With Nero Burning ROM you can encode audio files of an Audio CD in MP3 and MP3PRO formats. Set these options in the MP3PRO window that you can open in the Encode files window via the Settings button. Ensure that you have selected MP3/MP3PRO as the Output file format.  
The following encoding options are available in the Settings area:
Drop-down menu
Encoder quality
Specifies the encoder quality. Fast, Medium and Highest are available. These settings specify whether you place more value on fast encoding (Fast) or more value on a superior psychoacoustic encoder model for the very best results (Highest).
Check box
Constant Bit Rate
Selects a constant bit rate, i.e. the data flow per unit of time and the quality of the saved data are the same over the entire audio file. If the bit rate is small, less data is transferred. The file is then small, but quality is lower. If the bit rate is high, more data is transferred. The file size is then large, but the quality is high. The default value of 128 for MP3 files is near CD quality.
In the drop-down menu, select the Options that affect the bit rate: scan rate per second in kBit, scan frequency per second in Hertz and the channels.
Check box
Variable Bit Rate
Selects variable bit rate, i.e. the data flow per unit of time – and thus the quantity of the saved data – adapts to the dynamics of the audio file. The bit rate, for example, can be lowered at quieter points in the track.
Select your desired quality level in the drop-down menu.
Note that not all players are able to show the correct length of MP3 files that were recorded with variable bit rate.
The following encoding options are available:
Check box
Enable MP3PRO
Specifies the encoder method used. If you select the check box, an MP3PRO file is created (*.mp3 file extension).
If you clear the check box, an MP3 file is created (also *.mp3 file extension).
Opens the advanced area where you can configure Expert Features.
The following encoding options are available in the Expert Features area:
Check box
Original bit
Sets the original bit in the music file, which differentiates between the copy and the original.
Check box selected: Original bit = 1, i.e. original.
Check box cleared: Original bit = 0, i.e. copy.
Check box
Private bit
Sets the private bit in the music file. It is reserved for the user and is only used for informational purposes.
Check box
Copyright bit
Sets the copyright bit in the music file, which identifies protected content.
Check box selected: Copyright bit = 1, i.e. protected.
Check box cleared: Copyright bit = 0, i.e. unprotected.
Check box
Write CRC
Also sets a checksum in the music file to ensure that transfer errors are detected.
Check box
Allow intensity stereo coding
Uses a special codec at high frequencies that only saves the directional information and volume.
Check box
Allow downmix
Mixes two stereo channels down to one mono signal. This option is useful when the output data is of such poor quality that a poor stereo signal is to be expected. An increase in quality is expected when downmixing to mono.
Drop-down menu
Selects a padding type for MP3 blocks. We recommend padding type ISO that pads MP3 blocks according to the ISO specification.
The MPEG-1 Audio Layer 3 audio format is used to reduce the size of audio files to a fraction of their original size (factor 1:10) with little loss of quality. You can estimate about 1 MB per minute as opposed to 10 MB for the original files. This value and the quality can vary depending on the complexity of the audio signal. The bit rate used can be used as a measure of quality. The higher the bit rate, the better the quality, but also the more memory required.
The compressor/decompressor encodes the data for recording or saving digitally and then decodes it for playback. Various software codecs are available such as Cinapak, Indeo, Quicktime, Video for Windows, etc. Hardware codecs include MPEG, H.261, Motion JPEG, etc.

MP3 and MP3PRO Encoding Options