我們打算容許 iOS 裝置播放 4K 的錄製內容，本來已經在 QA 環境設定好 VOS 的編碼格式並在 iPhone + iOS14 下測試通過，不過最近 QA 發現 iPhone 在連接 HDMI 時播放 4K 頻道或錄製內容都會顯示播放失敗。

該錯誤大致如下：

Optional(Error Domain=AVFoundationErrorDomain Code=-11848 "Cannot Open" UserInfo={NSUnderlyingError=0x156d78f30 {Error Domain=NSOSStatusErrorDomain Code=-12925 "(null)"}, NSLocalizedFailureReason=The media cannot be used on this device., NSLocalizedDescription=Cannot Open})

雖然錯誤訊息指明裝置無法播放該媒體，卻沒有指出是哪一個原因導致無法播放。從網上的搜尋結果沒有得到任何有用的資料，然而我在調查系統記錄時發現一些決定性的資訊。

無論是在 iOS、MacOS 甚至是 Safari 播放 HLS 串流時，系統都會透過 mediaserverd 進行解碼，而 -12925 的錯誤也是由 mediaserverd 所拋出的：

mediaserverd <SEGPUMP> segPumpSetCurrentAlternate: 0x1050ec000 0x1050ec000: Attempting SwitchToAlternate bw 896000, at time nan, duration nan 1
mediaserverd <SEGPUMP> segPumpCreateHTTPRequest: Byte pump 0x1050ec020 created new request 0x10a627280 for session ref 0x10a640040
mediaserverd <SEGPUMP> segPumpSendIndexFileRequest: 0x1050ec000:1 session 0x10a640040 index req []
mediaserverd <SEGPUMP> segPumpSetCurrentAlternate: starting audio group on stream index 1, switching from stream index 0, isTrial=0
mediaserverd <<<< FigStreamPlayer >>>> fpfs_ReportCriticalThroughFigLog: [QE Critical]fpfs_ReportVariantSwitchStart: [0x10d50e370]: <0x10411d000>: (11): starting switch up from [<FigAlternate:[0x0]>] to [<FigAlternate:[0x108ea4a80] [Peak 896000] [640x360] [AudioGroup audio1] [hvc1.1.6.L90.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 25.000]>]
mediaserverd <<<< FigStreamPlayer >>>> fpfs_SetAlternateWithContext: <0x10411d000|I/VBQ.01> substream 1 will change gear
mediaserverd <<<< FigStreamPlayer >>>> fpfs_PostNotificationFromDispatch: posting AlternateStreamChanged on 0x10411d000 (0x10a644820)
mediaserverd <<<< FigStreamPlayer >>>> fpfs_StopPlayingItem: [0x10d50e370|P/IP] <0x10411d000|I/VBQ.01> Pausing, err=-12925
mediaserverd <<<< FigStreamPlayer >>>> fpfs_haltStream: item <0x10411d000|I/VBQ.01> pump 0x1050ec000
mediaserverd <<<< FigStreamPlayer >>>> fpfs_haltStream: item <0x10411d000|I/VBQ.01> pump 0x0
mediaserverd <<<< FigStreamPlayer >>>> fpfs_ResetAudioHardwareFormat: [0x10d50e370|P/IP] No more audio rendering, set preferred channel count back to stereo

不過這裡並未看出任何主要線索，只能知道是開啟第一個 alternate 時就已經出現錯誤而停止播放。但是在這段記錯前，原來有一段標示為 HLS-FASB 的記錄清楚說明了系統播放器會套用的篩選器、播放串流的 alternate 以及套用篩選器的順序及結果。以下是在 iPhone 中播放 4K 頻道 (正常播放) 時的 FASB 篩選器記錄：

