Information Technology Reference
In-Depth Information
For the above reasons, the conclusions of many of the listening tests carried out on audio compressors are simply
invalid. The author has read descriptions of many such tests, and in almost all cases little is said about the
loudspeakers used or about what steps were taken to measure their spatial accuracy. This is simply unscientific
and has led to the use of compression being more widespread than is appropriate.
A type of flat panel loudspeaker which uses chaotic bending modes of the diaphragm has recently been introduced.
It should be noted that these speakers are distributed sources and and so are quite unsuitable for assessing
compression quality.
Whilst compression may be adequate to deliver post-produced audio to a consumer with mediocre loudspeakers,
these results underline that it has no place in a high-quality production environment. When assessing codecs,
loudspeakers having poor diffraction design will conceal artifacts. When mixing for a compressed delivery system, it
will be necessary to include the codec in the monitor feeds so that the results can be compensated. Where high-
quality stereo is required, either full bit rate PCM or lossless (packing) techniques must be used.
Figure 4.49:
Using compression to test loudspeakers. The better the loudspeaker, the less compression can be
used before it becomes audible.
An interesting corollary of the above is that compressors can be used to test stereo loudspeakers. The
arrangement of
Figure 4.49
is used. Starting with the highest bit rate possible, the speakers are switched between
the source material and the output of the codec. The compression factor is increased until the effects noted above
are heard. When the effects are just audible, the bit rate of the compressor is at the information capacity of the
loudspeakers. Domestic bookshelf loudspeakers measure approximately 100 kbits/s. Traditional square box hifi
speakers measure around 200 kbits/s but a pair of state-of-the-art speakers will go above 500 kbits/s.
[
31
]
Cao, Y., Sridharan, S. and Moody, M., Co-talker separation using the cocktail party effect. J. Audio Eng. Soc.,
44, No. 12, 1084-1096 (1996)
