Sound on Sound, August 2011

ManualsBrandsSonnox ManualsSoftwarePro-Codec Plug-In (Native)

input signal, or listen to the difference between the input

and encoded signals (in other words, the audio that the data

compression is chucking out) by clicking on the Diff button.

Although part of what audio data‑compression schemes

do is completely lossless (in other words, removing data

redundancy on a purely mathematical level), this typically only

cuts file sizes roughly in half. To carve anything else away, you

need to start reducing the resolution of the audio information,

which inevitably adds noise and/or distortion to the signal.

The purpose of psychoacoustic encoding is to funnel these

potentially undesirable artifacts into areas of the frequency

spectrum where the human hearing system is least likely to

perceive them, and it’s because this bit of smoke and mirrors

is so effective that typical MP3 files can be 10‑12 times smaller

than their uncompressed PCM source files. However, the more

you try to squish the data, the more the processing artifacts,

as showcased in the Difference signal, start to impinge

unmusically on the listening experience. The aim of this

plug‑in is to help you decide which codec produces the most

acceptable trade‑off between audio enjoyment and file size.

While final quality judgements should clearly be made

by ear, Fraunhofer Pro‑Codec does, nonetheless, provide

a great deal of supplementary visual information to support

the auditioning process. Central to this is the multi‑function

display area that dominates the centre of the GUI. By default,

this shows a zoomable, real‑time FFT frequency analysis (the

yellow line) of either a single selected input channel or the

mono sum of all of them. The red shaded region beneath the

yellow line represents the spectral content of the Difference

signal, while a set of green lines above it indicates the Noise

to Mask Ratio (NMR), a theoretical measurement of how

effectively any data‑compression nasties are being concealed

or ‘masked’ by the spectral energy of the music itself. The

higher up the display the NMR line reaches in any given

spectral band, the more likely it is that the listener will detect

processing artifacts in that frequency region. Although Sonnox

admit that a given individual’s subjective impression of artifact

levels inevitably won’t correlate exactly to the calculated NMR

readings, the dancing green line is still useful for focusing your

attention towards the most likely spectral ‘danger areas’, as far

as encoding quality is concerned.

When you’re working with a stereo input, the Display

Type button accesses no fewer than three circular meters.

The left‑hand one is a Lissajous display with pink and yellow

traces, allowing comparison of the input and output stereo

Not sure you can tell the difference? Pro‑Codec can set your

mind at rest with its two blind‑testing modes: AB and ABX.

The first simply lets you toggle between auditioning any pair

of codecs (or any codec and the input signal) from a single

button. Normally the plug‑in’s GUI makes it visually clear

which signal you’re listening to, but if you don’t want that

distraction, the Hide button banishes all visual cues. The ABX

mode provides a more formal extension of this idea, offering

a proper ABX testing scheme which will inform you in concrete

terms whether your preference for one of any pair of the

available signals is statistically significant. I love this feature

— so much so, in fact, that I wish Sonnox would issue it on its

own as a stand‑alone plug‑in, so that I could test other plug‑in

processes with it too!

Blind Testing