mediaserverd    <<HLS-FASB>> fasb_log: [0x10a70ade0:I/VNB.01]
<FigAlternateSelectionBoss:0x10a70ade0 [filterCount 13] [alternateCount 4] [filteredAlternateCount 3] [mediaSelectionArrayCount 2]>
   Monitors:
   {
     [FigAlternateFilterMonitorForHDCP currentMaxProtectedHDCPLevel:-1 currentEPM:(null)]
     [FigAlternateFilterMonitorForNotification<0x10a7335d0> "DisplayVideoRangeChanged" state:5 currentFilter:[FigAlternateFilter <PreferredVideoFormat:0x108ec8380> priority:800]]
     [FigAlternateFilterMonitorForNotification<0x1095fda70> "DisplayVideoRangeChanged" state:1 currentFilter:[FigAlternateFilter <FrameRateBucketCap:0x109513200> priority:700]]
     [FigAlternateFilterMonitorForNotification<0x108e941c0> "DisplayVideoRangeChanged" state:5 currentFilter:[FigAlternateFilter <SupportedVideoRange:0x10a729640> priority:1000]]
     [FigAlternateFilterMonitorForNotification<0x109582400> "PowerStateChanged" state:0 currentFilter:(null)]
   }
   Filters:
   {
mediaserverd    <<HLS-FASB>> fasb_log: [0x10a70ade0:I/VNB.01]
       4 ->  4: [FigSimpleAlternateFilter <SuppressVP9:0x10a42b1f0> priority:1100]
       4 ->  4: [FigSimpleAlternateFilter <SupportedVideoRange:0x10a729640> priority:1000 mode:[Internal (HDR OK)]]
       4 ->  4: [FigSimpleAlternateFilter <SupportedAudioFormat:0x10a427cf0> priority:1000 ac3IsDecodable:YES atmosIsDecodable:YES ec3IsDecodable:YES, ac3CanPassthrough:NO]
       4 ->  4: [FigSimpleAlternateFilter <MediaValidation:0x108ff8b30> priority:1000 allowUnknownCodecs:NO]
       4 ->  4: [FigSimpleAlternateFilter <AllowedCPC:0x10a45bf40> priority:1000 systemCPC:0xffffffffffffffff]
       4 ->  4: [FigMediaSelectionAudibleAlternateFilter <MediaSelectionAudible: 0x10a74dbf0> priority: 950 persistantIDs: 0]
       4 ->  4: [FigSimpleAlternateFilter <ScanModePreference:0x10a413ed0> priority:820 contiguous]
       4 ->  4: [FigHDCPAlternateFilter <HDCP: 0x10a734b20> priority: 810 hdcp0:Unknown hdcp1:Unknown]
       4 ->  4: [FigSimpleAlternateFilter <PreferredVideoFormat:0x108ec8380> priority:800 preferredRange:PQ preferredFormat:Maximum]
       4 ->  4: [CombinedAudioPreferenceFilter <CombinedAudioPreference: 0x10a71bba0> priority:750 channels:16 sampleRateConstraints:none preferLossyEncodings=NO preferredAudioFormat:Unknown ac3CanPassthrough:NO permitSpatialization=YES spatialAudioResolutionCutoffSize:1280.000000X720.000000 contentType=moov spatialSources=mlti]
       4 ->  4: [FigSimpleAlternateFilter <PreferBestFormatForVideoRange:0x108eaf060> priority:700 videoRange:SDR bestFormat:HEVC]
       4 ->  3: [FigSimpleAlternateFilter <DisplaySize:0x108ee7fc0> priority:700 displaySize:[2048x1152]]
       3 ->  3: [FigSimpleAlternateFilter <FrameRateBucketCap:0x109513200> priority:700 framerateBucketCap:60fpsBucket]
   }
   Alternates:
   {
     <FigAlternate:[0x103736550] [Peak 896000] [640x360] [AudioGroup audio1] [hvc1.1.6.L90.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 25.000]>
     <FigAlternate:[0x109559bf0] [Peak 2096000] [1280x720] [AudioGroup audio1] [hvc1.1.6.L120.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
     <FigAlternate:[0x109562fe0] [Peak 5096000] [1920x1080] [AudioGroup audio1] [hvc1.1.6.L123.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
     <FigAlternate:[0x109588640] [Peak 15096000] [3840x2160] [AudioGroup audio1] [hvc1.1.6.L153.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
   }
   Filtered Alternates:
   {
     <FigAlternate:[0x103736550] [Peak 896000] [640x360] [AudioGroup audio1] [hvc1.1.6.L90.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 25.000]>
     <FigAlternate:[0x109559bf0] [Peak 2096000] [1280x720] [AudioGroup audio1] [hvc1.1.6.L120.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
     <FigAlternate:[0x109562fe0] [Peak 5096000] [1920x1080] [AudioGroup audio1] [hvc1.1.6.L123.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
   }
LOG COMPLETE

在記錄中可以看到，串流中共有 4 個 alternate，而篩選器 11 至 13 分別是指系統最高可接受 SDR 的 HEVC、不多於 2048x1152 的解析度及不高於 60fps 的幀率，所以篩選結果是 4K 解析度的那個 alternate 因為超出 2048x1152 的解析度而被篩選掉，但其餘三個 alternate 仍能播放，只是最高只能顯示 1080p 的解析度而已。

