Since 1.4.2, when playing FMP4 files, level switching occurred when the network condition was bad. but playback was not possible.

[kakao-wise-kim]

What version of Hls.js are you using?

1.4.2 to 1.4.10

What browser (including version) are you using?

chrome 116.0.5845.96

What OS (including version) are you using?

mac os 13.4.1

Test stream

If you need a link, please leave your email address.

Configuration

{
  "debug": true,
  "enableWorker": true,
  "lowLatencyMode": true,
  "backBufferLength": 90,
  "liveSyncDuration": 4
}

Additional player setup steps

I can see this in the hls.js demo, so there's nothing to set up.

Checklist

• The issue observed is not already reported by searching on Github under https://github.com/video-dev/hls.js/issues
• The issue occurs in the stable client (latest release) on https://hlsjs.video-dev.org/demo and not just on my page
• The issue occurs in the latest client (main branch) on https://hlsjs-dev.video-dev.org/demo and not just on my page
• The stream has correct Access-Control-Allow-Origin headers (CORS)
• There are no network errors such as 404s in the browser console when trying to play the stream

Steps to reproduce

I can reproduce the symptom when playing adaptive live videos with audio and video separated (m4s files) and multiple quality.

Step1 - Run the live video in a normal environment. Play for a sufficiently long time (enough time for a file of more than 9 segments to be downloaded and played).
Step2 - After a period of time, set a random delay on network. I usually use the Network Conditions option in Chrome browser.
Step3 - Limit the Network Condition until you encounter "Type: 'networkError', details: 'fragLoadError'" or "type: 'mediaError', details: 'bufferStalledError'" Buffer as an error. (Repeat the normal and limited settings, but don't do it too fast)

Expected behaviour

The video should switch to a lower quality and play.

What actually happened?

In this case, Video File tries to download a file in normal time, but Audio File tries to download a old time file.
As a result, playback stops, and if you try to retry playback, it doesn't work.
Therefore, the video buffer will continue to fill first. The Audio buffer is filled, but the time difference with the Video buffer is very large. Or, Audio buffer is not filled. Sometimes It try to download audio files from a long time ago. If the file doesn't exist due to a CDN TTL timeout, you'll get a 404.

Console output

If you give me your email address, I'll forward the log to a file.

