[RTTY] USB Sound Card

[Kok Chen]

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