現在讓我們來看看連接 HDMI 後的 HLS-FASB 記錄：

mediaserverd    <<HLS-FASB>> fasb_log: [0x108e902d0:I/VBQ.01]
<FigAlternateSelectionBoss:0x108e902d0 [filterCount 14] [alternateCount 4] [filteredAlternateCount 0] [mediaSelectionArrayCount 2]>
   Monitors:
   {
     [FigAlternateFilterMonitorForHDCP currentMaxProtectedHDCPLevel:-1 currentEPM:(null)]
     [FigAlternateFilterMonitorForNotification<0x10a6ef960> "DisplayVideoRangeChanged" state:5 currentFilter:[FigAlternateFilter <PreferredVideoFormat:0x10a6dcd50> priority:800]]
     [FigAlternateFilterMonitorForNotification<0x108fca140> "DisplayVideoRangeChanged" state:1 currentFilter:[FigAlternateFilter <FrameRateBucketCap:0x10a67e600> priority:700]]
     [FigAlternateFilterMonitorForNotification<0x10a6efd60> "DisplayVideoRangeChanged" state:5 currentFilter:[FigAlternateFilter <SupportedVideoRange:0x10a63fbd0> priority:1000]]
     [FigAlternateFilterMonitorForNotification<0x10a67e8e0> "PowerStateChanged" state:0 currentFilter:(null)]
   }
   Filters:
   {
       4 ->  4: [FigSimpleAlternateFilter <SuppressVP9:0x1095a5660> priority:1100]
       4 ->  4: [FigSimpleAlternateFilter <SupportedVideoRange:0x10a63fbd0> priority:1000 mode:[Internal (HDR OK)]]
       4 ->  0: [FigSimpleAlternateFilter <NeroSupportedVideoFormat:0x10a650c80> priority:1000 preferredRange:SDR preferredFormat:AVC]
       0 ->  0: [FigSimpleAlternateFilter <SupportedAudioFormat:0x109521590> priority:1000 ac3IsDecodable:YES atmosIsDecodable:NO ec3IsDecodable:YES, ac3CanPassthrough:NO]
       0 ->  0: [FigSimpleAlternateFilter <MediaValidation:0x109505220> priority:1000 allowUnknownCodecs:NO]
       0 ->  0: [FigSimpleAlternateFilter <AllowedCPC:0x109586db0> priority:1000 systemCPC:0xffffffffffffffff]
       0 ->  0: [FigMediaSelectionAudibleAlternateFilter <MediaSelectionAudible: 0x10a67d420> priority: 950 persistantIDs: 0]
       0 ->  0: [FigSimpleAlternateFilter <ScanModePreference:0x10a628e90> priority:820 contiguous]
       0 ->  0: [FigHDCPAlternateFilter <HDCP: 0x10a6ef8d0> priority: 810 hdcp0:Unknown hdcp1:Unknown]
       0 ->  0: [FigSimpleAlternateFilter <PreferredVideoFormat:0x10a6dcd50> priority:800 preferredRange:PQ preferredFormat:Maximum]
       0 ->  0: [CombinedAudioPreferenceFilter <CombinedAudioPreference: 0x10a67e1b0> priority:750 channels:2 sampleRateConstraints:none preferLossyEncodings=NO preferredAudioFormat:Unknown ac3CanPassthrough:NO permitSpatialization=NO spatialAudioResolutionCutoffSize:1280.000000X720.000000 contentType=moov spatialSources=none]
       0 ->  0: [FigSimpleAlternateFilter <PreferBestFormatForVideoRange:0x10a624a80> priority:700 videoRange:SDR bestFormat:Unknown]
       0 ->  0: [FigSimpleAlternateFilter <DisplaySize:0x10a640100> priority:700 displaySize:[1920x1080]]
       0 ->  0: [FigSimpleAlternateFilter <FrameRateBucketCap:0x10a67e600> priority:700 framerateBucketCap:60fpsBucket]
   }
   Alternates:
   {
     <FigAlternate:[0x108ea4a80] [Peak 896000] [640x360] [AudioGroup audio1] [hvc1.1.6.L90.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 25.000]>
     <FigAlternate:[0x1095aafb0] [Peak 2096000] [1280x720] [AudioGroup audio1] [hvc1.1.6.L120.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
     <FigAlternate:[0x109559bf0] [Peak 5096000] [1920x1080] [AudioGroup audio1] [hvc1.1.6.L123.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
     <FigAlternate:[0x109588640] [Peak 15096000] [3840x2160] [AudioGroup audio1] [hvc1.1.6.L153.B0,mp4a.40.2] [VideoRange SDR] [FrameRate 50.000]>
   }
   Filtered Alternates:
   {
   }
LOG COMPLETE

從這次的篩選記錄可以看到，系統現在支援的格式只有 SDR 格式的 AVC，最高 1920x1080 解析度及最高 60fps 的幀率，而由於所有 alternate 均是 HEVC 編碼，所以所有 alternate 都在經過第 3 個篩選器時被篩選掉了，以致最後沒有可以播放的 alternate 而導致 -12925 的錯誤，也就是說 iOS 支援 HEVC 解碼只限使用內建輸出，當連接 HDMI 時是不支援 HEVC 解碼的。