (QMS) which is an extension of Variable Refresh Rate (VRR) focused for content material playback situations. QMS permits a supply machine to take care of a relentless knowledge charge on the HDMI hyperlink even throughout transmission of content material with totally different body charges. It does so by adjusting the quantity of non-visible padding knowledge whereas protecting the quantity of seen video knowledge fixed. As a result of fixed HDMI knowledge charge, the HDMI transmitter and receiver don’t have to resynchronize, resulting in a seamless/glitch-free transition as illustrated in Determine 4.
HDMI QMS is positioned to be the perfect answer to handle the issue we’re presenting. Sadly, at current, this know-how is comparatively new and adoption into supply and sink gadgets will take time.
Aside from the above HDMI specification dependent options, it’s potential for an software like Netflix to govern the presentation time stamp worth of every video body to attenuate the impact of judder i.e. the appliance can current video frames to the underlying supply machine platform at a cadence that may assist the supply machine to attenuate the judder related to FRC on the HDMI output hyperlink.
Allow us to perceive this concept with the assistance of an instance. Let’s return to the identical 24 to 50 fps FRC situation that was lined earlier. However, as an alternative of fascinated by the FRC charge per second (24 ⇒ 50 fps), let’s increase the FRC calculation time interval to three seconds (24*3 = 72 ⇒50*3 = 150 fps). For content material with a local body charge of 24 fps, the supply machine must get 72 frames from the streaming software in a interval of three seconds. Now as an alternative of sending 24 frames per second at an everyday per second cadence, for every 3 second interval the Netflix software can determine to ship 25 frames within the first 2 seconds (25 x 2 = 50) and 22 frames within the third second thereby nonetheless sending a complete of 72 (50+22) frames in 3 seconds. This method creates a good FRC within the first 2 seconds (25 frames replicated twice evenly) and within the third second the supply machine can do a 22 to 50 fps FRC which is able to create much less visible judder in comparison with the 24->50 fps FRC given a extra even body replication sample. This idea is illustrated in Determine 5 under.
NOTE: This answer was developed by David Zheng within the Accomplice Expertise Know-how crew at Netflix. Be careful for an upcoming article going into additional particulars of this answer.
Given the potential options accessible to make use of and the related advantages and limitations, the Netflix software working on a supply machine adapts to make use of one in all these approaches based mostly on components reminiscent of supply and sink machine capabilities, consumer preferences and the precise use case throughout the Netflix software. Let’s stroll by way of every of those elements briefly.
Each supply machine that integrates the Netflix software is required to let the appliance know if it and the related sink machine have the flexibility to ship and obtain video content material at its native body charge. As well as, a supply machine is required to tell whether or not it may possibly assist QMS and carry out a seamless playback begin of any content material at its native body charge on the related HDMI hyperlink.
As mentioned within the introduction part, the presence of a system setting like “Match Content material Body Price” sometimes signifies {that a} supply machine is able to this function.
Even when a supply machine and the related sink can assist Native content material body charge streaming (seamless or non-seamless), a consumer may need chosen not to do that through the supply machine system settings e.g. “Match Content material Body Price” set to “By no means”. Or they could have indicated a choice of doing this solely when the native content material body charge play begin can occur in a seamless method e.g. “Match Content material Body Price” set to “Seamless”.
The Netflix software must know this consumer choice with the intention to honor their choice. Therefore, supply gadgets are anticipated to relay this consumer choice to the Netflix software to assist with this run-time resolution making.
Regardless of supply machine functionality and the consumer preferences collectively indicating that the Native Content material Body Price streaming must be enabled, the Netflix software can determine to disable this function for particular member experiences. For instance, when the consumer is shopping Netflix content material within the residence UI, we can not play Netflix trailers of their Native body charge as a result of following causes:
- If utilizing Resolution # 1, when the Netflix trailers are encoded in various content material body charges, switching between trailers will end in display screen blanking, thereby making the UI shopping unusable.
- If utilizing Resolution # 2, sending Netflix trailers of their Native body charge would imply that the related UI parts (motion of cursor, asset choice and so forth) would even be displayed on the lowered body charge and this may end in a sluggish UI shopping expertise. It is because on HDMI output from the supply machine, each graphics (Netflix software UI) and video parts will exit on the identical body charge (native content material body charge of the trailer) after being blended collectively on the supply machine.
To deal with these points we observe an method as proven in Determine 6 under the place we allow the Native Body Price playback expertise solely when the consumer selects a title and watches it in full display screen with minimal graphical UI components.
This text introduced options that purpose to enhance the content material playback expertise on HDMI supply gadgets. The breadth of accessible technical options, consumer selectable preferences, machine capabilities and the appliance of every of those permutations within the context of assorted in-app member journeys characterize a typical engineering and product resolution framework at Netflix. Right here at Netflix, our purpose is to maximise immersion for our members by way of introduction of latest options that may enhance their viewing expertise and maintain them totally engaged in our content material.
We want to acknowledge the onerous work of a variety of groups that got here collectively to ship the options being mentioned on this doc. These embrace Core UI and JS Participant improvement, Netflix Software Software program improvement, AV Take a look at and Tooling (earlier article from this crew), Accomplice Engineering and Product groups within the Consumer Engineering group and our knowledge science buddies within the Data Science and Engineering group at Netflix. Diagrams on this article are courtesy of our Accomplice Enterprise Platform XD crew.
