[net.audio] Digital vs. analog

kimr@sri-unix (07/16/82)

For you digital audio fans, I have a thought question.

There best current information on "audible differences" (admittedly
reported by golden-ears, but incorporated in products by manufacturers
such as NAD and Cotter) is that the fastest signal rise time the ear
is sensitive to is in the 9 to 14 microsecond range.

A digital signal recorded at 50kHz has a maximum possible rise time of
20 microseconds.

Admittedly, analog recordings have their problems, but rise time limitations
are not among them.  Why isn't our technology capable of supporting, say
100kHz sampling frequencies?  My favorite solution for the digital disk
(assuming the 100kHz a-to-d's are the limiting factor) is to record the
program using analog technology and then "half speed master" the digital
disk at 50kHz and play it back at 100kHz.

I, for one, will never purchase a digital recording, disk, or disk player
until the sampling rates are improved.

                                              Kim Rochat

jj (07/19/82)

	I must admit that I find it difficult to reply to the article by
Kim Rochat, although not because the arguements are difficult to refute.
They are, in fact, not that difficult to refute, to another signal processing
person.
	Disgegarding the opinion statements about Golden Ears, I will
begin my comments by addressing the statements about rise time in analog
equipment.  It is indeed limited, and quite severly, (in a record, for example)
after several passes through the playback device. (This statement holds for
magnetic recording, also, but a discussion of the causes is beyond this
note).  In addition, records, which do generate signals with a very
high rise time, most often do so (even in the case of the best records and
cartridges)  as a result of tracing distortion or mistracking.
	Even if the signal being reproduced does have an undistorted
risetime on the order of 10us, the transducer that reproduces it and
the travel through the ear will spread it out, due both to filtering
and dispersion.  There have been several, although few and far between, papers
that have discussed the variability of DISTORTION with respect to signal
risetime.  Most transducers, do, in fact, have measurable non-linearities
that arise during highly slewed parts of the signal.  These non-linearities
have been shown to be perceptable.<i.e.  how does the test that you refer
to avoid measuring THAT as a perceptable effect?>


	Regarding the recording of digital at half speed:
There are indeed converters that are capable of converting at
100kHx.  The converter is not the problem.  STORAGE of the information is
the problem.  Your scheme of slowing down the digitization of the
signal will do the following:
1) Make no difference whatsoever in the Digital sense<assuming the
equipment is working correctly>
2) Introduce a distortion of about 2% <minimum> due to the
nonlinearities in the magnetic recording process.
3) Limit the dynamic range to 60-65dB  OR introduce low frequency distortion
as a result of compression/expansion.
4) Introduce the most annoying (to me) problem with magnetic recording tape:
Head Scrape/Flutter.  This bizzare process is the result of sliding a tape,
with a finite coeffecient of friction, against the tape head.  It has nothing
to do with the flutter due the drive system, etc.  This is the process that
causes the "closed in' sound that is often attributed to magnetic recording.
5) Double the storage required for the signal.
	To summarize.  Your method of making digital recordings would
eleminate the beneficial aspects of digital recording, without making
any change in the problems involved.

thomas (07/19/82)

raeh My thanks to Jim Johnson for the comments on the 'rise time/sampling
frequency' issue.  I can flame for quite a while about people who think
they can hear 50kHz (or even 100kHz), but I'll refrain.  (I often wonder
if all the AM radio signals pulsing through their ears drive them crazy.)

I want to emphatically second the point that any analog recording stage
in the process totally destroys the digital advantage (except its
archival quality, more on this later).  The Fleetwood Mac album "Tusk"
(pre-released in platinum, as explained in net.records) was recorded
analog (so they could play their 32-track mixdown games), then transferred
to a digital form for editting and (I think) mastering.  ON THE RECORD
you could hear the tape hiss from the single analog stage.  GROSS!!!

Why then, you say, did they do the digital stage at all?  The way I heard
the story is that the analog MASTER tapes for "Rumours" (these are the
originals, no way to recreate them) suffered during the process of producing
the 10 million copies they sold that the high end was severely depressed
in the later pressings!  They wanted the 'never changes no matter how
many times you play it or copy it' quality of the digital recording
medium for this reason.

To me, this is the overweening reason to go digital, even if there were no
other differences.  A copy of a digital recording is EXACTLY the same as
the original, no added noise or distortion.  Tom Stockham (a pioneer in
digital recording technology) often puts it this way:
	"Digital recording is to analog recording as books are to
	oral tradition."

Too much, I'll stop now.
=Spencer Thomas

pdh (08/04/82)

I, too have heard of a number of good arguments against digital recording
techniques using the 50Khz sampling rates.  The best one I have been able to
come up with so far is that, at that sampling rate, potential channel-to-
channel phasing misalignment at much over 15KHz COULD be tremendous, possibly
as high as 20 or 30 degrees.  This phase misalignment would vary with
frequency, due to the way the A/D's work, and so the final result is a
piece of music with very vague and poorly defined imaging.  I have noticed
this in every digital album I have heard to date (about 30).  The big
deal with this is that the problem won't go inaudibly away until the sampling
rate is up around 300KHz or more, which is WAY out of the range of technology.
One possible solution to the problem would be to digitize the signal from
the microphones onto several different digital master tapes, and then use
a computer to correlate the signals, and assure proper phase alignment.  This
should work by statistically making the maximum phase alignment proportional
to the number of master tapes being correlated into a single master.

				Peter Henry
				hplabs!pdh

jj (08/04/82)

	The different articles posted to the net about digital audio
demonstrate a very interesting phenominon, i.e. that of complaining about
(I lack the vocabulary to use the proper word here) various
alleged problems that exist in digital audio, without either a correct
statement of the so-called problem or a reasonable comparison with the
performance of the current analog systems.  The complaints about the phase
response of digital systems are just plain WRONG!  I can't attack
the argument on technical grounds because it doesn't have any basis
in either theory or practice.  There have been hardware problems that
have, in the past, led to such problems, but I do not consider
that a problem with even a set of the most well known hardware constitutes a
flaw in the concept.  

	
	The problem alluded to in the article that this responds to alludes
to phase problems.  The correspondant does not mention whether this is a
fixed, random, or time varying phase problem.  Neither is the magnitude of
the "phase problem" compared with the phase response of a tape deck
such as that used to master a disc.  
	To clear things up a bit..  
	If the "phase problem" is random, it will cause the system
to have a total snr of about 10dB, regardless of the number of
bits, etc.  Since this is clearly not the case, I dispose of this case.
	If the phase is time varying (at a fast rate--) it will cause
modulation effects that have similar results (no snr).
	If the phase varies very slowly, it has the same effect as
a tape deck.
	If the phase is non-tracking, but fixed, it doesn't matter, since
it's like a perfectly aligned tape deck with NO head skew.(Fat chance)

	I am becoming tired of defending digital audio systems against
people who have complaints which are either wrong or taken out of
context.  Is there someone else out there with an interest in digital
audio?  If so, and your are interested in continuing the debate,
mail me.  If no, I will abandon this group to the uninformed digit haters
who seem to think that any new technology should be perfect,and who disregard
the performance of current systems in their haste to tear down a new idea.
I guess that new ideas will always suffer from conservatives who
fear that they will become obsolete. (Very likely, if they refuse to become informed.)