What Is The Use Of Extended Sample Rates; Interfacing Extended Sample Rates - Prism Sound ADA-8XR Operation Manual

Prism Sound ADA-8XR Multi-channel A/D D/A Converter Operation Manual - Revision 1.00
converted data, up to a maximum just short of half the selected rate. In the case of D/A or
D/D Paths, the choice of sample rate is usually made for us since incoming data has already
been sampled at a particular rate.

7.2.2 What is the use of extended sample rates?

Whether the inclusion of extended audio frequencies in an A/D conversion (i.e. above the
20kHz-ish imposed for, say, 44.1kHz sampling) is beneficial is still being debated. The
general listener opinion is that it really does give improved sound quality, although it is worth
bearing in mind that nearly all comparative tests have tended to involve setups where 'A' and
'B' differ by far more than just the sample rate. Assuming, then, that there is an improvement,
there is much disagreement as to why this might be. There is little or no 'scientific' evidence
to suggest that anyone can hear sounds above about 25kHz, with sensitivity above 20kHz
being non-existent for the vast majority. There has been much discussion as to whether the
presence of high frequencies which would be imperceptible alone, can cause changes to the
rendition of lower frequencies.
Perhaps a more likely explanation for the improvement in perceived quality at higher sample
rates cites implementation limitations of the digital (or analogue) anti-aliasing filters within
current 'off-the-shelf' converter devices. These filters have traditionally been designed with
very sudden high-frequency roll-off in order to maximise the available audio bandwidth at
barely-adequate sample rates. This requirement, coupled with practical limitations in filter
complexity, have led to compromised designs with significant ripple in the passband (and
sometimes inadequate attenuation in the stop-band). In addition, these very steep filters
often result in unwanted temporal distortion: i.e. discrete pre- and post- echo effects. There is
speculation as to why and how these might be noticeable, but experiments carried out with
'no-compromise' filter designs (at traditional sample rates) which eliminate these echoes,
suggest that their removal renders the sampled signal subjectively indistinguishable from the
original analogue. So perhaps very extended sample rates are not strictly necessary; on the
other hand, an inadequate filter operating at extended rates produces echoes proportionately
nearer in time to the actual signal than it does at lower rates, which are likely to be less
noticeable. If this theory proves to be correct, it will cast doubt on the usefulness of the '4x'
rates (e.g. 176.4kHz or 192kHz) and above, since the transition band is already comfortably
wide enough at 96kHz to eliminate echoes, with cheaply-realisable filters rolling off
comfortably beyond audible frequencies.

7.2.3 Interfacing extended sample rates

When extended sample rates were first experimented with, it was difficult to operate the
preferred AES3 interface format at twice its normal rate (not that difficult, actually, but existing
interface devices wouldn't work), and so early 96kHz equipment used two AES3 interfaces
each running at 48kHz; instead of carrying a stereo signal, each interface carried one channel
with its 'odd' and 'even' samples placed as if 'left' and 'right'.
Although offensive to engineering types, this approach had unexpected advantages
operationally – you could use your existing AES3 routers and multi-channel recorders (albeit
with half the channel density) to work at 96kHz.
This method of interfacing between equipment is referred to variously as 'Two-wire' or
'Split96' AES3.
Later, manufacturers began to implement extended sample-rate interfaces by operating an
AES3 interface at twice the 'normal' speed. This method of interfacing is usually referred to as
'Double-speed' or 'One-wire' AES3.
Direct interconnection of the two interfaces is not possible, although the effects of connecting
half of a Split96 output to a 'normal' input have occasionally caused embarrassing confusion!
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