On Aug 4, 2010, at 12:49 PM, Joe Subich, W4TV wrote:
> Separate microphone input and analog Line Input which can be
> selected by the operating system. Those cards with separate
> mic and line inputs are "three port" cards - Mic In, Line In
> and Line Out (or speaker/headphone output).
With the inexpensive sound cards, the different inputs are not connected to
separate A/D converters. You cannot use the Microphone input at the same time
as the Line-in, for example. There is only a single A/D in the chip, and that
common A/D converter is switched between the different input pins of the chip
(often with different preamplification gains).
An additional thing to look out for is that while these chipsets provide a
stereo input when Line-in is selected, the microphone input is often mono only.
For example, on Mac OS X, a Micronas
(http://www.semiconductorstore.com/pdf/newsite/Micronashal/uac355xb_2pi.pdf)
based sound card appears as a stereo sound card when Line-in is selected, but
drops to become a mono sound card when the microphone input is selected. These
switched inputs are called "Sources" on Mac OS X to distinguish them from real
multi-channel or multi-stream inputs.
73
Chen, W7AY
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