On Tuesday 02 April 2013 10:29:48 Ralf Mardorf did opine:
> Peder I'm 46 years old and worked most of my live as audio engineer, so
One thing that has a huge bearing on "the sound" in the digital realm, a
word that I don't believe I have caught yet in this thread, which as
predicted will go on long after I'm gone, is aliasing.
Digitizers that can run at 192k samples don't have to worry a lot about the
brick wall filters to keep the aliasing under control because while they
will also do it, the products are them selves well beyond the range of
human ears, plus we don't normally feed them with signals that have any
great amount of energy above 25 khz anyway.
But at 44.1k samples/sec, the requirements of a brick wall anti-aliasing
filter are so stringent that they, while controlling most of the aliasing,
introduce their own phasing artifacts. Going to 48k samples will result in
a filter with a considerable softer cut off, removing enough of those
artifacts and ringing from the filter so it makes sense that its going to
sound better, and that most ears can easily hear the diff in ABX testing.
A 2nd and 3rd contribution, where op-amps are used in the analog circuitry
leading to the digitizer, most likely involved in realizing those filters,
is crossover distortion in the op-amps output stage at that point where the
signal passes thru the zero value point. Problems there I can see on the
scope, and I can hear quite easily despite the distortion analyzer sitting
there on the back of the bench telling me the distortion in the
distribution amplifier card I am servicing after an EMP from a lightning
strike blew the opamps, was less than .05%. A value my ears could easily
hear, but the scope really had to be worked hard to be able to see it.
It was a card I designed, to be both relatively low power consumption and
to be able to supply near 30 volts peak to peak so it could handle peaks of
+ 12 dbm or more since the house std level was 0 dbm.
So I used a cmos op-amp which uses very little current at normal signal
levels. The TLO74/84 family. These were great, and could make 29 volts P-
P on a +-15 volt supply rail at around .007% distortion.
But as I found in the real world where mother nature throws million volt
lightning bolts around, and the audio cables going 200 feet to the news
dept make a real good antenna to pick that up & shove it back into the
outputs of those op-amp's, they can and will, long before an outright
failure, develop this "crossover" distortion.
But in the long view, they were a 10,000% improvement over the audiophiles
pet op-amp, the 5532. Its fairly decent at low levels, but it runs so hot
it goes to hell in a hand basket. So you socket them, put clip-on heat
sinks and fans on them, and keep a tube of them in the closet when you have
an audio board with about 50 of them in it.
Another is the 4532?. That thing is horrible when you push it as its
nothing but a quad of 741's. Slew rate limits rare up and bite you at the
higher frequencies at output levels below 1 volt p-p. Crossover distortion
starts showing up at 1 volt output at only 1khz frequency. How that chip
Ramco was in love with ever got into the audio business in the first place
is totally beyond me. But there are thousands of them in use, mucking up
the sound, in broadcast facilities all over, or were till it went 99%
digital with hi-def tv. Radio is still the same old same old.
Distortion is often in the ear of the listener, and mine are going to pot,
but they can still hear /that/ stuff.
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