HEVC (and MP4) audio bitrate over 192kBit/s?

FunkyB wrote on 9/13/2019, 8:43 AM

I recently purchased the HEVC codec plugin for MEP to be able to output high video quality with lower file sizes than MPEG-4. I've been using both, and was hoping someone could answer two questions:

1. I'd like to set the audio bitrate to 256 or 320, which is what my source video usually is. In the manual, on pg. 353, Audio Codec, AAC, it says "The bit rate can be set between 6 and 512 kbit/s." However, I only have 96, 128, 160 and 192 as options in the dropdown. (Exception: with "MPEG-4 (AVCHD)", the audio is set to AC3 which goes up to 384.) With "MPEG-4 (MP4)" and "HEVC", how can I set it over 192?

2. (Just out of curiosity; not an issue.) I only have limited experience with HEVC from using Handbrake and other software that uses libx265. In that codec, the quality setting is a "Constant Rate Factor" number, where between 18 and 22 is recommended. MEP's codec seems to take a different approach, where you specify the bitrate, etc.. Is there any correspondence between these systems, or are they two completely different flavors of HEVC that just work differently? Also, other software does let me set the AAC audio bitrate up to 512 with HEVC.

Comments

johnebaker wrote on 9/13/2019, 11:49 AM

@FunkyB

As you have not said which version of MEP you are using, I would assume you are reading from the MEP 2019 manual as this one is the only one I can find where:

. . . . Audio Codec, AAC, it says "The bit rate can be set between 6 and 512 kbit/s." . . . .

is on page 353.

However, there is one problem. that particular section is referring to MPEG-4 h.264 and MPEG-Simple - not HEVC.

The HEVC settings are on page 343/4 and gives no guidelines to the Bitrate settings available for audio.

A bitrate of 192 kbs for stereo audio is more than sufficient, any more the difference is audibly undetectable.

. . . . "MPEG-4 (AVCHD)", the audio is set to AC3 which goes up to 384 . . . .

That is because AVCHD supports 5.1 Surround Sound and needs the higher bitrates to handle the 6 channels.

. . . . the quality setting is a "Constant Rate Factor" number, where between 18 and 22 is recommended. MEP's codec seems to take a different approach . . . .

CRF and Bitrate are not that same thing, the Coding quality option determines the CRF values used.

HTH

John EB

Scenestealer wrote on 9/13/2019, 8:10 PM

. . . . "MPEG-4 (AVCHD)", the audio is set to AC3 which goes up to 384 . . . .

That is because AVCHD supports 5.1 Surround Sound and needs the higher bitrates to handle the 6 channels.

Yes, and also AAC is an Advanced Audio Codec which presumably is more efficient at lower bitrates, similar to HEVC.

FunkyB wrote on 9/15/2019, 1:50 AM

Thank you but that doesn't answer my question. The manual says that bitrates up to 512kbps can be set for AAC, so why aren't they showing up in the dropdown list? Or is there a way to manually enter a value? Do options above 192 show up for other people?

Yes, that page is under the h264 section, but those bitrates don't show up for me with h264 either. Also, as it states that h265 is the successor, which uses the same AAC audo, the options should be the same as for h264 unless stated otherwise.

What bitrate one considers optimal for a given video depends on a lot of factors, which is why there are settings in the first place. No other software I've seen, even freeware, caps audio at 192 for h264/265. This plugin cost extra on top of the base software so I'm not asking too much that an advertised feature be available. Maybe time to open a support ticket.

johnebaker wrote on 9/15/2019, 4:44 AM

@FunkyB

. . . . The manual says that bitrates up to 512kbps can be set for AAC, so why aren't they showing up in the dropdown list? . . . .

As I commented previously and hopefully this emphasis will make it clear:

that particular section is referring to MPEG-4 h.264 and MPEG-Simple - not HEVC.

The HEVC settings are on page 343/4 and gives no guidelines to the Bitrate settings available for audio.

A bitrate of 192 kbs for stereo audio is more than sufficient, any more the difference is audibly undetectable.

. . . . No other software I've seen, even freeware, caps audio at 192 for h264/265 . . . .

They do, however what they do is not at issue we are referring to MEP.

The aim you have set is . . . . . to output high video quality with lower file sizes . . . . which the preset export formats in MEP do. They are optimised for a combination of best video/audio quality with a file size small enough to achieve that quality.

Because both h.264 and h.265 use the same audio format that does not mean that 2 very different encoding formats should have the same Bitrate settings.

Additionally the MP4 file format is not relevant as it is a container format and can contain many different video and audio codec formats internally.

The h.265 is a very efficient codec for compression video and audio to lower bitrates - h.265 has a much lower video bitrate compared to h.264 for the same perceived quality, hence it's full name - High Efficiency Video Coding.

The different format export settings in MEP are optimised settings and increasing these is not recommended for 2 reasons:

it is very easy to make the finished video export quality worse
setting the bitrates too high can result in unplayable video on some devices

The final destination/viewing device also has to be taken into account - for example: if uploading to Youtube or other similar sites they will create various resolution/bitrate versions of the video to enable smooth streaming for the viewer depending on their Internet connection and these can go as low as 240p.

HTH

John EB