Chrome media internals output

Timestamp	Property	Value
0:00:00	created	"2023-08-22 00:17:15.559 UTC"
0:00:00	origin_url	"https://hlsjs.video-dev.org/"
0:00:00	kFrameUrl	"https://hlsjs.video-dev.org/demo/?src=https%3A%2F%2Fsbox-live-orisa-noacl.play.kakao.com%2Fsld3phtgn4ufqwrt002vtlsxm%2Fmain%2Fadaptive.m3u8&demoConfig=eyJlbmFibGVTdHJlYW1pbmciOnRydWUsImF1dG9SZWNvdmVyRXJyb3IiOnRydWUsInN0b3BPblN0YWxsIjpmYWxzZSwiZHVtcGZNUDQiOmZhbHNlLCJsZXZlbENhcHBpbmciOi0xLCJsaW1pdE1ldHJpY3MiOi0xfQ=="
0:00:00	kFrameTitle	"hls.js demo"
0:00:00	url	"blob:https://hlsjs.video-dev.org/81f016ae-dcef-459c-a318-9289092bdc8e"
0:00:00	kTextTracks	[{"kind":"Metadata","label":"id3","language":""}]
0:00:00	info	"ChunkDemuxer"
0:00:00	kRendererName	"RendererImpl"
0:00:00	pipeline_state	"kStarting"
0:00:01	kVideoTracks	[{"alpha mode":"is_opaque","codec":"h264","coded size":"1280x720","color space":{"matrix":"BT709","primaries":"BT709","range":"LIMITED","transfer":"BT709"},"encryption scheme":"Unencrypted","has extra data":false,"hdr metadata":"unset","natural size":"1280x720","orientation":"0°","profile":"h264 high","visible rect":"0,0 1280x720"}]
0:00:01	kAudioTracks	[{"bytes per channel":4,"bytes per frame":8,"channel layout":"STEREO","channels":2,"codec":"aac","codec delay":0,"discard decoder delay":false,"encryption scheme":"Unencrypted","has extra data":false,"profile":"unknown","sample format":"Signed 32-bit","samples per second":44100,"seek preroll":"0us"}]
0:00:01	kIsAudioDecryptingDemuxerStream	FALSE
0:00:01	kAudioDecoderName	"FFmpegAudioDecoder"
0:00:01	kIsPlatformAudioDecoder	FALSE
0:00:01	info	"Selected FFmpegAudioDecoder for audio decoding, config: codec: aac, profile: unknown, bytes_per_channel: 4, channel_layout: STEREO, channels: 2, samples_per_second: 44100, sample_format: Signed 32-bit, bytes_per_frame: 8, seek_preroll: 0us, codec_delay: 0, has extra data: false, encryption scheme: Unencrypted, discard decoder delay: false, target_output_channel_layout: STEREO, target_output_sample_format: Unknown sample format, has aac extra data: true"
0:00:01	debug	"Video rendering in low delay mode."
0:00:01	info	"Cannot select DecryptingVideoDecoder for video decoding"
0:00:01	kIsVideoDecryptingDemuxerStream	FALSE
0:00:01	kVideoDecoderName	"VDAVideoDecoder"
0:00:01	kIsPlatformVideoDecoder	TRUE
0:00:01	info	"Selected VDAVideoDecoder for video decoding, config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [1280,720], visible rect: [0,0,1280,720], natural size: [1280,720], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:00:01	pipeline_state	"kPlaying"
0:00:01	seek_target	6.001399
0:00:01	pipeline_state	"kSeeking"
0:00:01	pipeline_state	"kPlaying"
0:00:01	dimensions	"1280x720"
0:00:01	kResolution	"1280x720"
0:00:01	info	"Effective playback rate changed from 0 to 1"
0:00:01	event	"kPlay"
0:00:01	duration	12
0:00:01	pipeline_buffering_state	{"for_suspended_start":false,"state":"BUFFERING_HAVE_ENOUGH"}
0:00:03	info	"video decoder config changed midstream, new config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [1920,1080], visible rect: [0,0,1920,1080], natural size: [1920,1080], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:00:03	kIsVideoDecryptingDemuxerStream	FALSE
0:00:03	kVideoDecoderName	"VDAVideoDecoder"
0:00:03	kIsPlatformVideoDecoder	TRUE
0:00:03	info	"Selected VDAVideoDecoder for video decoding, config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [1920,1080], visible rect: [0,0,1920,1080], natural size: [1920,1080], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:00:03	dimensions	"1920x1080"
0:00:03	kResolution	"1920x1080"
0:00:03	duration	14
0:00:05	duration	16
0:00:07	duration	18
0:00:09	duration	20
0:00:11	duration	22
0:00:13	duration	24
0:00:15	duration	26
0:00:17	duration	28
0:00:19	duration	30
0:00:21	duration	32
0:00:23	video_buffering_state	{"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:23	duration	34
0:00:25	info	"video decoder config changed midstream, new config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [1280,720], visible rect: [0,0,1280,720], natural size: [1280,720], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:00:25	kIsVideoDecryptingDemuxerStream	FALSE
0:00:25	kVideoDecoderName	"VDAVideoDecoder"
0:00:25	kIsPlatformVideoDecoder	TRUE
0:00:25	info	"Selected VDAVideoDecoder for video decoding, config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [1280,720], visible rect: [0,0,1280,720], natural size: [1280,720], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:00:25	dimensions	"1280x720"
0:00:25	kResolution	"1280x720"
0:00:25	duration	36
0:00:26	pipeline_buffering_state	{"for_suspended_start":false,"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:27	duration	38
0:00:29	duration	40
0:00:30	pipeline_buffering_state	{"for_suspended_start":false,"state":"BUFFERING_HAVE_ENOUGH"}
0:00:31	video_buffering_state	{"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:31	duration	42
0:00:33	duration	44
0:00:34	pipeline_buffering_state	{"for_suspended_start":false,"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:35	seek_target	42.0101
0:00:35	pipeline_state	"kSeeking"
0:00:35	duration	46
0:00:35	audio_buffering_state	{"state":"BUFFERING_HAVE_NOTHING"}
0:00:37	duration	48
0:00:39	duration	50
0:00:41	pipeline_state	"kPlaying"
0:00:41	duration	52
0:00:41	pipeline_buffering_state	{"for_suspended_start":false,"state":"BUFFERING_HAVE_ENOUGH"}
0:00:43	duration	54
0:00:43	video_buffering_state	{"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:45	duration	56
0:00:46	pipeline_buffering_state	{"for_suspended_start":false,"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:47	seek_target	54.000399
0:00:47	pipeline_state	"kSeeking"
0:00:47	duration	58
0:00:47	audio_buffering_state	{"state":"BUFFERING_HAVE_NOTHING"}
0:00:49	duration	60
