April 13, 2024

by Akshay Garg, Roger Quero

Maximizing immersion for our members is a vital objective for the Netflix product and engineering groups to maintain our members entertained and absolutely engaged in our content material. Leveraging a very good mixture of mature and cutting-edge shopper machine applied sciences to ship a easy playback expertise with glitch-free in-app transitions is a vital step in direction of reaching this objective. 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 gadgets.

You probably have a streaming machine linked to your TV, similar to 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 price. Machine producers usually name this function “Match Content material Body Fee”, “Auto modify show refresh price” or one thing related. For those who’ve ever questioned what these options are and the way they will enhance your viewing expertise, hold studying — the next sections cowl the fundamentals of this function and clarify the main points of how the Netflix utility makes use of it.

Netflix’s content material catalog consists of video captured and encoded in certainly one 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 on…) the content material is delivered after which decoded at its native body price, which is the body price it was captured and encoded in. After the decode step, the supply machine converts it to the HDMI output body price which was configured primarily based on the capabilities of the HDMI enter port of the linked sink machine (TV, AVR, Monitor and so on). Usually, the output body price over HDMI is robotically set to 50fps for PAL areas and 60fps for NTSC areas.

Netflix provides restricted excessive body price 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 primarily signifies that more often than not, our content material goes by means of a course of referred to as body price conversion (aka FRC) on the supply machine which converts the content material from its native body price to match the HDMI output body price 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 price sounds logical and easy. In reality, FRC works effectively when the output body price is an integer a number of of the native body price ( ex. 24→48, 25→50, 30→60, 24→120, and so on…). 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 on…), which manifests as uneven video playback as illustrated under:

With Judder
With out Judder

It is very important be aware that the severity of the judder is dependent upon the replication sample. Because of this, 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 should 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 unique frames are derived as follows:
  • 22 frames are duplicated ( complete of 44 frames )
  • 2 frames are repeated thrice ( complete of 6 frames )
Determine 2: Instance of a 24 to 50fps body price conversion algorithm

As a assessment, 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 have now a greater understanding of the difficulty, let’s assessment 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 deal with this problem for as many gadgets as attainable. Every distinctive answer leverages present or new supply machine capabilities and comes with numerous tradeoffs.

The primary answer we explored and not too long ago enabled leverages the potential of present supply & sink gadgets to vary the outgoing body price on the HDMI hyperlink. As soon as this function is enabled within the system settings, gadgets will match the HDMI output body price with the content material body price, both precisely or an integer a number of, with out consumer intervention.

Whereas this appears like the proper answer, gadgets that assist older HDMI applied sciences e.g. HDMI v<2.1, can’t change the body price with out additionally altering the HDMI knowledge price. This ends in what is usually referred as an “HDMI bonk” which causes the TV to show a clean display momentarily. Not solely is that this a disruptive expertise for members, however the length of the clean display varies relying on how briskly the supply and sink gadgets can resynchronize. Determine 3 under is an instance of how this transition seems:

Determine 3: Native body price expertise with display blanking

Enhancements within the current HDMI requirements (HDMI 2.1+) now enable a supply machine to ship the video content material at its native body price with no need an HDMI resynchronization. That is attainable by means 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 keep up a relentless knowledge price 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 knowledge whereas conserving the quantity of seen video knowledge fixed. As a result of fixed HDMI knowledge price, 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 perfect answer to deal with the issue we’re presenting. Sadly, at current, this know-how is comparatively new and adoption into supply and sink gadgets will take time.

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

Aside from the above HDMI specification dependent options, it’s attainable for an utility like Netflix to control the presentation time stamp worth of every video body to reduce 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 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 lined earlier. However, as a substitute of interested by the FRC price per second (24 ⇒ 50 fps), let’s broaden the FRC calculation time interval to three seconds (24*3 = 72 ⇒50*3 = 150 fps). For content material with a local body price of 24 fps, the supply machine must get 72 frames from the streaming utility in a interval of three seconds. Now as a substitute of sending 24 frames per second at an everyday per second cadence, for every 3 second interval the Netflix utility 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 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 under.

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

NOTE: This answer was developed by David Zheng within the Associate Expertise Expertise group at Netflix. Be careful for an upcoming article going into additional particulars of this answer.

Given the attainable options out there to make use of and the related advantages and limitations, the Netflix utility operating on a supply machine adapts to make use of certainly one of these approaches primarily based on components similar to supply and sink machine capabilities, consumer preferences and the particular use case throughout the Netflix utility. Let’s stroll by means of every of those facets briefly.

Each supply machine that integrates the Netflix utility is required to let the appliance know if it and the linked sink machine have the power to ship and obtain video content material at its native body price. As well as, a supply machine 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 price on the linked HDMI hyperlink.

As mentioned within the introduction part, the presence of a system setting like “Match Content material Body Fee” usually signifies {that a} supply machine is able to this function.

Even when a supply machine and the linked sink can assist Native content material body price 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 Fee” set to “By no means”. Or they may have indicated a choice of doing this solely when the native content material body price play begin can occur in a seamless method e.g. “Match Content material Body Fee” set to “Seamless”.

The Netflix utility must know this consumer choice with a purpose to honor their choice. Therefore, supply gadgets are anticipated to relay this consumer choice to the Netflix utility to assist with this run-time resolution making.

Despite supply machine functionality and the consumer preferences collectively indicating that the Native Content material Body Fee streaming ought to be enabled, the Netflix utility can determine to disable this function for particular member experiences. For instance, when the consumer is looking Netflix content material within the house UI, we can not play Netflix trailers of their Native body price as a result of following causes:

  • If utilizing Answer # 1, when the Netflix trailers are encoded in various content material body charges, switching between trailers will lead to display blanking, thereby making the UI looking unusable.
  • If utilizing Answer # 2, sending Netflix trailers of their Native body price would imply that the related UI elements (motion of cursor, asset choice and so on) would even be displayed on the diminished body price and it will lead to a sluggish UI looking expertise. It’s because on HDMI output from the supply machine, each graphics (Netflix utility UI) and video elements will exit on the identical body price (native content material body price of the trailer) after being blended collectively on the supply machine.

To deal with these points we comply with an method as proven in Determine 6 under the place we allow the Native Body Fee playback expertise solely when the consumer selects a title and watches it in full display with minimal graphical UI parts.

Determine 6: Native Body Fee utilization inside Netflix utility

This text introduced options that intention 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 varied in-app member journeys signify a typical engineering and product resolution framework at Netflix. Right here at Netflix, our objective is to maximise immersion for our members by means of introduction of recent options that can enhance their viewing expertise and hold them absolutely engaged in our content material.

We wish to acknowledge the laborious work of quite a lot 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 group), Associate Engineering and Product groups within the Consumer Engineering group and our knowledge science pals within the Data Science and Engineering group at Netflix. Diagrams on this article are courtesy of our Associate Enterprise Platform XD group.