neroAacEnc.exe and 6ch encoding efficiency ... ? Options

[Sagittaire]

I have little ac3 6ch 448 Kbps. I make these convertions:
file wav 6 chanels 6ch.wav
file wav 2 chanels 2ch.wav (default matrix 6ch->2ch conversion for BS)
six files wav FR.wav, FL.wav, C.wav, LFE.wav, SR.wav and SL.wav

1) Convertion with neroAacEnc.exe
I use this command line for all wav file
neroAacEnc.exe -q 0.3 -lc -if input.wav -of output.mp4

and I find these size for each mp4 files
2ch done 2497 Ko and 114 Kbps
6h done 8764 Ko
FL done 1574 Ko
FR done 1569 Ko
C done 1567 Ko
LFE done 725 Ko
SL done 1481 Ko
SR done 1533 Ko

well it's really curious:
- same quality level for individual chanel done less bitrate than 6ch conversion (8.32 Mo vs 8.55 Mo)
- for same quality level bitrate for 6ch and 2ch sould be simple scalling something like between x2.0-x2.5 ... here it's 3.5


2) Convertion with FAAC.exe
I use this command line for all wav file
faac.exe input.wav -o output.mp4 --tns -q 100

and I find these size for each mp4 files
2ch done 2552 Ko and 116 Kbps
6h done 6559 Ko
FL done 1212 Ko
FR done 1211 Ko
C done 844 Ko
LFE done 2497 Ko
SL done 1233 Ko
SR done 1240 Ko

well here it's normal:
- same quality level for individual chanel done more bitrate than 6ch conversion (7.00 Mo vs 6.40 Mo)
- for same quality level bitrate for 6ch and 2ch sould be simple scaling something like between x2.0-x2.5 ... here it's 2.57

If quality level is the same for each encoding (1.0, 2.0, 5.1 ... etc) then bitrate for q 0.3 6ch file should be in 5000-6500 Ko interal. Chanel coupling seem work for 2.0 but not for 5.1. Bug for multichanel or different quality level ... ???

This post has been edited by Sagittaire: May 6 2006, 02:31

[Dzamburu]

-q 0.3 is almost 70kbs, and i faac q 100 is about 120kbs correct me if i wrong. 70 is to small for 5.1 encoding and 120 is is normal like you say. So for HE-AAC is only recommend to use 96-192 for LC 192-max. I think in that case and 2pass can't help with that small bitrates

This post has been edited by Dzamburu: May 6 2006, 01:05