Native Body Price Playback. This text talks a few novel HDMI… | by Netflix Know-how Weblog | Jun, 2023

by Akshay Garg, Roger Quero

Maximizing immersion for our members is a vital purpose for the Netflix product and engineering groups to maintain our members entertained and totally engaged in our content material. Leveraging an excellent mixture of mature and cutting-edge consumer machine applied sciences to ship a easy playback expertise with glitch-free in-app transitions is a vital step in direction of attaining 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 units.

You probably 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. For those who’ve ever questioned what these options are and the way they’ll enhance your viewing expertise, preserve studying — the next sections cowl the fundamentals of this function and clarify the small print of how the Netflix software makes use of it.

Netflix’s content material catalog consists of video captured and encoded in one in every of 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, Sport 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 mechanically set to 50fps for PAL areas and 60fps for NTSC areas.

Netflix presents restricted excessive body charge content material (50fps or 60fps), however the majority of our catalog and viewing hours will be attributed to members watching 23.97 to 30fps content material. This primarily signifies that more often than not, our content material goes by way of a course of referred to 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.

Determine 1 : 3:2 pulldown approach to transform 24FPS content material to 60FPS

Changing the content material and transmitting it over HDMI on the output body charge sounds logical and easy. In reality, FRC works properly 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…). However, FRC introduces a visible artifact referred to 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 beneath:

With Judder
With out Judder

It is very important notice that the severity of the judder is dependent upon the replication sample. Because of this, judder is extra distinguished in PAL areas due to the method of changing 24fps content material to 50fps over HDMI (see Determine 2):

  • Whole 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 ( whole of 44 frames )
  • 2 frames are repeated thrice ( whole of 6 frames )
Determine 2: Instance of a 24 to 50fps body charge conversion algorithm

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 distinguished judder.

Now that we’ve a greater understanding of the difficulty, 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 difficulty for as many units as attainable. Every distinctive resolution leverages present or new supply machine capabilities and comes with numerous tradeoffs.

The primary resolution we explored and not too long ago enabled leverages the aptitude of present supply & sink units to vary the outgoing body charge on the HDMI hyperlink. As soon as this function is enabled within the system settings, units 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 appears like the proper resolution, units that assist older HDMI applied sciences e.g. HDMI v<2.1, can’t change the body charge with out additionally altering the HDMI information charge. This ends in what is usually 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 length of the clean display screen varies relying on how briskly the supply and sink units can resynchronize. Determine 3 beneath is an instance of how this transition seems to be:

Determine 3: Native body charge expertise with display screen blanking

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 while not having an HDMI resynchronization. That is attainable by way of an revolutionary know-how referred to 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 continuing information charge on the HDMI hyperlink even throughout transmission of content material with completely different body charges. It does so by adjusting the quantity of non-visible padding information whereas conserving the quantity of seen video information fixed. As a result of fixed HDMI information charge, the HDMI transmitter and receiver don’t must resynchronize, resulting in a seamless/glitch-free transition as illustrated in Determine 4.

HDMI QMS is positioned to be the best resolution to handle the issue we’re presenting. Sadly, at current, this know-how is comparatively new and adoption into supply and sink units will take time.

Determine 4: Native body charge expertise with out display screen blanking utilizing HDMI QMS

Other than the above HDMI specification dependent options, it’s attainable for an software like Netflix to control 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 eager about 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 strategy creates an excellent 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 can 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 beneath.

Determine 5: FRC Algorithm from Resolution#3 for twenty-four to 50 fps conversion

NOTE: This resolution 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 resolution.

Given the attainable options out there to make use of and the related advantages and limitations, the Netflix software operating on a supply machine adapts to make use of one in every of these approaches based mostly on elements 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 points 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 could 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 might 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 may have indicated a desire 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 desire. Therefore, supply units are anticipated to relay this consumer desire to the Netflix software to assist with this run-time choice making.

Despite supply machine functionality and the consumer preferences collectively indicating that the Native Content material Body Price streaming ought to be enabled, the Netflix software can determine to disable this function for particular member experiences. For instance, when the consumer is looking Netflix content material within the dwelling UI, we can’t play Netflix trailers of their Native body charge because of the 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 looking unusable.
  • If utilizing Resolution # 2, sending Netflix trailers of their Native body charge would imply that the related UI elements (motion of cursor, asset choice and so forth) would even be displayed on the decreased body charge and this can end in a sluggish UI looking expertise. It’s because on HDMI output from the supply machine, each graphics (Netflix software UI) and video elements will exit on the similar body charge (native content material body charge of the trailer) after being blended collectively on the supply machine.

To deal with these points we comply with an strategy as proven in Determine 6 beneath 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.

Determine 6: Native Body Price utilization inside Netflix software

This text introduced options that goal to enhance the content material playback expertise on HDMI supply units. The breadth of accessible technical options, consumer selectable preferences, machine capabilities and the appliance of every of those permutations within the context of varied in-app member journeys symbolize a typical engineering and product choice 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 preserve them totally engaged in our content material.

We wish to acknowledge the exhausting work of numerous groups that got here collectively to ship the options being mentioned on this doc. These embody Core UI and JS Participant improvement, Netflix Utility Software program improvement, AV Check and Tooling (earlier article from this crew), Accomplice Engineering and Product groups within the Consumer Engineering group and our information science associates within the Data Science and Engineering group at Netflix. Diagrams on this article are courtesy of our Accomplice Enterprise Platform XD crew.