The nmr indicator, 1 the nmr indicator – Sonnox Fraunhofer Pro-Codec User Manual
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5.3 Online Auditioning and Configuring Codecs
5 THE PRO-CODEC PLUG-IN
5.3.1 The NMR Indicator
Also featured in the FFT display is the
NMR (Noise to Mask Ratio) indicator line. This
provides an indication of the frequency areas where the difference between the codec
output and the original input might be audible. The
NMR indicator can be disabled by
clicking on the
NMR button (above the display).
All lossy codecs will produce a very slightly different output from their input. Sometimes, if
you listen to the difference signal it can sound significant; however, the very nature of a
perceptual coder is that this difference signal 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) this
NMR can
turn red, indicating that the induced artefacts are potentially audible.
There are potentially audible artefacts in these frequency bands.
The listening environment, and the training and sensitivity of the listener’s ears are obvious
and variable factors to take into account regarding the audibility of these artefacts.
The
NMR display is a useful indication when auditioning the MP3 or AAC codec families.
If there are areas where the
NMR line is above 0 dB, those are the frequency areas of
interest. The display will highlight these frequency areas in red. It is possible, for example,
to make very quick comparisons of a selection of different bit rates while auditioning
music.
The
NMR calculation is less accurate for parametric codecs (these use enhancements
such as Parametric Stereo or Spectral Band Replication). HE-AAC and HE-AACv2 use
parametric enhancements to achieve very high compression ratios. The
NMR indicator is
enabled for these codecs by default, because it can still give an indication to the engineer