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AU Roundtrip AAC is another tool that can be used to compare an AAC file to the original source audio file to check for clipping. It includes clip and peak detection as well as a simple listening test environment. The audio unit plug-in can be used in any audio unit host applications, such as Logic or AU Lab. AU Lab is available as a free download at apple.com/itunes/mastered-for-itunes and is mainly designed as an audio unit host for developers. But it has an application for audio unit auditioning like this one as well.
So after the program is launched, I am going to go up and I'm going to say give us a new file, and in this case what we are going to see is the routing for Input and Output, and you can just go and select exactly where you want. I am just going to keep where it is in Digital Input and Built-in Line Output and create a document. Now this documented in fact gives us an Input Channel, but the Input Channel that it's giving us is the actual input from the microphone, and this is the input going into this computer.
We don't want this in this case, so what we are going to do is we are going to mute it by clicking on the 1 there. What we do want is another channel, but it's not exactly what you think. We are going to go up here to the Edit window and hit Add Audio Unit Generator. Now it's going to give us choice of exactly what we can use as a track here, and we are going to say AUAudioFilePlayer, because really what we want to do is we want to play a file. So there is a generator window, and now what we are going to do is go to Affects > Apple, and you can see, what we're seeing here is all our various plug-ins that are available on this program, but we really only want the one, and it's down at the bottom, Roundtrip AAC.
Now this gives us a second window. So now what we want to do is get a file to playback, so we are going to go up here to our original master of Simply Falling and drag it over into the window, and now we play it from here. (music playing) And we can actually take it to wherever we want and set the region, and if we'd like we can even loop it as well. But the real trick here is this window here, it's the Roundtrip AAC Generator Window, and if we actually go down and look at Show Details, this gives us the interesting part of the whole thing, and this is actually going to show us the distortion that we might see in any peaks or overloads, and even down to the sample and inter-sample level.
Now watch when we play. (music playing) Now take notice what just happened there. We had two different pieces of distortion that happened. We had one on the source file and one on the encode file, and this is the really cool trick about Roundtrip AAC, it allows us to listen to what the source file, the original 48K 24-bit, in this case file, and what that encode might be in this, the AAC encode that might be on the Apple Store after they encode it.
What we are seeing here is there were two clips, there's one on the source file and another on the encode, and it comes down and tells us exactly where those clips were. It says we had our left channel clip, and there was one inter-sample clip that happened. That's not very much actually, it's not something that we really hear. Inter-sample clips are interesting because there are really peaks that happen in between each sample. It is not something that we hear as distortion as much as you put enough of them together and suddenly it doesn't sound clean anymore.
So we want to stay away from those if possible, and of course, any sample clips, we definitely want to stay away from because we can definitely hear those. So let's play a little more. (music playing) I am going to move up to a place that I know was kind of loud here, and let's play this. (music playing) And we can see we have some additional clips that happened, and let's go back and listen between the source and the encode again, and see if you can hear the difference.
(music playing) And what's cool about this is in fact, it's almost instantaneous that it happens, there is no latency in between the source monitor, and the encode monitor, which is pretty neat. The other thing that Roundtrip AAC gives us is a listening test.
If we'd click on this window here, we can see it sets up a blind AB test. Now most of the time you don't need this at all, and I'd say the only time you'd probably use it if we had a bunch of people around there trying to figure out who had the biggest ears, or the most golden ears, but nonetheless, this allows you to in a blind test to figure out on an average who could hear the most correct number of Cycles. And this gives us two modes here, there is a Test Cycle mode, and there is Training mode, and really the Training mode is just so we can kind of figure this out.
I am going to play with it just so watch. (music playing) That's what our source sounds like. Now we can listen to A and B, and again this is blind between the Source and the Encode. (music playing) Okay, I think this is A, and it tells me it's correct. So we're back to listening to the Source (music playing) Okay, now I think this is B.
And you can see it tells us we are incorrect. If I uncheck Training mode, we will actually do this 20 times and at the end of the 20th time, it will take an average of all those to see exactly how good our ears are. But once again, this doesn't really prove anything except how well you can hear the AAC encoder or if you can't hear it at all. So as you can see, AU Roundtrip shows you where any overloads are occurring, in either the Source file or the Encode. iTunes won't reject the file with distortion or clipping, but if you know where it's at, you can fix it before it's submitted so your listeners get the best sounding songs available.
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