To answer my own question,
When setting the sample rate, I must also set "mChannelsPerFrame" to
1, then it works.
No idea if that is the correct thing to do.
I also have found another quirk.
It seems if I ask for a small enough chunk of samples,
MovieAudioExtractionFillBuffer returns with no error, but the data is
all 0. I have to trace this down more since it could be me doing
something odd, so unless there is another undocumented limitation,
don't look into this too closely till I can be sure its not me :)
On 22-May-06, at 3:29 PM, Steve Israelson wrote:
- I want to extract and convert the audio from a single track.
How do I specify this? I can see how to specify left or right
etc, but that mixes all left and rights together, but I want
say the left of a single track only.
There are two ways.
1) iterate through the tracks in the source movie, get the track
layout, and set the channel labels you want to exclude to
kAudioChannelLabel_Unused. Note that if you leave, say,
multiple "Left's" labelled as "Left", they will still all mix
into a single "Left", unless you specify:
2) kQTMovieAudioExtractionMoviePropertyID_AllChannelsDiscrete on
the MovieAudioExtractionSession. This causes QuickTime to do
_no_ mixing. You get all the source channels in all the audio
tracks in the movie (in order) if you set this property.
So, if the track in question is stereo, and all others are set to
"unused", then I just pick out the left or right track as needed.
Just pick out the left or right _channel_ as needed. I think
that's what you meant though.
Is there any other track layouts, like say 5.1 where I have more
options than just left and right PER TRACK?
I'm not sure I understand the question. QuickTime audio tracks
now allow any number of channels of audio, and those channels can
be labelled using any of the AudioChannelLabels supported by
CoreAudio (see CoreAudioTypes.h). You need to be prepared for
strange things -- for instance, you could have an audio track with
10 channels of audio, each one labelled "Right". QTPro Users can
relabel audio channels at will in the Movie Jinfo window.
Ok, so here is what I am doing:
MovieAudioExtractionBegin(...)
MovieAudioExtractionGetProperty
(kQTMovieAudioExtractionAudioPropertyID_AudioStreamBasicDescription)
// here I set the sample rate to what I want: 44,100
MovieAudioExtractionSetProperty(...) // for above sample rate change
// choose left channel
AudioChannelLayout layout;
layout.mChannelLayoutTag = kAudioChannelLayoutTag_UseChannelBitmap;
layout.mChannelBitmap = kAudioChannelBit_Right;
MovieAudioExtractionSetProperty(...)
// finally, extract some data:
MovieAudioExtractionFillBuffer()
And it then returns an improper call order error. Huh?
Should I set it to left, then set the sample rate?
Essentially all I want is ONE specific channel of audio at a sample
rate I specify. I am using "left" for testing, but the user can
choose any channel.
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