HE AACv2 Audio profile on MediaInfo

Question

I have a SDI input encoder with audio configuration as

Coding : AAC/HE-AAC
Coding Mode : MPEG-4 HE AAC v2
Mode : Stereo (2/0)
Sameple Rate : 48KHz
ES Bitrate : 64Kbps
Transport Format : ADTS

And, When I captured a encoder's output and put the file into MediaInfo, The MediaInfo shows like this.

Audio
ID                                       : 392 (0x188)
Menu ID                                  : 9 (0x9)
Format                                   : AAC
Format/Info                              : Advanced Audio Codec
Format version                           : Version 4
Format profile                           : HE-AACv2 / HE-AAC / LC
Muxing mode                              : ADTS
Duration                                 : 14 s 336 ms
Bit rate mode                            : Variable
Channel(s)                               : 2 channels / 1 channel / 1 channel
Channel positions                        : Front: L R / Front: C / Front: C
Sampling rate                            : 48.0 kHz / 48.0 kHz / 24.0 kHz
Frame rate                               : 23.438 FPS (1024 SPF)
Compression mode                         : Lossy
Delay relative to video                  : -696 ms
Language                                 : Korean

It looks like there are totally 4 channels in stream. And also contains HE-AACv2, HE-AAC, and LC. and different sampling rates.

I'm wondering about, Where are these 2 additional channels from? Is this normal result?

And Also, The Bit rate mode is shown as 'Variable'. not Constant. Is this okay?

Answer 1

The output may be misleading, right.

Such MediaInfo output means that there are legacy layers:

• a HE-AACv2 decoder sees 2Ch@48kHz
• a legacy HE-AAC decoder sees 1Ch@48kHz (and plays it fine, just without 1 of the channels)
• a legacy AAC decoder sees 1Ch@24kHz (and plays it fine, just without half of the frequency)

CBR AAC does not exist in reality (all frames have a specific size), but MediaInfo flags AAC as CBR is it with similar frame sizes (not the case here). AAC bit rate mode is not related to the "official" target from your encoder and/or the CBR underlying layer (e.g. an underlying layer may pad the stream in order to get it with constant bit rate in order to be transported in a thing needing constant bit rate)

Jérôme, developer of MediaInfo