Re: Data Acquistion Standards

[David Isherwood <David.Isherwood@xxxxxxxxx>]

Hi all,

On 14 Jun 01, at 15:43, Bradley Wood Libbey wrote:

<snip>

> However, I am concerned about noise. I suspect that a majority of the
> noise actually exists as acoustic energy, but I don't want my
> measurement
> system to be adding to this.  The first possible source of
> noise is from
> aliased high frequency signals.  What is the standard in
> auditory research
> for antialiasing filters?  If they are necessary, do most
> researchers use
> stand alone filters or do they simply acquire digital data on
> acquisition
> systems with built in filters?

Firstly, previous discusions have talked of the use of Sound Blaster cards
for sample acquistion. Using such a device as a microphone amplifier and A/D
means that any artifacts associated with the anti-aliasing filter will be
very small when taking a holistic view of the noise and non-liniarities
inherent in the system as a whole.
The amount of artifacts created by the anti-aliasing filter will depend on
the sampling frequency used. For 44.1/48kHz the usual negative effects of
any low-pass filter (e.g. ripple, phase distortions, ringing) are mainly
present in frequencies above 18kHz and are thus of little effect given the
small amount of energy present in speech at these frequencies. I am not
fully familiar with your experimental needs and computing resources, but on
modern PCs the falling costs of audio cards that sample at higher sampling
rates (upto 96kHz) mean that if this is an issue, then sampling at higher
rates and then software resamling to lower sampling frequencies will give
better results as the effect of the anti-aliasing filter will be confined to
the frequencies you are dumping.

> Secondly is the analog circuitry or the analog to digital converter
> noisier on an inexpensive card than on a "professional" acquisition
> system?

The short answer is yes, but the longer one will depend on the source
signal. If speech is being recorded under anechoic conditions then 'noise'
created by the acquisition system will become an issue.
One of the main problems for digital audio acquisition boards on computers
is the amount of RF energy produced by the other components in a computer
inducing noise in the input amplifier and A/D stage. This can be kept to a
minimum by choosing a card which is well shielded or by placing the card
away from 'noisy' sources (e.g. hard drives). Better still would be to use
an external A/D and D/A connected to a digital input on the computer(even
cheap Sound Blasters have these now).

There are other fundamental issues that can effect speech acquisition but I
can't make recomendations without knowing more about your needs.

Cheers
Dave

David Isherwood
Research Engineer
Speech and Audio Systems Laboratory
Nokia Research Center

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