| 00:00 | AU RoundTrip AAC is another tool that can be
used to compare an AAC file to the original
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| 00:05 | source audio file to check for clipping.
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| 00:08 | It includes clip and peak detection, as
well as a simple listening test environment.
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| 00:12 | An audio unit plug-in can be used in any audio
unit host application such as Logic or AULab.
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| 00:19 | The AULab is available as a free download
at apple.com/itunes/mastered-for-itunes.
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| 00:26 | It's mainly designed as an audio unit host
for developers, but it has an application
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| 00:30 | for audio unit auditioning
like this one as well.
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| 00:33 | So after the program is launched, I'm going
to go up and I'm going to say give us a new
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| 00:38 | file, and in this case what we're going to
see is the routing for input and output, and
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| 00:44 | you can just go and select exactly where you
want, and we're just going to keep it where
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| 00:48 | it is in digital input and built-in
line output and create a document.
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| 00:53 | Now, this document in fact gives us an input
channel, but the input channel that it's giving
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| 00:58 | us is the actual input from the microphone,
and this is the input going into this computer.
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| 01:05 | We don't want this in this case, so what we're going to
do is we're going to mute it by clicking on the 1 there.
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| 01:11 | What we do want is another channel,
but it's not exactly what you think.
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| 01:15 | I'm going to go up here to the Edit
window and hit Add Audio Unit Generator.
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| 01:20 | Now it's going to give us a choice of
exactly what we can use as a track here, and we're
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| 01:25 | going to say AUAudioFilePlayer, because really
what we want to do is we want to play a file,
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| 01:32 | so there is a Generator window.
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| 01:34 | Now what we're going to do is go to Effects
and go down to Apple, and you can see what
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| 01:38 | we're just seeing here is all our various
plug-ins that are available in this program,
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| 01:43 | but we really only want the one, and
it's down at the bottom, RoundTrip AAC.
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| 01:47 | Now this gives us a second window.
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| 01:50 | So, now what we want to do is get a file to
play back, so we're going to go up here to
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| 01:55 | our original master of Simply Falling and drag it
over into the window, and then we play it from here.
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| 02:02 | (music playing)
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| 02:05 | And we can actually take it to wherever we
want and set the region, and if we like, we
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| 02:11 | can even loop it as well.
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| 02:14 | But the real trick here is this window here,
it's the RoundTrip AAC Generator window, and
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| 02:19 | if we actually go down and look at Show
Details, this gives us the interesting part of the
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| 02:24 | whole thing, and this is actually going to
show us the distortion that we might see in
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| 02:29 | any peaks or overloads, and even down
to the sample and inter-sample level.
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| 02:34 | Watch when we play...
(music playing)
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| 02:43 | ...and take notice what just happened there.
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| 02:46 | We had two different pieces
of distortion that happened.
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| 02:49 | We had one on the source
file and one on the encode file.
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| 02:52 | And this is the really cool trick about
RoundTrip AAC, it allows us to listen to what the source
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| 02:59 | file, the original 48k 24-bit in this case
file, and what the encode might be, and this
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| 03:06 | is the AAC encode that might be on
the Apple store after they encode it.
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| 03:11 | What we're seeing here is there are two clips.
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| 03:14 | There is one on the source file and another
on the encode, and it comes down and it tells
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| 03:18 | us exactly where those clips were.
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| 03:20 | It says we had a left channel clip, and
it was one Inter-sample clip that happened.
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| 03:26 | That's not very much actually, and
it's not something that we really hear.
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| 03:30 | Inter-sample clips are interesting because there
are really peaks that happen in between each sample.
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| 03:34 | It's not something that we hear as
distortion as much as you put enough of them together
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| 03:40 | and suddenly it doesn't sound clean anymore.
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| 03:42 | So we want to stay away from those if possible,
and of course any sample clips we definitely
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| 03:47 | want to stay away from because
we can definitely hear those.
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| 03:49 | So let's play a little more.
(music playing)
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| 03:54 | I'm going to move up to a place that I know
was kind of loud here, and let's play this.
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| 03:59 | (music playing)
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| 04:12 | Now we can see we have some additional
clips that happen, and let's go back and listen
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| 04:17 | between the source and the encode again
and see if we can hear the difference.
|
| 04:21 | (music playing)
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| 04:40 | What's cool about this is, in fact,
it's almost instantaneous that it happens.
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| 04:44 | There is no latency in between the source
monitor and the encode monitor, which is pretty neat.
|
| 04:50 | The other thing that RoundTrip
AAC gives us is a listening test.
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| 04:54 | If we click on his window here, we
can see it sets up a blind A-B test.
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| 04:59 | Now, most of the time you don't need this at
all, and I'd say the only time you'd probably
|
| 05:03 | use it is if we had a bunch of people around
there that were trying to figure out who had
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| 05:06 | the biggest ears or the most golden ears.
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| 05:09 | But nonetheless, this allows you to in a
blind test, figure out on an average who could hear
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| 05:16 | the most correct number of cycles.
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| 05:20 | And this gives us two modes here, there is a
Test Cycle mode, and there's Training mode,
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| 05:26 | and really, the Training mode is just so
we can kind of figure this out. So watch.
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| 05:30 | (music playing)
That's what our source sounds like.
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| 05:37 | Now we can listen to A and B, and again,
this is blind between the source and the encode.
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| 05:43 | (music playing)
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| 05:51 | Okay, I think this is A,
and it tells me it's correct.
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| 05:59 | So we're back to listening to the source.
(music playing)
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| 06:05 | Okay, now I think this is B.
(music playing)
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| 06:09 | And you can see it tells us we're incorrect.
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| 06:13 | If I uncheck Training mode, we'll actually
do this 20 times, and at the end of the 20th
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| 06:18 | time it will take an average of all
those to see exactly how good our ears are.
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| 06:24 | And once again, this doesn't really prove
anything except how well you can hear the
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| 06:29 | AAC encoder or if you can't hear it at all.
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| 06:32 | So as you can see, AU RoundTrip shows you
where any overloads are occurring on either
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| 06:36 | the source file or the encode.
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| 06:39 | iTunes won't reject the file
with distortion or clipping.
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| 06:42 | If you know where it's at, you can fix it
before it is submitted so your listeners get
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| 06:45 | the best sounding songs available.
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| 06:50 |
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