Re: (no subject)
Re: (no subject)
- Subject: Re: (no subject)
- From: "Dan Morgan" <email@hidden>
- Date: Tue, 06 Dec 2005 17:14:28 -0500
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) 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?
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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