0:00:51	duration	62
0:00:52	pipeline_state	"kPlaying"
0:00:52	pipeline_buffering_state	{"for_suspended_start":false,"state":"BUFFERING_HAVE_ENOUGH"}
0:00:53	duration	64
0:00:54	video_buffering_state	{"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:55	duration	66
0:00:57	pipeline_buffering_state	{"for_suspended_start":false,"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:00:57	duration	68
0:00:59	seek_target	66.0013
0:00:59	pipeline_state	"kSeeking"
0:00:59	duration	70
0:00:59	audio_buffering_state	{"state":"BUFFERING_HAVE_NOTHING"}
0:01:01	duration	72
0:01:03	duration	74
0:01:05	pipeline_state	"kPlaying"
0:01:05	debug	"Audio buffer splice at PTS=7987680us. Trimmed tail of overlapped buffer (PTS=7967708us) by 3248us."
0:01:05	duration	76
0:01:05	pipeline_buffering_state	{"for_suspended_start":false,"state":"BUFFERING_HAVE_ENOUGH"}
0:01:07	info	"video decoder config changed midstream, new config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [428,240], visible rect: [0,0,428,240], natural size: [428,241], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:01:07	kIsVideoDecryptingDemuxerStream	FALSE
0:01:07	kVideoDecoderName	"VDAVideoDecoder"
0:01:07	kIsPlatformVideoDecoder	TRUE
0:01:07	info	"Selected VDAVideoDecoder for video decoding, config: codec: h264, profile: h264 high, level: not available, alpha_mode: is_opaque, coded size: [428,240], visible rect: [0,0,428,240], natural size: [428,241], has extra data: false, encryption scheme: Unencrypted, rotation: 0°, flipped: 0, color space: {primaries:BT709, transfer:BT709, matrix:BT709, range:LIMITED}"
0:01:07	pipeline_buffering_state	{"for_suspended_start":false,"reason":"DEMUXER_UNDERFLOW","state":"BUFFERING_HAVE_NOTHING"}
0:01:07	duration	77.983333
0:01:09	seek_target	74.984732
0:01:09	pipeline_state	"kSeeking"
0:01:09	duration	79.983333
0:01:09	video_buffering_state	{"state":"BUFFERING_HAVE_NOTHING"}
0:01:11	duration	81.983332
0:01:11	duration	81.983333
0:01:13	duration	83.983332
0:01:13	duration	83.983333
0:01:15	seek_target	80.985133
0:01:15	pipeline_state	"kPlaying"
0:01:15	pipeline_state	"kSeeking"
0:01:15	duration	85.983332
0:01:15	duration	85.983333
0:01:17	duration	87.983332
0:01:17	duration	87.983333
0:01:19	duration	89.983332
0:01:19	duration	89.983333
0:01:21	seek_target	86.984633
0:01:21	pipeline_state	"kPlaying"
0:01:21	pipeline_state	"kSeeking"
0:01:21	duration	91.983332
0:01:21	duration	91.983333
0:01:23	duration	93.983332
0:01:24	duration	94
0:01:25	duration	95.983332
0:01:25	duration	95.983333
0:01:27	seek_target	92.984332
0:01:27	pipeline_state	"kPlaying"
0:01:27	pipeline_state	"kSeeking"
0:01:27	duration	97.983332
0:01:27	duration	97.983333
0:01:29	duration	99.983333
0:01:31	duration	101.983332
0:01:31	duration	101.983333
0:01:33	seek_target	98.983633
0:01:33	pipeline_state	"kPlaying"
0:01:33	pipeline_state	"kSeeking"
0:01:33	duration	103.983332
0:01:34	duration	104
0:01:35	duration	106
0:01:37	duration	108
0:01:39	seek_target	105.0013
0:01:39	pipeline_state	"kPlaying"
0:01:39	pipeline_state	"kSeeking"
0:01:39	duration	110
0:01:41	duration	112
0:01:43	duration	114
0:01:45	seek_target	111.0005
0:01:45	pipeline_state	"kPlaying"
0:01:45	pipeline_state	"kSeeking"
0:01:45	duration	116
0:01:47	duration	117.983332
0:01:48	duration	118
0:01:49	duration	120
0:01:51	seek_target	117.0003
0:01:51	pipeline_state	"kPlaying"
0:01:51	pipeline_state	"kSeeking"
0:01:51	duration	122
0:01:53	duration	124
0:01:55	duration	126
0:01:57	seek_target	123.001399
0:01:57	pipeline_state	"kPlaying"
0:01:57	pipeline_state	"kSeeking"
0:01:57	duration	128
0:01:59	duration	130
0:02:01	duration	132
0:02:03	seek_target	129.012899
0:02:03	pipeline_state	"kPlaying"
0:02:03	pipeline_state	"kSeeking"
0:02:03	duration	134
0:02:05	duration	136
0:02:07	duration	138
0:02:09	seek_target	135.000399
0:02:09	pipeline_state	"kPlaying"
0:02:09	pipeline_state	"kSeeking"
0:02:09	duration	140
0:02:11	duration	142
0:02:13	duration	144
0:02:15	seek_target	141.0013
0:02:15	pipeline_state	"kPlaying"
0:02:15	pipeline_state	"kSeeking"
0:02:15	duration	146
0:02:17	duration	148
0:02:19	duration	150
0:02:21	seek_target	147.001399
0:02:21	pipeline_state	"kPlaying"
0:02:21	pipeline_state	"kSeeking"
0:02:21	duration	152
0:02:23	duration	154
0:02:25	duration	156
0:02:27	seek_target	153.001399
0:02:27	pipeline_state	"kPlaying"
0:02:27	pipeline_state	"kSeeking"
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0:02:29	duration	160
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0:02:33	seek_target	159.0003
0:02:33	pipeline_state	"kPlaying"
0:02:33	pipeline_state	"kSeeking"
0:02:33	duration	164
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0:02:39	seek_target	165.0013
0:02:39	pipeline_state	"kPlaying"
0:02:39	pipeline_state	"kSeeking"
0:02:39	duration	170
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0:02:45	seek_target	171.016399
0:02:45	pipeline_state	"kPlaying"
0:02:45	pipeline_state	"kSeeking"
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0:02:51	pipeline_state	"kPlaying"
0:02:51	pipeline_state	"kSeeking"
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0:02:57	seek_target	183.0043
0:02:57	pipeline_state	"kPlaying"
0:02:57	pipeline_state	"kSeeking"
0:02:57	duration	188
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0:03:01	duration	192
0:03:03	seek_target	189.0015
0:03:03	pipeline_state	"kPlaying"
0:03:03	pipeline_state	"kSeeking"
0:03:03	duration	194
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0:03:09	seek_target	195.0013
0:03:09	pipeline_state	"kPlaying"
0:03:09	pipeline_state	"kSeeking"
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0:03:15	seek_target	201.000399
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0:03:15	duration	206
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[kakao-wise-kim]

