AAC only allows 48Kb/s encoding

[chodgetts]

Hello,
I am not sure if this an issue with Glasscoder or my AAC encoding setup, or something else, but I seem to be unable to get GlassGUI to allow more than 48Kb/s when using AAC as the encoding option.

I have tried various AAC encoders - but the dropdown menu only allows for the maximum of 48Kb/s

Just wondering if I am doing something wrong, or if that is all its able to achieve?

Thanks in advance.

[ElvishArtisan]

We max out at 48 kbps for HE-AAC because that is really the highest bit-rate that makes sense for that audio object type (using the '1 bit per audio sample' rubric discussed below). The full rationale is rather complex and involves some knowledge of the internal encoder processes, but bottom line, Fraunhofer recommends AAC-LC instead of HE-AAC for those higher bit-rates.

From the Fraunhofer encoder docs:

2.11 Supported Bitrates

The FDK AAC Encoder provides a wide range of supported bitrates. The minimum and maximum
allowed bitrate depends on the Audio Object Type. For AAC-LC the minimum bitrate is the bitrate
that is required to write the most basic and minimal valid bitstream. It consists of the bitstream
format header information and other static/mandatory information within the AAC payload. The
maximum AAC framesize allowed by the MPEG-4 standard determines the maximum allowed
bitrate for AAC-LC. For HE-AAC and HE-AAC v2 a library internal look-up table is used.
A good working point in terms of audio quality, sampling rate and bitrate, is at 1 to 1.5 bits/au-
dio sample for AAC-LC, 0.625 bits/audio sample for dualrate HE-AAC, 1.125 bits/audio sample for
downsampled HE-AAC and 0.5 bits/audio sample for HE-AAC v2. For example for one channel
with a sampling frequency of 48 kHz, the range from 48 kbit/s to 72 kbit/s achieves reasonable
audio quality for AAC-LC.
For HE-AAC and HE-AAC v2 the lowest possible audio input sampling frequency is 16 kHz be-
cause then the AAC-LC core encoder operates in dual rate mode at its lowest possible sampling
frequency, which is 8 kHz. HE-AAC v2 requires stereo input audio data.
Please note that in HE-AAC or HE-AAC v2 mode the encoder supports much higher bitrates
than are appropriate for HE-AAC or HE-AAC v2. For example, at a bitrate of more than 64 kbit/s
for a stereo audio signal at 44.1 kHz it usually makes sense to use AAC-LC, which will produce
better audio quality at that bitrate than HE-AAC or HE-AAC v2.