September 28, 2023


Technology Room

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

7 min read

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 a great mixture of mature and cutting-edge shopper gadget applied sciences to ship a clean playback expertise with glitch-free in-app transitions is a vital 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 shopper streaming units.

When you’ve got a streaming gadget related to your TV, corresponding to a Roku Set Prime Field (STB) or an Amazon FireTV Stick, you will have come throughout an choice within the gadget show setting pertaining to content material body charge. Gadget producers typically name this characteristic “Match Content material Body Charge”, “Auto alter show refresh charge” or one thing related. If you happen to’ve ever puzzled what these options are and the way they’ll enhance your viewing expertise, hold studying — the next sections cowl the fundamentals of this characteristic 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 every of numerous body charges starting from 23.97 to 60 frames per second (fps). When a member chooses to observe a film or a TV present on a supply gadget (ex. Set-top field, Streaming stick, Recreation Console, and many others…) 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 gadget converts it to the HDMI output body charge which was configured primarily based on the capabilities of the HDMI enter port of the related sink gadget (TV, AVR, Monitor and many others). Typically, the output body charge over HDMI is robotically set to 50fps for PAL areas and 60fps for NTSC areas.

Netflix gives restricted excessive body charge content material (50fps or 60fps), however the majority of our catalog and viewing hours might be attributed to members watching 23.97 to 30fps content material. This primarily implies that more often than not, our content material goes via a course of known as body charge conversion (aka FRC) on the supply gadget 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 simple. In truth, FRC works effectively 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 many others…). However, FRC introduces a visible artifact known as Judder when non-integer a number of conversion is required (ex. 24→60, 25→60, and many others…), which manifests as uneven video playback as illustrated under:

With Judder
With out Judder

It is very important observe that the severity of the judder is dependent upon 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):

  • Whole of fifty frames should be transmitted over HDMI per second
  • Supply gadget 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 3 times ( complete of 6 frames )
Determine 2: Instance of a 24 to 50fps body charge conversion algorithm

As a overview, judder is extra pronounced when the frequency of the variety of repeated frames is inconsistent and unfold out e.g. within the state of affairs talked about above, the body replication issue varies between 2 and three leading to a extra outstanding judder.

Now that we’ve a greater understanding of the problem, let’s overview the options that Netflix has invested in. Because of the fragmented nature of gadget capabilities within the ecosystem, we explored a number of options to handle this challenge for as many units as attainable. Every distinctive resolution leverages current or new supply gadget capabilities and comes with numerous tradeoffs.

The primary resolution we explored and just lately enabled leverages the aptitude of current supply & sink units to alter the outgoing body charge on the HDMI hyperlink. As soon as this characteristic 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 person intervention.

Whereas this appears like the right 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 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 length of the clean display screen varies relying on how briskly the supply and sink units can resynchronize. Determine 3 under is an instance of how this transition appears to be like:

Determine 3: Native body charge expertise with display screen blanking

Enhancements within the latest HDMI requirements (HDMI 2.1+) now permit a supply gadget to ship the video content material at its native body charge without having an HDMI resynchronization. That is attainable via an modern 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 gadget to keep up a continuing 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 holding the quantity of seen video knowledge fixed. Because of the fixed HDMI knowledge 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

Aside from the above HDMI specification dependent options, it’s attainable for an software like Netflix to govern the presentation time stamp worth of every video body to reduce the impact of judder i.e. the applying can current video frames to the underlying supply gadget platform at a cadence that may assist the supply gadget to reduce 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 state of affairs that was coated earlier. However, as a substitute of occupied with 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 gadget must get 72 frames from the streaming software in a interval of three seconds. Now as a substitute of sending 24 frames per second at a daily 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 gadget 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 under.

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 staff at Netflix. Be careful for an upcoming article going into additional particulars of this resolution.

Given the attainable options obtainable to make use of and the related advantages and limitations, the Netflix software operating on a supply gadget adapts to make use of one in every of these approaches primarily based on components corresponding to supply and sink gadget capabilities, person preferences and the particular use case inside the Netflix software. Let’s stroll via every of those facets briefly.

Each supply gadget that integrates the Netflix software is required to let the applying know if it and the related sink gadget have the flexibility to ship and obtain video content material at its native body charge. As well as, a supply gadget is required to tell whether or not it will probably 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 Charge” sometimes signifies {that a} supply gadget is able to this characteristic.

Even when a supply gadget and the related sink can assist Native content material body charge streaming (seamless or non-seamless), a person might need chosen not to do that by way of the supply gadget system settings e.g. “Match Content material Body Charge” 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 Charge” set to “Seamless”.

The Netflix software must know this person choice in an effort to honor their choice. Therefore, supply units are anticipated to relay this person choice to the Netflix software to assist with this run-time resolution making.

Despite supply gadget functionality and the person preferences collectively indicating that the Native Content material Body Charge streaming needs to be enabled, the Netflix software can determine to disable this characteristic for particular member experiences. For example, when the person is shopping Netflix content material within the dwelling 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 lead to 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 many others) would even be displayed on the decreased body charge and it will lead to a sluggish UI shopping expertise. It is because on HDMI output from the supply gadget, each graphics (Netflix software UI) and video parts will exit on the similar body charge (native content material body charge of the trailer) after being blended collectively on the supply gadget.

To deal with these points we observe an strategy as proven in Determine 6 under the place we allow the Native Body Charge playback expertise solely when the person selects a title and watches it in full display screen with minimal graphical UI components.

Determine 6: Native Body Charge utilization inside Netflix software

This text offered options that purpose to enhance the content material playback expertise on HDMI supply units. The breadth of accessible technical options, person selectable preferences, gadget capabilities and the applying of every of those permutations within the context of assorted in-app member journeys signify a typical engineering and product resolution framework at Netflix. Right here at Netflix, our purpose is to maximise immersion for our members via introduction of recent options that may enhance their viewing expertise and hold them totally engaged in our content material.

We want to acknowledge the exhausting work of quite a few groups that got here collectively to ship the options being mentioned on this doc. These embrace Core UI and JS Participant growth, Netflix Utility Software program growth, AV Check and Tooling (earlier article from this staff), 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 staff.

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