Sonnox Fraunhofer Codec Toolbox - Manual (Page 10)

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6.1.1 NMR Meter
The NMR METER (Noise to Mask Ratio) provides an indication of the frequency areas where the
difference between the codec output and the original input might be audible.
All lossy codecs will produce a very slightly different output from their input. The very nature of a
perceptual coder is that this difference should be inaudible (i.e. masked by the output signal). You
can choose to trade off more data compression (and smaller files) against increased audibility of
artefacts and codec-induced noise. Theory states that this codec-induced noise should be
inaudible when the NMR indicator is green.
Under some circumstances (codec, frequency and input signal dependent) one or more of the
NMR LEDs will turn orange, indicating the frequency range where encoding artefacts and codec-
induced noise might be audible. The listening environment, training and sensitivity of the listener’s
ears are also variable factors that must be taken into account.
It is possible to make very quick comparisons of a selection of different bit rates while auditioning
music and monitoring the NMR METER for such potential artefacts.
The NMR calculation is less accurate for parametric codecs (those that use enhancements such as
Parametric Stereo or Spectral Band Replication). HE-AAC and HE-AAC v2 use parametric
enhancements to achieve very high compression ratios. The NMR METER is still enabled for these
codecs, because it can still give an indication of the frequency areas that might require
examination.
Note that the NMR METER is not present if the selected codec is lossless (i.e. mp3-HD or HD-
AAC). The NMR meter is not valid or displayed for frequencies above 16 kHz, and is not supported
at a sample rate of 32 kHz.
NMR Frequency Ranges (to the nearest 1Hz)
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