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It is assumed that your pc sound card or sound module is capable of full duplex operation. That is two-way simultaneous audio flow.
Software wise, my two pcs have for instance a freeware audio recording program installed called Audacity. A short-form instruction guide follows below. There are versions for Linux and any Windows except W95/NT.
In fairness to them, other recording programs are available.ISRA full build instructions
Two holes are drilled in the box short side, for two stereo audio co-ax cables of wanted length. Mine were short but up to a metre each is reasonable as explained further below.
A top hole is drilled in the box lid as shown for the 3.5mm stereo headphones jack socket.
Optional four small stick-on feet as obtainable from Wilko, are pressed on underneath the box.
There are six components soldered on to whatever type of mounted circuit veroboard, tag strips, or tag board suits the user. The prototype used a cut-to-size tag board.
There are two resistors of 2k2 each and two resistors of 2k7 each. There are two of each, either tantalum bead or miniature electrolytic capacitors of 10mfd for passing bass frequencies. Ensure they are both wired the correct way round, with their +ve legs facing the pc mic socket to block the outputted stereo phantom DCs of +3V.
Board tags or pins also provide convenient soldering anchors for the ends of the appropriate length stereo co-ax cables fed through the box side holes. Small tie-wraps on the cables inside the box, just behind the co-ax entry holes, provide useful anti-stress anchors.
Solder two 3.5mm stereo jack plugs to the free ends of the co-ax leads. For convenience add a coloured ident marker to the mic plug, as this being the signal input.
Add correctly wired links between the headphones jack pinouts and the mounted circuit board.
It might be useful to hand draw the circuit on a folded slip of paper and leave it in the box for future reference. Double check all wiring and finally screw down the box lid.
Insert stereo plugs the correct way round into the pc speaker output and mic input jack sockets and plug in the headphones.
1) Depending on program version, the stereo recording & playback level bar should be seen somewhere at the upper right side. Position wherever for convenience and stretch the bar's right side to the far right, for better visual clarity of the scales. This setting will be remembered next time Audacity is launched.
2) Press the Record button and stereo tracks will appear below. Pull the bottom track edge down, to fill the screen for better visual clarity. This change will not be remembered. Tracks can be deleted anytime by pressing an audio track's upper X symbol.
3) Repeatedly press the (- magnifier) sub button, to change visible recording time on the tracks from seconds to minutes. This will compress the recorded sound on the tracks further over to the left. This change will not be remembered.
In most computers, the Audacity recording fader interacts with the pc mic fader. Adjust this so as not to over-peak on levels of the Audacity recording red bars. Over-peaking would overdrive the pc soundcard causing distortion to the recording.
Also ensure the pc software mic boost is switched off.
Therefore the maximum red bar peak levels to aim for, should be no higher than -10dB, but in practice could be just about anything depending on the dynamic range or loudness of the source being recorded, volume level and individual computer characteristics.
If a strong echo caused within the pc soundcard is heard on Record, set the Record fader much lower and raise Playback volume much higher. This action may not completely kill echo but will certainly reduce it. The tradeoff here results in a lower level, but a cleaner recording.
It is just a fact of life that some soundcards and sound modules are more prone to crosstalk between the two-way audio flows from loop-back, than others.
My Win7 laptop has this effect, but my Linux netbook does not. In this situation it should be possible to have the speaker output of one pc feeding the mic input of another, via the ISRA having longer fitted cables.
Continuing on, pressing Stop will terminate the recording. Left mouse wipe the now static recorded sound waveform to darker grey. Press the Play button, green playback bars will appear and sound will come out of the pc speaker socket to headphones.
Any Effects then required such as fade in and fade out for the completed recording, will only work when the wanted sound section in the track, has been highlighted with the mouse to a darker grey.
Do trial runs and make notes to establish a feel for what happens to the program and pc, during recording and the final quality of playback.
The finished wanted sound track is finally exported to a folder to save, when using your own "filename.wav" for Windows or "filename.ogg" for Linux.Sound Sample
Enjoy listening to the finished recordings, either through the headphones or an external stereo loudspeaker amplifier.
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