YTUHD

Unlocks 1440p (2K) and 2160p (4K) resolutions in iOS YouTube app.

Backstory

For a few years, YouTube had been testing 2K/4K resolutions on iOS as A/B (Alpha/Beta testing). The first group of users will see 2K/4K options while the others won't. There are certain prerequisites for those options to show:

1. Whether the iOS device support VP9 video decoding, which implies on Apple's end to be at least on iOS 14, and later YouTube says iOS 15 is the minimum requirement.
2. Whether YouTube decides on their end to include those options for that particular iOS device. The obviously slow devices are excluded.

YTUHD attempts to bypass those restrictions for all 64-bit devices running iOS 11 or higher.

VP9

Hardware accelerated VP9 decoder is technically added as of iOS 14 and YouTube has been utilizing it through a private entitlement com.apple.coremedia.allow-alternate-video-decoder-selection (alternatives: com.apple.coremedia.allow-specifying-alternate-video-decoder-selection and ``com.apple.developer.coremedia.allow-alternate-video-decoder-selection`) (All apps are equal is a lie). This decoder handles up to 4K, but only for A12 devices and later.

Those old devices don't get AppleAVD driver (/System/Library/Extensions/AppleAVD.kext) which is essential for VP9 decoding to work. The driver availability is checked inside /System/Library/VideoDecoders/AVD.videodecoder. Provided that you can extract a functional AVD.videodecoder binary from a dyld shared cache, you will still encounter the error AVDRegister - AppleAVDCheckPlatform() returned FALSE trying to load it.

Fortunately, YouTube app has a fallback to software decoding, which is not as efficient as hardware, but it works. It can be utilized by disabling server ABR, as can be seen from the code.

Update July 2025: Disabling server ABR is no longer necessary. See the section below.

AV1

YouTube introduced the AV1 codec around late 2018. This codec is also capable of 2K/4K resolutions. YTUHD also attempts to enable AV1 as seen in the code.

Starting from YouTube version 20.47.3, the software VP9 decoder (HAMVPXVideoDecoder) has been removed entirely. YTUHD reimplements this class as YTUHDVPXVideoDecoder using libvpx (statically linked), so VP9 continues to work on A11 and earlier devices with new YouTube versions. On A12 and later, the hardware VideoToolbox VP9 decoder is used instead. See NOTICES for the libvpx BSD-3-Clause license attribution.

Server ABR

If you look at the source code, there is an enforcement to not use server ABR. ABR stands for Adaptive BitRate. Its purpose is to fetch the available formats (resolutions) of a video.

It is unknown how YouTube exactly decides which formats to serve when the server ABR is enabled. YTUHD has no control over that and has to disable it and relies on the client code that reliably allows for 2K/4K formats. More specifically, it enables the VP9 software streaming filter so that those formats will not be filtered out.

Update July 2025: The author discovered a technique to enable 2K/4K formats without disabling server ABR. From the code, the idea is to force software decoder over hardware decoder only when a decoder is initialized by hooking SupportsCodec function (this function actually is unnamed in YouTube binary, but is labeled here for clarity) that toggles the return value. Specifically, SupportsCodec returns NO for AV1, and for VP9 when there is no hardware VP9 decoder support (A11 and earlier on new YouTube, or when the native libvpx-backed HAMVPXVideoDecoder handles it on older YouTube). In all other cases it returns YES.

iOS version

The history has shaped YTUHD to spoof the device as iOS 15 (or higher) for those running lower. The user agent gets changed from spoofing for YouTube server to respond with VP9 formats and all the goodies.

Sideloading

Normally when an app is sideloaded, the private entitlements get removed (including com.apple.coremedia.allow-alternate-video-decoder-selection) and the app won't be allowed to access the hardware VP9 decoder. As for sideloaded YouTube, you will end up with only the software VP9 decoder, which can drain battery significantly. There is no known solution to bypass this, unless you can use TrollStore on your device to install the sideloaded YouTube IPA. TrollStore preserves the entitlements of the app.

Update July 2025: libundirect is now used by YTUHD. If you want to sideload YouTube IPA with YTUHD that uses libundirect, the simplest solution is to add SIDELOAD=1 when building the package with Theos.

AV1 Software Decoding

For old YouTube versions (< 20) that have no built-in AV1 software decoder, YTUHD now reimplements AV1 decoding as YTUHDDav1dVideoDecoder using dav1d (statically linked). This enables AV1 playback on devices without hardware AV1 support (A16 and earlier). See NOTICES for the dav1d BSD-2-Clause license attribution.

Building

Building YTUHD requires Theos. The VP9 software decoder (YTUHDVPXVideoDecoder) and AV1 software decoder (YTUHDDav1dVideoDecoder) statically link libvpx and dav1d respectively, which are built automatically on first make.

Building dav1d requires meson and ninja. Install them via Homebrew before building:

brew install meson ninja

Then build as usual.

To rebuild the static libraries explicitly:

make libvpx   # rebuilds vendor/libvpx_ios/libvpx.a
make dav1d    # rebuilds vendor/dav1d_ios/libdav1d.a