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The 3ivx DirectShow Audio Encoder encodes audio into the MPEG-4 Low Complexity AAC (LC-AAC) format. It supports up to 5.1 channels of Audio.
The AAC audio stream would normally be muxed into the MP4 File Format with the 3ivx DirectShow Media Muxer.
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Configuration Options
Average Bitrate Per Channel
Use the slider to set the approximate Average Bitrate (ABR) in kilobits per second (kbps) per channel (i.e. using 64 averages at 128 kbps for stereo).
Use ABR
If checked, the Audio Encoder will use the Average Bitrate specified.
Variable Bitrate Quality
Use the slider to set the quantizer quality (the default of 100% averages at approximately 128 kbps for normal 44.1khz stereo audio).
Use VBR
If checked, the audio will be encoded with the Quality specified and a Variable Bitrate.
Frequency Cutoff
Set the Frequency Cutoff in Hz.
Note: The text field does not have any effect currently, the slider works fine.
Use Cutoff
If checked, the Audio Encoder will use the Cutoff Frequency specified otherwise the cutoff will be calculated automatically based on the sample rate.
Use M/S Stereo
Mid/Side Stereo coding can achieve a significant saving in bitrate which is lossless as opposed to MP3's Intensity Stereo coding. Recommended
If checked Mid/Side Stereo encoding will be used.
Use Temporal Noise Shaping
Temporal Noise Shaping (TNS) was introduced into AAC to overcome a limitation in conventional transform coding schemes often encountered with signals that varied heavily over time, especially speech signals.
TNS uses frequency domain prediction to soften quantization noise over time.
If checked Temporal Noise Shaping will be used.
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| Table: How Quality and Cutoff affect VBR |
| Quality (%) |
Cutoff (Hz) |
Bitrate at 44.1 kHz |
| 150 |
20,000 |
~184 kbps |
| 140 |
19,000 |
~169 kbps |
| 130 |
18,000 |
~157 kbps |
| 120 |
17,000 |
~145 kbps |
| 110 |
16,000 |
~132 kbps |
| 100 |
15,000 |
~120 kbps |
| 90 |
14,000 |
~109 kbps |
| 80 |
13,000 |
~96 kbps |
| 70 |
12,000 |
~85 kbps |
| 60 |
11,000 |
~74 kbps |
| 50 |
10,000 |
~64 kbps |
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This table should give some orientation for useful Quality and Cutoff settings, based on FAAC 1.21.2. The VBR bitrates refer to an average sounding 44.1 kHz stereo file (ct_reference.wav in this case). |
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VBR, ABR and Frequency Cutoff
VBR final bitrate depends on Quality and Cutoff, so you should only vary the default Quality of 100% and Cutoff of 15,000 Hz if you know what you're doing or want to experiment with other Cutoff Frequencies at a given quality setting.
This behaviour is also used for Average Bitrate (ABR) encoding. The outcome is adjusted by using a specific Cutoff Frequency for each Average Bitrate and varying the quantizer quality internally until the desired overall bitrate is reached. Nevertheless the usual remarks about ABR vs. VBR apply here, so this setting is only recommended for special purposes where a predictable bitrate is more important than sound quality, e.g. live streaming or obtaining exact file sizes. |
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