In article , Rob
wrote:
Jim Lesurf wrote:
In article 4a6881f4.21074781@localhost, Don Pearce
wrote:


http://www.audiomisc.co.uk/Linux/Sou...stenAgain.html

This discusses how I setup a (Ubuntu) Linux box to access the BBC
iPlayer, and provides some measurements on the results that look at
some aspects of the BBC aac/aac+ radio iPlayer.


Interesting - although I don't follow a lot of the technical text.

For reasons I don't need to understand, I take it it's better not to use
any volume control on the computer that affects the volume of a digital
output other than set it at maximum?

Alas, not quite. The trick is to establish the settings for the computer
and its software that pass though digital values with no rescaling of the
amplitude of the values. This may be 'maximum', but that depends entirely
on how the programmers wrote the software, etc. If they don't tell you, you
are left to guess, or experiment, or measure.

With the specific systems I tried - that were Linux based - I found that
setting levels to '100%' (or '0dB') did this. So that is a good bet. But as
shown by the problem I found with headphones you can't be certain in
advance that some pinheaded software or hardware engineer hasn't upset this
and some other value is better.


FWIW if I can blag a borrow I will try out some other USB 'soundcards'
to see how they work (if at all!) with Linux. Then report on what I
find out. A number of these have appeared recently. But I've only seen
limited results based on windows and macs.


If I get round to it, and you can return it at some point, you can
gladly borrow an Opcode Sonicport (bus powered USB DAC). Works natively
on a Mac and Windows Vista (but not XP). I got it from a recording place
round the back of Denmark St - I'd guess it had a sort of claim to
goodness at some point.

I may be interested at some point. Afraid I've not heard of it. Is it a
currently available USB DAC? How does it identify itself on a windows box?


The DACMagic is excellent in my opinion.


[snip]

Is it too much to ask for a standard?

Afraid I don't know what you are asking here.

Anyhow, do you mean this:

http://www.cambridgeaudio.com/summar...&Title=Summary

Yes.

It's £250. Ebay has DACs for £30 - Mini HIFI SPDIF+OPTICAL
DAC--PCM1793+DIR9001+OPA2134. What's the critical part of the component
here? It's not the DAC itself presumably - it's the analogue
amplification?

No, the vital part in this context is the chip or other interface that
receives the USB audio. It has to identify itself to the computer in a way
that allows the data transfers to proceed correctly.

I (still!) have problems understanding why one digital component
necessarily sounds different compared to another *unless* it's to do
with analogue amplification. And I don't really follow that - just seems
to make sense.

The problem with USB DACs is as above. But with computer based systems all
kinds of other things can go wrong. For example the stream of values may be
'resampled' or otherwise fudged about by the system. So that what comes out
isn't what went in, even as a series of digital values.

I was tempted by the fit-pc2 as it is said to run with Linux, is small
and silent, and I can use an external DAC. But I've since been told it
doesn't currently work with Ubuntu 9.04 and has various hardware
snags. So no cigar unless the snags are sorted.


Ah - that's me wrong again! The netbook has an Atom processor and is
pretty clunky with Windows XP - and as I say, uses a fan which does kick
in frequently.

The good news is that many Linux distrbutions are somewhat less demanding
than Windows, so don't stress machines as much. But that may not matter
much if you then run tasks that are demanding. e.g. Processing video is
likely to be demanding regardless of OS. But I'd suspect that even a very
low spec machine should cope with simply passing around LPCM with no
processing without having to struggle.

Seems those Fit-PC2s have some sort of clever convection
cooling perhaps. Macs have very impressive cooling IME. Even the
desktop iMac (dual core Intel, fast HD, and decent graphics) has 3 fans
that 'trickle' at around 1200rpm and produce a very unobtrusive sshhh.
The HD is silent. Room temp in here is 23C, CPU is 38C.

The Shuttle I am using has a large, low speed fan that is meant to be
'quiet'. But I disconnected it to ensure silence. 8-] This is OK as what
I use the machine for isn't demanding for long periods, so only runs the
CPU at a small fraction of the level it can nominally deliver.

I also had a SSD fitted, not a traditional Magnetic HD. So no HD noises,
either.

Slainte,

Jim

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