Edit (Amplify, Noise Reduce, Selectively Silence, etc.) the Audio in a Video Recording on the iPhone/iPad
When you are recording video, it's hard to correct audio problems on the fly. It's easy to have sound that is way too low, too loud, too noisy, capture something you don't want, etc. The easy solution is to just get rid of it (or lower it way down) and add a musical sound track. A more professional solution is to separate out the audio and modify it with a wave editor. There you can make decisions about volume, clarity, and content. Mix the results back into your video and you have a stronger project. On a desktop we would open our video with an audio editor, make our audio corrections, then re-mix together the audio and video with a movie editor. That process is pretty much the same on iOS (iPad/iPhone/iPod).
Preparing the Video
There are two main video formats in iOS, mov and mp4. MP4 movies are usually exported from apps, and MOV videos are recorded from the device camera(s). There are two audio editors (TwistedWave and Hokusai) that will import in the audio from a video. The problem being that they import AAC audio from MP4 videos, so we need to convert the MOVs (from our camera) to MP4s. The Simple Video Converter with Dropbox app ($2.99) from Dexwell Technology LLP will do this. The app's primary purpose seems to be reducing the size/quality of videos so they upload faster. That's not quite why we need it. We don't care what it does to the video (we are going to mix the original video back in), we just want it to open the sound in an audio editor. If the video you are working with is from your iPad/iPhone, launch the app. Otherwise skip to importing the video into TwistedWave or Hokusai.
Tap the blue + icon, then select "Choose from gallery." Select the video. Finally, tap Use.
Normally we would want to avoid this dialog, since videos imported this way are automatically re-compressed (resulting in a loss of quality and time). In this case we don't have a choice and the video won't be used anyway. When the compression progress bar is complete, tap on the underlaying screen.
Notice the format is Quicktime (mov) and there are High, Medium, and Low options. Tap on MPEG4 (mp4) and turn the "Save to gallery" switch to ON.
The video is going from 1280x720 to 480x320. Tap Convert. If you forgot to flip the camera roll switch, don't worry you can save it after the conversion. For now we are ready for the next step.
Transfering Video from the Camera Roll Without Losing Quality
When you are importing video from the camera roll in iOS to an app, often it is re-compressed (resulting in a loss in quality and time to do the compression). If you have the option of trimming the video, the video will definitely be compressed again. Luckily this seems determined by the app developer, and some apps will access the video directly. For instance the Dropbox app will upload a video without loss. If you use Dropbox, though you have to both upload and download the video to send it to another app (inefficient in time and bandwidth). Try the iTransfer app instead.
With iTransfer you can send media directly from your camera roll to an app via "Open In...." Get it from the App Store and launch it.
Under Local Files > Photo Album > Camera Roll, select the video we created. It should have a mp4 extension. In the Preview pane, Tap "Open In" in the bottom button bar. Choose an audio editor app (either TwistedWave or Hokusai).
TwistedWave has fewer filters and no noise correction. Hokusai can be cheaper depending on which features you enable via In-App Purchase. Balance this with Hokusai problems with longer duration audio (30min+), weak noise correction, an lack of audio formats (no mp3). Follow the section on your editor.
TwistedWave Audio Editor
Select TwistedWave in the "Open In" dialog, and the app will load the audio from the movie. When editing audio from a video it is important that you don't change the overall duration or the timing of the audio segments. Modifying them will make your audio not sync the video when we put everything back together. Think old kung-fu movie!
So... if you want to remove pop, click, or a segment where a baby starts crying, don't cut it out. Doing this would change the overall length and move everything afterward forward in time. Select the segment then under Effects (Gears icon, lower left corner), tap Silence.
Let's look at our example... Here you can hardly see any data at all, definitely there is a volume problem. It is big enough a problem that a video editor probably couldn't fix it alone.
Select the entire audio. Under Effects, tap Normalize.
Normalize amplifies the audio so that the loudest portion is brought up to a certain level. Most Professional audio editor apply a level of 80% (~-3dB) for head room. For computer work I tend to discount the need for that and set it at 100% (0dB). Applying Normalize increases volume without cutting off any of the audio.
