[LAD] Xiph.org - Video:Digital Show and Tell - No difference between analog and digitally processed sound.

[Jörn Nettingsmeier]

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:
>> 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.

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

Meister für Veranstaltungstechnik (Bühne/Studio)
Tonmeister VDT

http://stackingdwarves.net