Wells JASON Owner's Manual page 10

OWNERS MANUAL FOR WEISS JASON CD TRANSPORT
ringing due to the high steepness. In the early days of digital audio these side effects have been
recognized as beeing one of the main culprits for digital audio to sound bad.
So engineers looked for ways to enhance those filters. They can't be eliminated because we are
talking laws of physics here. But what if we run the whole thing at higher sampling rates? Like
96kHz or so? With 96kHz we can allow frequencies up to 48kHz, so the reconstruction filter can
have a transition band between 20kHz and 48kHz, a very much relaxed frequency response
indeed. So let's run the whole at 96kHz or even higher! Well – the CD stays at 44.1kHz. So in
order to have that analog lowpass filter (the reconstruction filter) to run at a relaxed frequency
response we have to change the sampling frequency before the D/A process. Here is where the
Upsampler comes in. It takes the 44.1kHz from the CD and upsamples it to 88.2kHz or 176.4kHz
or even higher. The output of the upsampler is then fed to the D/A converters which in turn feeds
the reconstruction filter.
All modern audio D/A converter chips have such an upsampler (or oversampler) already built into
the chip. One particular chip, for instance, upsamples the signal by a factor of eight, i.e. 44.1kHz
ends up at 352.8kHz. Such a high sampling frequency relaxes the job of the reconstruction filter
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very much, it can be built with a simple 3 order filter.
So, how come that upsamplers are such a big thing in High-End Hi-Fi circles? The problem with the
upsamplers is that they are filters again, digital ones, but still filters. So in essence the problem of
the analog reconstruction filter has been transferred to the digital domain into the upsampler
filters. The big advantage when doing it in the digital domain is that it can be done with a linear
phase response, which means that there are no strange phase shifts near 20kHz and the ringing
can also be controlled to some extent. Digital filters in turn have other problems and of course
have quite a few degrees of freedom for the designer to specifiy. This means that the quality of
digital filters can vary at least as much as the quality of analog filters can. So for a High-End Hi-Fi
designer it is a question whether the oversampling filter built into the D/A chips lives up to his/her
expectations. If not, he/she can chose to design his/her own upsampler and bypass part of or the
whole oversampler in the D/A chip. This gives the High-End Hi-Fi designer yet another degree of
freedom to optimize the sonic quality of the product.
For the MEDEA we have decided to do part of the upsampling (the most critical part in fact) in the
Digital Signal Processor (DSP) chip external to the D/A chip.
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Page: Date: 10/04 /dw