23 May
2013
23 May
'13
9:38 a.m.
hi john,
On 05/22/2013 09:44 PM, John Rigg wrote:
On Wed, May 22, 2013 at 10:38:37AM -0400, Bill Gribble
wrote:
this comment raised my eyebrows a little bit. can you explain what you
mean by "decimation filter"? the way i understand it, decimation means
chopping off bits, usually by shifting the data words, and possibly
adding dither. how can this be a problem at fs/2? no new frequency
components are introduced (apart from additional quantisation noise,
which must necessarily be band-limited to fs/2), and the input of the
decimation stage will already be band-limited as well.
otoh, if you mean sample rate down-conversion, i understand your
comment, but then you picked an unfortunate term.
moreover, i'd expect src circuits with only -12dB at fs/s to be unusable
in practise, because the aliasing artefacts would be obvious. it means
the top octave from 10-20hkz would be polluted with junk at -24 to
-12dB, unless of course there are some oversampling tricks going on and
the effective fs is higher during down conversion. although i must
confess i don't know anything about DAC and SRC design - if someone can
explain this in more detail, i'm all ears.
best,
jörn
--
Jörn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
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There are real effects due to clock jitter on
both the A/D and D/A end that can cause small but measurable
distortions.
Not to mention audible if it's severe enough. Decimation filters
that only give 6 or 12dB attenuation at fs/2 (typical in many pro
audio ADC chips) can allow audible aliasing too. I wouldn't expect
an oscilloscope to have enough resolution to detect these effects,
but a good spectrum analyser and/or a good pair of ears often can.