Now you can see the data, and hear more of the background noise too. Since none of the data was cut off Normalize only increased the volume so far. Let's amplify the audio another 10dB and let some of the audio cutoff.
Now you can actually hear something. This is pretty quick and dirty. You can remove the audio pops, apply noise reduction (from another app), and generally futze with it until you are happy. When you are done tap the upper right Share button. Make sure the format is AAC, then tap "Open In..." for the next step.
Hokusai Audio Editor
There is a lot that is good about this app, so let's quickly run through the problem areas first. I've crashed this app quite a few times. It especially has a problem with long duration audio recordings (30+ mins), so voice recordings of meetings will be a problem. You can't save as mp3. In fact it doesn't support most sound formats (just wav and mp4). Now the good.
You enable features via IAP, so depending on what you need the app costs from free to $9.99 (the same cost as TwistedWave). At the high end you get filters and noise reduction features not found in TwistedWave. For free you get Ads (unfortunately), basic audio editing (cut, paste, and trim), Dropbox support, Open In support, Audio Recording, Multiple Tracks, Convert to Stereo, Convert to Mono, Fade In/Out, Gain (Amplify), Normalize, Silence, Reverse, White Noise, and Synthesiser.
With the Tools Pack ($2.99) you get AudioCopy (Garageband support), AudioPaste, Noise Gate (not that effective), Peak Limiter, and Vocal Levels Smoother. Buying anything IAP removes the Ads.
I don't think the other Packs are very useful, so $2.99 gets you a full featured audio editor with no ads!
The UI for selecting audio is a bit awkward. Tap and hold at the beginning of where you want the selection to start, then drag right or left. As you drag a blue selection area highlights. Once you have a selection you can reposition the whole selection, but not resize it. Tap on the wave area to deselect and move the playhead.
We are going to be working on the entire track so let's enable a gesture that will help us selecting everything. Go to the Settings app then select Hokusai. Under Advanced Gestures, flip the on switch for Select Visible: Double-tap track. All set let's begin.
In iTransfer's Open In dialog select Hokusai. Hokusai loads in the audio from the movie and shows you the waveform data. Here it looks almost empty.
Double tap the wave form to select everything (only if you enable the gesture in Settings). The cut&paste bar appears.
Tap "More..." in the bar.
Under Effects, tap on Normalise.
Now we can see the audio. Normalise amplifies the audio without any cutoff, however background noise may prevent it from increasing the volume enough. Add two more Gain Effects at 200% to compensate.
I think we are ready now to move our new audio to the next step. Tap the "My Documents" button in the upper, left corner.
Tap the Sharing button (upper,left). Select the project we worked on, then tap Export (bottom).
Be sure the the format is set to MPEG-4, and "After export" is set to "Open in Other Application". Tap Done.
Now we are ready to put everything back together in a video editor app.
Video Editor App: Apple's iMovie
There are many good video editors in the App Store, however we need one that accepts audio assets via "Open In". The answer is Apple's iMovie. iMovie allows you to work with one videoclip at a time, and one extra audio track (in addition to the audio in the video). That extra audio track is usually music as a background soundtrack. We are going to use that track to sync together our newly edited audio with the original video. Once we are done we can export our project back into the camera roll and edit the movie within a larger project.
Hopefully we haven't changed the duration or the timing of the audio so audio and video syncs back together.
From our audio editor (either TwistedWave or Hokusai) share the audio via "Open In". Tap on iMovie in the dialog box.
iMovie will import the audio and ask to add it to a new or existing project.
Create a new project.
Tap the Movie icon to see videos in your Camera Roll. Select the original video.
Tap the blue add arrow. Don't trim the movie! The movie (blue box) will be added within the audio track (green box) in the timeline. One video and two audio tracks. Double tap the blue box to adjust the Clip Settings.
It doesn't matter here since the original audio was so low, but turn off the audio in the video clip. Double tap the green bloc to adjust the audio clip.
iMovie does allow you to boost the audio internally (125%), just not as much as an audio editor (200%+). If you play the movie you should see the video and audio sync correctly. Lets export it to the Camera Roll. Tap the Projects button (first, upper left).
Tap the Sharing button, and select Camera Roll. We are done!