MD: Listening to compression (was: md-l-digest V2 #855)

[Eric Woudenberg, Minidisc.org Editor]

Hi Howard,

"Howard Chu" <[EMAIL PROTECTED]> writes:

> re: quality of MD, ATRAC, LP2, LP4, MP3: this is very easy to measure both
> objectively and subjectively:
> 
> First, we have to agree on a "high quality" standard to start from. I
> remember all of the analogue vs digital flame wars of days gone by,
> but for now let's agree that 16 bit PCM at 44.1khz as present on a CD
> is "high quality."
> 
> Use a bit-accurate CD-ROM drive and rip an audio file off a CD onto
> your PC.  This is the original, "perfect" data source. Encode it as an
> MP3 using any bitrate you care to test, and then decode it back to PCM
> format. If you invert this file and add it to the original WAV file,
> the result will be the difference between the two signals, the "error"
> between the original and the MP3 file. You can listen to this error
> signal and measure its RMS value, to give both subjective and
> objective quality measures.

As you state, the error signal (or its square) is a valid quantitative
measure of the loss incurred by the coder. But even though you can
listen to the error as an audio signal, it isn't a valid subjective
measure since the error in question is never normally heard
standalone, rather only as a deviation in the presence of the full
audio signal (and is hence being masked by the signal).

General aside on subjective evaluation of coders:

Automatic measurement of the subjective quality of perceptual encoders
is a current research topic and involves some degree of modeling the
human auditory system in the measurement phase. I've always thought
that this was a bit of a tail-chasing problem, since if you've got a
model with superior accuracy in the measurement system, why not use
that model in the encoder as well and further reduce the perceptual
loss?

An automatatic system that produced reliable subjective scores for
audio and speech coders would be a great boon to developers, since
they could save the considerable time and money spent using human
subjects. (They could also save wear and tear on their subjects -- at
Lucent I would occasionally participate in experimental evaluations of
cell-phone speech coder samples (as a favor to colleagues). I found it
to be painstaking and frequently frustrating work).

Rick


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