The most recent main branch does not reproduce this behavior. There are a few PRs related to the audio buffer that have been addressed, but it looks like these will be released to the new 1.5 version.
Hopefully we'll see a release in the near future.

[robwalch]

Could this gave been fixed by #5742?

[kakao-wise-kim]

@robwalch

What version should I use to use the version with #5742 applied? This PR looks like it's supposed to apply to 1.5. Has it been applied to 1.4.11-Canary-9471 by any chance? I still have the same symptoms in 1.4.11-Canary.

The reason I ask is that I'm using hls.js from npm and combine it with external modules, so it's very easy to test im my test environments if you know the version.

[robwalch]

Hi @kakao-wise-kim,

Yes, dev and the latest canary has it:

https://0f22780e.hls-js-dev.pages.dev/demo/ (v1.4.11-0.canary.9471)

[kakao-wise-kim]

Hi @RobWatch

This version also made a similar action.
But It has not same symptoms with a version 1.4.2 to 1.4.10.
This version has try to download normal time audio segments when a network throttling situation.

But, video segments download action was not normal. It was try to download video segments what a high level. And, It was canceled. and seeked playback timeline. And try downed level video segment downloading again. But It also maybe high level on a throttle network states. When this process is repeated, playback stops and only video segments are downloaded in the background.

After playback stops, playback will not start even if you click the Play button. It have to refresh to get back to normal.

[robwalch]

What kind of throttling are you performing exactly?

[kakao-wise-kim]

@robwalch
Use the Network Condition tool that exists in chrome development tools.
The live stream consists of 1 audio and 5 video levels.
Most of the issues occur when throttling occurs sharply. For example, when I set the Network Condition from No Throttling to 750k.

[kakao-wise-kim]

@robwalch
This afternoon I ran more tests and saw the same symptoms as before.

From my analysis, the symptoms are as follows

• Buffering occurs.
• It download several video segments first.
• After the video segments have buffered a certain amount, I download the audio segments.
• At this point, it returns a 404 because there is no audio segment file left because it is live.
• The player takes a hang.

I increased the TTL of the CDN to avoid the 404, and here are the results of my tests.

• Buffering occurs.
• It download several video segments first.
• The audio download is successful.
• It plays.

The time of requested for audio download here varied from case to case: 10 seconds, 20 seconds, 30 seconds, over a minute, etc.

I have a question here.

