> Date: Sun, 23 Aug 2015 19:11:17 +0200 > From: Gunnar Arndt <madenhacker@xxxxxxxxx> > To: alsa-user@xxxxxxxxxxxxxxxxxxxxx > > Hi Alsa users, > > > I have the following idea, which may be of interest for other users, > too: I would like to abuse the 'normal' unbalanced stereo input of my > on-board sound as a balanced mono input. > I have a measurement microphone with an integrated amplifier and a > balanced mono output which should be connected to consumer sound devices > with an `ordinary' unbalanced input (I have added a very short > explanation on the term `balanced' to the post scriptum). > > My equipment: > > * Ubuntu 14.04.3 with latest updates, including Kernel 3.19 and ALSA 1.0.25. > * HD-Audio-based Realtek AL1150 codec on an Asrock X99 Extreme4 > mainboard - with unbalanced stereo input, of course. > * Earthworks M30BX microphone with a balanced mono output (an XLR plug). > * A DMX out cable (XLR to 3.5mm stereo) connects the mic correctly to > the line input of the sound card: Ground to ground, balanced > non-inverting to unbalanced left, balanced inverting to unbalanced right. > ... In the analog domain, one issue will be the signal levels. Line level is usually a few hundred millivolts. Mike level is usually on the order of a few millivolts. When going through a stereo line input, you might not have enough gain. There is also some likelihood that the line input's front-end's noise floor might be too high for your mike signals. Another analog domain issue is that noise and other common mode excursions might be of greater amplitude than signal and might be larger than your input channel can handle. A true balanced input channel, perhaps with a transformer front end, would be more likely to handle larger common mode noise than desired differential signal. The cheapest analog hardware method to convert from balanced to unbalanced requires two conditions: 1) the balanced output must come from a transformer (coil of wire on a ferrous core); _AND_ 2) you are willing to sacrifice a little noise floor in exchange for economy. That solution is to just ground one of the balanced wires and use the other as signal. Another analog hardware solution would be to use an audio isolation transformer in front of your digitizer. Radio Shack used to sell a fairly cheap audio isolation transformer that worked surprisingly well. A metal box for shielding is probably a good idea. For the digital domain solution, if your use case is non-real-time, you could record a stereo signal into a 2-channel WAV file and then post-process it into a mono WAV file by taking the difference. Sox might have a filter for that. Otherwise, a C program wouldn't take long to write to do that. An alternative to saving the (noisy) stereo file would be to output RAW samples and do the conversion using Linux pipes--but that would probably introduce considerable latency, depending on buffer sizes. HTH Robert ------------------------------------------------------------------------------ _______________________________________________ Alsa-user mailing list Alsa-user@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/alsa-user
