Re: (no subject)
Re: (no subject)
- Subject: Re: (no subject)
- From: Brad Ford <email@hidden>
- Date: Tue, 6 Dec 2005 15:06:10 -0800
On Dec 6, 2005, at 2:14 PM, Dan Morgan wrote:
Brad -
Thank you very much for your reply.
You are right that I am recording at 24 bits.
I suspected that this might be a problem, so I did try first
converting
to 16 bits (and a sample rate of 16kHz)
???? Sample rate has nothing to do with it. You should leave sample
rate alone. It's silly to downsample something that's ultimately
going to be compressed with Lossless. In fact, it's silly to convert
to 16-bits to compress with greater efficiency to lossless, because
at the end of the day, you've got a lossly recording. If you're just
trying to make a smaller file, use AAC. AAC makes very small files
and sounds very good.
and then applying an audio unit
format converter at the post-conversion callback to change from
lpcm to
alac format. Did I do a reasonable thing?
I'm not really sure what you're doing. BTW, WhackedTV uses Sequence
Grabber, which is a QuickTime API, so you might be better served
directing questions to the quicktime-api list.
It sounds like you've set up sequence grabber to not capture a movie,
then you're using the post-conversion callback to feed an audio unit
chain of your own? If so, then yes, that's a perfectly legitimate
thing to do, except that you're making more work for yourself. You
could set the output format 'alac', then sit on the post-conversion
callback and get the lossless packets delivered to you, already
compressed for you by SGAudioChannel, which uses an AudioConverter.
HTH.
-Brad Ford
QuickTime Engineering
Dan
From: Brad Ford <email@hidden>
To: Dan Morgan <email@hidden>
CC: email@hidden
Subject: Re: (no subject)
Date: Tue, 6 Dec 2005 12:06:09 -0800
On Dec 6, 2005, at 11:34 AM, Dan Morgan wrote:
I have read that apple lossless will give about a factor of two
compression,
but I was not seeing this when I used it. The audio being
recorded was the
sound of a conference room. I tried a variety of different
approaches in
my code and could not get it to compress, so I tried running
WhackedTV
with output format set to apple lossless, and it also did not
compress.
Has anyone noticed this before?
Is apple lossless tuned to particular types of data and just
does not work
well on other types of data?
Apple Lossless typically gets 2:1 compression ratios, ***provided
the source is 16-bit***. It's highly optimized for 16 bit. I've
seen it do even better than 2:1 with recordings with a lot of
silence. If you feed the compressor larger integer samples, such
as 20, 24, or 32- bit, the compression efficiency goes down.
I just verified with WhackedTV that when I record from a 16-bit
input device (the built in microphone on my powerbook) to
lossless, its data rate is right around 50% of the data rate of
uncompressed 16- bit. I suspect you're recording from a 24-bit
source.
-Brad Ford
QuickTime Engineering
Thanks,
Dan
P.S. Apologies if this has been discussed previously. I tried
searching the
archives but did not find this topic.
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