• Why does it only download the video segment first, after a certain amount of buffering, even though it is live?
• Is it possible to set the number of video segments or time to download first?
• It only downloads video segments up to a minute or more. I'd like to limit the amount of time that only videos are downloaded when buffering etc. occurs, is there an option I can set?
• Version 1.4.1 downloads audio and video together, are you thinking of changing this to be the same?

[robwalch]

The most recent main branch does not reproduce this behavior. There are a few PRs related to the audio buffer that have been addressed, but it looks like these will be released to the new 1.5 version.
Hopefully we'll see a release in the near future.

v1.5.0 will be in beta in the coming weeks, with a stable release some time in September.

Why does it only download the video segment first, after a certain amount of buffering, even though it is live?

Audio segment loading is handled by the audio-stream-controller doTickIdle() method. Audio segments should be loaded along side video segments (from media `currentTime), but they should not be loaded more than one segment or target duration past the end the video buffer.

Is it possible to set the number of video segments or time to download first?
It only downloads video segments up to a minute or more. I'd like to limit the amount of time that only videos are downloaded when buffering etc. occurs, is there an option I can set?

HLS.js fills the main (video) forward buffer to at least maxBufferLength and idles only after maxMaxBufferLength or maxBufferSize has been reached (whichever comes first).

The audio buffer should follow the end of the video buffer +1 segment within the same discontinuity sequence.

Version 1.4.1 downloads audio and video together, are you thinking of changing this to be the same?

All versions should perform this way. It is still unclear why that is not the case with your live stream - but only for certain versions from your perspective. If this is fixed in dev, then the issue is resolved.

Looking at your screenshots, the issue is with your audio playlist not updating. If your audio playlist is behind, there is no way for the player to load segments it does not know about. This could be an issue with playlist requests timing out, bad target duration values, misaligned media timestamps, or something else - we can't tell without a sample.

I have tested 1.4.10 with live streams that I have access to and throttles down to 750kbps/100ms-rtt with the lowest level also being around 500-600kbps and I could not reproduce what you are describing. The player may have stalled for a couple of segments as the average causes level 1 to be selected (second lowest) before lowest, but audio and video were in lock step.

[kakao-wise-kim]

@robwalch

hello. I made a mistake when I explained about reproduction testing earlier.
The audio is not 1 level. It is 3 levels. Video consists of 5 levels.

I found repeated one symptom here.
is that if I use only one audio, I won't have the problem.
However, if I configure multiple audio levels, the problem is reproduced when switching.
This problem is also happening in the recently updated version 1.4.12.

This is our Master Playlist Sample.

#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="a1",NAME="a0",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="2",LANGUAGE="ko",URI="a0_a1/a1.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="a2",NAME="a0",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="2",LANGUAGE="ko",URI="a0_a2/a2.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="a3",NAME="a0",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="2",LANGUAGE="ko",URI="a0_a3/a3.m3u8"

#EXT-X-STREAM-INF:AVERAGE-BANDWIDTH=3192000,BANDWIDTH=6384000,CODECS="avc3.640020,mp4a.40.2",RESOLUTION=1280x720,FRAME-RATE=60.000,AUDIO="a1"
v0_v3/v3.m3u8
#EXT-X-STREAM-INF:AVERAGE-BANDWIDTH=8192000,BANDWIDTH=16384000,CODECS="avc3.64002a,mp4a.40.2",RESOLUTION=1920x1080,FRAME-RATE=60.000,AUDIO="a1"
v0_v1/v1.m3u8
#EXT-X-STREAM-INF:AVERAGE-BANDWIDTH=1128000,BANDWIDTH=2256000,CODECS="avc3.64001f,mp4a.40.2",RESOLUTION=854x480,FRAME-RATE=30.000,AUDIO="a2"
v0_v5/v5.m3u8
#EXT-X-STREAM-INF:AVERAGE-BANDWIDTH=628000,BANDWIDTH=1256000,CODECS="avc3.64001e,mp4a.40.2",RESOLUTION=640x360,FRAME-RATE=30.000,AUDIO="a2"
v0_v7/v7.m3u8
#EXT-X-STREAM-INF:AVERAGE-BANDWIDTH=264000,BANDWIDTH=528000,CODECS="avc3.640015,mp4a.40.2",RESOLUTION=428x240,FRAME-RATE=30.000,AUDIO="a3"
v0_v8/v8.m3u8

If a Buffer Stall error occurs while audio level switching is occurring, the switched audio pts are set olded time the video pts.
If the audio fragments continue to download and looks similar to the video pts, playback will start.

This symptom does not occur every time. Often they will switch normally.
The symptom is mainly reproduced when a buffer stall occurs with level switching, as shown below.