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Re: Audio recording hardware (was: Re: audio recorder in linux ... what's its name?)

* Bob McGowan <bob_mcgowan@symantec.com> [070226 11:14]:
> Russell L. Harris wrote:
>> Within the past year or two there have become available a number of
>> alternatives to the computer sound card.  These eliminate the
>> pitfalls (especially the electrical noise) associated with sound
>> cards, as well as the problem of hardware and software
>> obsolescence.
>> Some are stand-alone flash memory recorders with broadcast-quality
>> microphone inputs (i.e., balanced XLR).  These typically have a usb
>> interface which can be used, after recording, to transfer the
>> digitized audio to the computer for editing or processing; or you
>> can make the transfer using a flash card reader.
>> Also now available are a number of recording interfaces with
>> broadcast-quality microphone inputs; these also typically interface
>> to the computer via usb or firewire.  Some claim to require no
>> proprietary software for recording.
> Could you provide some additional information or pointers to it,
> regarding your comments about sound card electrical noise, and
> perhaps some references to the hardware you mentioned?

Hi, Bob.

>From the standpoint of electrical noise, the interior of a computer is
extremely "dirty", because of the nature of digital signals (sharp
transitions, which correspond to high-frequency components).  

In the case of a microphone input, an analogue audio signal in the
millivolt or even microvolt range must be amplified by a factor of a
thousand or more before it is digitized.  Careful shielding and
grounding, together with the use of balanced circuitry, are essential
to prevent the analogue signal from being corrupted; the measures
require good engineering and typically are expensive. Once in the
digital realm, there is no major problem with integrity of the signal.

Recording analogue sound with a sound card plugged into a PC is
analogous to recording birds singing in the backyard while the kid
next door and his rock band are practicing in the garage.  Putting the
low-level amplification and the digitizing circuitry in a box external
to the PC chassis is much like doing your recording in a sound
isolation booth.

One of the best sources of information regarding the problems of
electrical noise and methods for handling the problem is the
literature of manufacturers of sound cards and audio interfaces.

Go to the web page of a broadcast-gear supplier such as www.bswusa.com
and view the on-line catalogue, or request a catalogue to be mailed to
you.  The catalogue lists a variety of broadcast-grade sound cards and
external audio interfaces.  Then go the web pages of the various

Again, if you are serious about signal integrity, the use of balanced
circuitry is essential.  Balanced lines consist of a twisted pair of
conductors with a shield, and typically use the Canon XLR connector or
a 1/4-inch TRS (tip, ring, sleeve) phone plug.  Apart from its use in
digital applications, coax cable (in which the "shield" is an integral
part of the signal path) is used only on non-critical consumer gear.


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