Computer-DAC
 

"Rob" wrote in message
om

On bit perfect: do you/anyone know how it's
measured/tested?

Align the two files at the sample level and subtract them. The result must
be zero.

Play a Dolby digital sound track through it and decode it with a Dolby
digital decoder. If you get noise, then its not bit perfect. If you get
music, it is bit perfect.

Computer-DAC
 

In article , Rob
wrote:
On 11/02/2010 12:14, David Pitt wrote:
Jim wrote:


In a fit of enthusiasm, and optimism, I have rushed out and bought a
DacMagic which is now connected via USB to the Mac mini. The Mac Mini
is not bit perfect in this mode, the DacMagic is fed at the bitrate
set by the configuration and not by the bitrate of the file being
played, with iTunes. Now does that matter, time will tell.


Well, if the file is the same as the 'iTunes forced mode' (44.1/16),
then I would hope that all's well. Except to say the Dacmagic
*upsamples* for what the bumpf maintains are Very Good Reasons. Just
have to let that one go ;-)

In principle there is nothing wrong with either 'upsampling' or
'resampling' the audio data. Such processes are common in DACs as part of
the process of generating an *analogue* output - hence situations where you
have to 'process' the data anyway.

This is different to a case where a computer blindly 'resamples' data when
you *don't* need it to. If the DAC can handle the source sample rate, then
it is the steam of source sample values that should be delivered to the DAC
without being 'fiddled about' by the computer.

Again in principle 'resampling' can be done very well, and may be
'harmless'. But in practice it may also be poorly done (as per the examples
on the webpages I gave). Hence needless resampling is just an added
opportunity for the data to be fouled up, and the sound degraded.

One Arnold B Krueger responded with, "Why would you be surprised if a
modern OS did transparent resampling?".

That is point, does it matter. I assume that monkeying around with the
digits is a "bad thing" but can resampling of the sort done in these
computer mixers be done sufficiently well.


Resampling/upsampling as done by a device like a Meridian DAC or the
DACMagic can work very well. The problem is that some computer systems may
crudely 'interpolate' and thus generate distortions.

Indeed, even if resampling is done well it can lead to problems for a
following DAC that doesn't know it was done. So the basic rule is that it
best avoided unless it is employed for a specific purpose, well done, and
the user knows the snags, etc. Yet with computer it tends to be done
without the user being knowing, or being told how the process is being
computed.

I have the feeling that this is all a testament to the standard way
commercial customers of computers are regarded by the OS/hardware makers.
Keep people in ignorance of how what they have bought works so they don't
realise when they may have been short-changed! I've moaned more than once
about the poor quality of 'reviews' in audio mags. But my impression is
that the 'reviews' in computer mags are even less useful.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Computer-DAC
 

In article , David Pitt
wrote:
Jim Lesurf wrote:

done, and the user knows the snags, etc. Yet with computer it tends to
be done without the user being knowing, or being told how the process
is being computed.

In a similar vein, this about Mac OS makes a good read, the author seems
to know what he is talking about.

http://scopeboy.com/scopeblog/?tag=resampling

The article is interesting. But it also indicates the problems.

Firstly the basic error of having a system which imposes resampling when it
may simply not be necessary. If you output to a DAC that can cope with a
range of sample rates then the default should be to send it the source rate
samples. Not impose a needless change with the chances that will degrade
the info.

Secondly, the above gives a link to what is described as if it were a
program, but is actually just sets of coefficient values for use in a
resampling process. No real details of how the process is done.

The problem is then as follows. Is it really the case that every new
resampled value is computed using around 3000 coefficient x input sample
multiply and adds? If so that seems an insanely over demanding method. That
kind of thing is OK for dedicated hardware but sheer blind brute force and
iggorance for a CPU in a general computer *when you could have left the
data alone*.

And what level of accuracy will you then get when using single precision.
How accurate are the coefficients give they aren't in floating format, nor
binary, etc?

So it does look to me like another example of where those working on
general computing just don't understand the problem, and apply irrelevant,
needless, or less than optimum processes.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html