I appear to be seeing a regression or limitation when converting 7.1 FLAC to Apple AAC with dBpoweramp Music Converter for macOS.
dBpoweramp version: Release 2026-09-13 (2)
Source file according to macOS afinfo:
Data format: 8 ch, 48000 Hz, flac, from 24-bit source
Channel layout: 7.1 (L R C LFE Rls Rrs Ls Rs)
I convert this using the Apple AAC encoder in dBpoweramp. The relevant settings are:
Target: Variable Bit Rate
VBR quality: 108 (~244 kbps estimated)
Quality/speed: Best quality (-q 127)
Format: AAC
SoundCheck: disabled
The resulting file is unexpectedly reduced to six channels. ffprobe reports:
codec_name=aac
profile=LC
sample_rate=48000
channels=6
channel_layout=5.1
bit_rate=445803
I initially had the Bit Depth (64 Bit Float) and Resample (48000 Hz) DSPs enabled. To rule those out, I repeated the conversion with all DSP effects removed. The result was still 6-channel 5.1 AAC.
I then tested the same source file directly with Apple's afconvert on the same Mac. When explicitly requesting 8-channel AAC_7_1_B, the Apple encoder successfully produces:
Data format: 8 ch, 48000 Hz, aac
Channel layout: 7.1 (C L R Ls Rs Rls Rrs LFE)
So the underlying Apple AAC/Core Audio encoder appears capable of encoding this source as 8-channel 7.1 AAC. The 8ch → 6ch reduction only occurs when using Apple AAC through dBpoweramp.
For reference, the successful Apple test was:
afconvert \
"03 - A Fragile Thing.flac" \
"/tmp/a-fragile-thing-apple-7.1.m4a" \
-f m4af \
-d 'aac '@48000 \
-c 8 \
-l AAC_7_1_B \
-s 3 \
-q 127 \
-b 512000
afinfo then confirms 8 channels and the 7.1 (C L R Ls Rs Rls Rrs LFE) layout.
I also remember successfully converting multichannel/7.1 material with dBpoweramp previously without it being reduced to 5.1, although I cannot currently verify which earlier release I was using.
Could you confirm whether Apple AAC output in the current macOS release is intentionally limited to 5.1, or whether this is a regression in the handling of 7.1 channel layouts?
I noticed that the recent release history mentions changes to Apple AAC ("Improved auto upsample logic"), so I thought it worth reporting in case the current behaviour is unintended.
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