Subject: Re: [linux-audio-dev] Writing a driver for this card: your thoughts?
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Tue May 28 2002 - 05:48:28 EEST
On Friday 24 May 2002 10:11 pm, Paul Davis wrote:
> >As to the AudioScience cards, they are common in radio stations and
> > recording studios.
> By my standards, anybody who puts A-D/D-A conversion inside their
> computer chassis is by definition not concerned with "the best in the
> business". The term "professional audio" for me doesn't include analog
> I/O for a computer card unless the converters are in a separate
> box. It might be good enough for a radio station, but for a recording
> studio?
Properly designed balanced in and out A/D and D/A converters can be completely
immune to the PC's internal noise. But it all goes to proper design. The
Antex SX series of cards likewise contain internal A/D converters. I have
measured the noise figures of the Antex SX-36 we have at WGCR using state of
the art audio systems analyzers, and the noise floor is below the LSB
threshold. All due to balanced I/O, sound PC layout techniques, and top of
the line components.
With unbalanced I/O all bets are off, of course.
But I still use an Echo Layla in our production studio -- and Layla's A/D is
out of the PC in a rackmount chassis. Balanced I/O on TRS 1/4 inch jacks.
But no Linux drivers yet.
> I think you're always better off with 3rd party converters myself, so
> you can make you're own choices on the quality level, and possibly
> have multiple options (Fostex,Frontier Designs,Apogee for example) as
> well as upgrade/downgrade paths.
In a high RF environment, unless the converters are optically isolated from
the PC, you might be asking for trouble. When I say 'high RF' I'm talking
10KW of AM transmitter fifteen feet away, with a measured RF field intensity
of 105V/m (the ANSI exposure limit is around 645V/m). This means a one meter
piece of wire that isn't properly grounded can develop 105V of RF energy. I
have suffered RF burns of appreciable intensity touching wires that weren't
connected to anything on either end -- they were just oriented along the
field gradient.
Also, due to the processing typical radio stations use in the air chain, in
many cases the quality of playback and record hardware for on-air use must be
up to the top quality of a recording studio. Typical radio stations use
sophisticated compressors, multiband limiters, and many other DSP-driven
techniques to maximize loudness -- and when the dynamics of the source
material go pianissimo, the compressors drive up the noise floor to
compensate. Noise in the outputs of a broadcast automation or
music-on-hard-drive system is verboten. This is particularly (and somewhat
paradoxically) true on an AM station, where the processing will be much more
aggressive -- the higher the average modulation, the greater the effective
coverage area for an AM station.
Having said that, the distortion percentages aren't nearly as important for
the exact same reason. But it is wrong to state that a recording studio will
automatically need higher quality than a radio station.
But in theory I agree with you on the topic of analog I/O into the PC -- but I
have just seen top quality hardware with on-board analog I/O that had no
measureable PC-induced noise too many times to ignore.
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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