At 20 kHz, the effects of
upsampling on THD+N were even more pronounced. Notice that the amplitude resolution
of the 20 kHz peak increased by 10 dB. The base of the peak was at - 110 dB with no upsampling, and at - 120 dB with upsampling turned on. In all tests, THD+N remained lower than
0.003%.
The measured frequency response was 20 Hz - 20
kHz ± 0.05 dB. That is superb!
When CD players were originally marketed, the
digital sampling frequency was 44.1 kHz and the word length was 16 bits. The
Digital-to-Analog decoders (DACs) were only
able to operate within those limitations.
However, with improvements in DACs, higher
frequencies were soon obtainable. Engineers realized that quantization noise, which is the result of
errors produced by the fact that analog voltage values in the music have to
be assigned the nearest 16 bit digital number to that value, rather than
always being the exact value, could be reduced by oversampling the 44.1 kHz
sampling to multiples of that, during the conversion to analog signals. This
noise is the difference between the analog signal value and the digital
number that represents it. The difference is small, and therefore, it is
very high frequency. Quantization
noise, even though it is not in the audible band, causes aliasing, which has
an effect on frequencies in the audible band, resulting in more
harmonic distortion.
At first, oversampling was 4X (176.4 kHz), and
then later, 8X (352.8 kHz).
The higher sampling frequency with
oversampling pushes the quantization noise so far away from the audible
band, filters in the output stage have a much easier task in removing the
noise.
Upsampling is basically the same process, but
is strictly in the digital domain, and was created to let DACs that are used
in the new high resolution music formats, such as
DVD-A and SACD, easily decode CD music.
The reason this requires its own special procedure is that high resolution
music formats are not integer multiples of 44.1 kHz. For example, DVD-A uses
96 kHz and 24 bit word lengths (a 2X integer multiple of 44.1 kHz would have
been 88.2 kHz). Likewise, SACD uses a number that is not an integer multiple
of 44.1 kHz.
However, the results are the same: reduction
of quantization noise in the analog signals sent from the CD player, to the
preamplifier.
Upsampling is a newer technology, and
sometimes it is used in a CD player to reduce quantization noise, even though the player only plays standard CDs rather
than high resolution music formats. Such is the case for the ONIX CD-3.
Conclusions
If you are in the market for a standard CD
player, the ONIX CD-3 is a must for your consideration. At $999, it
is one of the best values in audio for 2007.
- John E. Johnson, Jr. -