From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Platt Subject: Re: USB sound adapter for use with Tom's soundmodem? Date: Mon, 15 Dec 2008 11:00:15 -0800 Message-ID: <4946A93F.5060104@radagast.org> References: <49432397.8060304@weca.org> <4388.219.89.148.235.1229141951.squirrel@webmail02.lancs.ac.uk> <49443D5C.1020307@weca.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <49443D5C.1020307@weca.org> Sender: linux-hams-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Alan Crosswell Cc: Andrew Errington , linux-hams@vger.kernel.org Alan Crosswell wrote: > Do these cheapo USB things with only a Mic input have enough impedance > matching to handle a line level input? A line-level signal is at a substantially higher voltage level... it's usually 1 to 2 volts peak-to-peak. Microphone level is lower... perhaps 100 millivolts peak-to-peak. Driving a microphone input with a line-level signal may overdrive it into serious distortion. On some devices, the input is switchable between line and microphone levels (this is usually done via the software mixer). On others, the additional gain in the mic path cannot be defeated, and it's necessary to "turn down" the input signal to avoid overdriving. This "turn-down" can be done with a couple of resistors, or (if you want adjustable gain) via a potentiometer. There's a disadvantage to "padding" down the signal level and going in the higher-gain mic input - the resulting noise level will probably be somewhat higher than if you went into a properly- level-matched line input. Impedance matching per se is very probably not a problem in this case. Line-level drive circuits usually have a fairly low output impedance (a few tens or hundreds of ohms), and both mic and line-level inputs have relatively high input impedances (tens of thousands of ohms). The input impedance is so high that it does not load down the driving circuit by any significant amount. > The reviews seem to be focused on playback. I'm willing to invest the > $5 to find out but would rather not waste a week to find out.... My guess is that this approach would probably work just fine for AFSK packet, with most cheap USB audio dongles (plus a padding resistor or two), due to the characteristic of the signal... high audio level, lots of gain in the FM signal path, and no close-in interferers within the audio bandwidth. I'm not sure they'd work as well in (for example) weak-signal I/Q demodulation applications - their dynamic range might not be adequate.