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Identifying the best AAC bitrate version of a YouTube title

2 posts in this topic

Camera encoding noticeably poorer than YouTube encoding at same nominal bitrate

I was recently puzzled by the fact that subjectively the sound quality I could get from a camera recording made with the AAC sound codec at a nominal bitrate of 128kbps approx was not as good as the sound quality  from a YouTube file streaming with the same audio codec and at the same nominal audio bitrate. (To test the camera, a high quality stereo audio signal was fed to the external microphone socket, at an appropriate level, with the camera's automatic gain control disabled.)

I discovered from examining audio bitrate variations as the files were playing that my Panasonic Lumix DMC-G7 camera keeps the recording audio bitrate within a fairly narrow range whereas YouTube videos involve higher peaks in the audio bitrate. (The camera needs to process on the fly. Youtube encoding is an off-line process.) I felt the disparity had been explained.

 

YouTube practices regarding AAC audio bitrate

In my internet searches about YouTube bitrates I'd noticed that although these days the AAC audio stream for a 720p YouTube video is generally streamed at a nominal 128kbps there have been differences of opinion regarding what bitrates have been used in the last year or so, and also differences of opinion regarding the subjective quality of the AAC audio. See for example this thread on another forum:

 

 and this more recent thread:  https://www.reddit.com/r/youtube/comments/6a8a87/request_to_the_dev_team_on_youtube_we_can_tell/

 

Test upload [only in draft form, and unlisted]

In the last week I decided to upload my own very brief 1080p video file to YouTube, with a high audio quality sound stream (44.1kHz, 24-bit, LPCM), to see what YouTube would do with it. I found the results were quite interesting. For the highest resolution video version (1080p), YouTube compressed the sound to a nominal 128kbps AAC stream. However for a lesser video quality version (720p), YouTube compressed the sound to a nominal 192kbps AAC stream.

For a lot of music, a nominal 128kbps YouTube stream may sound reasonable enough. However I had chosen a demanding sound track, the FLAC version of a professional closely miked recording of a string quartet. The 192kbps version of this demanding music was definitely better sounding for my ears than the 128kbps version, and the video quality at 720p was quite adequate.

 

Firefox add-on

I've found a Firefox Browser add-on that allows the available audio codecs and nominal bitrates of a YouTube title to be examined. This could be used to help identify the best audio stream. Here is a link to the add-on: https://addons.mozilla.org/en-us/firefox/addon/youtube-video-and-audio-dow/

From some spot checking I've done, and as a general statement, it would appear that if a 192kbps AAC audio stream is available, it is usually associated with a 720p video stream, not a 1080p video stream.

 

 

 

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I've been searching for a convenient YouTube downloader that lists the audio codec and bitrate. This one looked useful: Gihosoft TubeGet (downloadable from  https://www.gihosoft.com/free-youtube-downloader.html?from=prod).   Use of this downloader confirms that it does indeed seem to be the case that the 720p version of a YouTube is typically the one labelled as having a 192kbps AAC stream (rather than a 128kbps AAC stream).

Regrettably, I have found that a YouTube file version labelled by TubeGet or other tools as having 192kbps AAC doesn't necessarily have such a stream when downloaded. This may be what was being referred to in the first link in my opening post where one of the contributors stated:
 

[–]willb3d 1 point 1 year ago 

Yes. Regardless of what tools you may use to examine current YouTube quality, you will find that as of early 2016 the M4A audio component in all MP4 videos has been reduced back down to 128kbps (with the 16kHz cutoff of high frequencies) - as it was circa 2012 or earlier. It is a marked regression in quality.

However, the quality of the Vorbis audio component in the WebM versions of the very same videos are at 192kbps (or possibly higher - only 192kbps has been confirmed, but at least that brings the cutoff up to 19kHz). There is a noticeable difference in fidelity.

 

The draft 1080p video file I uploaded to YouTube myself  recently currently appears privately on YouTube with a 720p video version and an associated audio stream that an analysis tool will show labelled as being 192kbps  AAC and by downloading I can confirm the audio is indeed at that approximate average  bitrate.

Something fishy is going on here. More investigation needed!

This is an important issue for me as I find that for listening to classical music a 192kbps AAC stream can be a distinct improvement over a 128kbps AAC stream. 

Example

The AAC sound for the 720p version of the YouTube video below doesn't sound too bad to my ears. However despite being labelled by TubeGet and other tools as 192kbps AAC, playing the downloaded file with Cyberlink PowerDVD will reveal that the AAC stream peaks at about 160kbps and at times is around 110kbps.  It is not 192kbps AAC:-

 

 

 

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