Robert, thank you for your message. I appreciate the additional information. Am 23.08.2015 um 23:20 schrieb Robert M. Riches Jr.: >> Date: Sun, 23 Aug 2015 19:11:17 +0200 >> From: Gunnar Arndt <madenhacker@xxxxxxxxx> >> To: alsa-user@xxxxxxxxxxxxxxxxxxxxx >> >> 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). >> ... > 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. The mic has an integrated amp which seems to deliver line level - at least, I have a clear signal from the mic on the line input, which is much stronger than the noise. > > 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. I know that this is the common way, but why would I do it that way if there IMHO is a better one? As I had explained in my initial message, there is noise which can be cancelled by using the line in as a fake-balanced. Btw, that noise is actually from the on-board wiring - it's there even if nothing is connected to line input. And with the fake balanced, it is cancelled, so I guess that Asrock have placed the wires for the left and right channels in close proximity to each other on the mainboard, allowing to use the advantages of fake-balanced signalling even there. > > 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. I'm not sure if I understand correctly what its purpose would be - unbalance the signal, like what I know as an opposite DI box? If you put under consideration that the noise comes from the mainboard for the most part, you'll agree that it would not make sense to unbalance the signal before reaching the mainboard, as some box would require. > > 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. As explained in my initial message, I had tested my approach with recording software successfully, and was looking for a (soft) real-time solution, which Clemens has provided in the meantime. Regards Gunnar ------------------------------------------------------------------------------ _______________________________________________ Alsa-user mailing list Alsa-user@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/alsa-user
