First, I recommend you use Wasapi whenever possible, because it offers loopback (I'll explain this in a bit). Wasapi has been around since Vista, so this should be available to pretty much any computer these days unless you're still using XP. Not sure how Audacity handles Wasapi, but in Gold Wave, if you set your system to wasapi/core audio and check recording devices, you will see things like "loopback speakers - (audio device name)". This means that anything that you hear over that audio device will be recorded. It's a lot like stereo mix, but even if you have stereo mix and it works well, I still recommend Loopback because stereo mix tends to add noise, distortion or worse from the sound card's internal circuitry. This is best illustrated on my laptop which does have stereo mix (it's called rec. playback), but it's very quiet and adjusting the volume only affects the left channel. It is also very hissy. Loopback is different because it completely bypasses the sound card's hardware. It's a digital capture. What you hear is exactly what you get.
The only thing Loopback won't be able to capture is the stuff you get on some sound cards that have hardware or 0-latency monitoring. Most Realtechs in desktop machines from my experience allow you to adjust monitor levels for microphone, line-in etc. in addition to changing speaker volume. Since these are hardware features, stereo mix can pick them up but Loopback cannot. In order to hear them over Loopback you have to use software to send the audio to the sound card, such as Virtual Audio Cable or Windows listen feature which Cae_Jones described:
CAE_Jones wrote:Go into recording devices (open the context menu for the volume control in the system tray, and press r), select your microphone, and go into properties (alt+p, I think?). You need to find the "listen to this device" checkbox (I think it's on the second tab) and check it. If done correctly, you should hear your voice through your speakers.
The only problem with transferring sound via software is lag. Your microphone audio will be delayed slightly through your headphones and you may find this distracting. If you don't have good headphones or microphone setup, you may hear some echoing or bleed-through caused by the delay. Your microphone will also be slightly behind the game audio, though I doubt it will be enough to really worry about. It works for some people who just want to record, but if you're a perfectionist like me, you'll want to do it differently.
What I do when I record my playthroughs is to record both parts separately. I talk over the game like I normally would, but I record loopback for the game, and record my microphone in a separate file. This way I have more control in postproduction. I generally leave the Loopback recording as is with no processing, but will process my microphone by removing some noise, applying eq and compression, and doing other things that I think make it sound better. Then I mix them together and adjust the balance between the two. Unfortunately I sometimes run into issues with Loopback and the microphone going out of sync so I have to re-sync them every so often, and that is a royal pain. But that's how I do it. I don't monitor my mic at all while recording, so I have to test things before I start to make sure they sound good, and hope nothing stupid happens. I wish I could monitor, but that would be picked up over Loopback, and I want to keep the game and microphone separate until postproduction is done.
I wonder if there's a program that could record audio from a specific application only? Hypothetically it's possible, since the Windows volume mixer allows adjusting the volume of individual applications, and there is software that can route an application's audio to a different device.
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