Re: (no subject)
Re: (no subject)
- Subject: Re: (no subject)
- From: Doug Wyatt <email@hidden>
- Date: Tue, 6 Dec 2005 15:50:25 -0800
If you're using AudioConverter, it's much simpler to simply set
mFormatFlags on the destination lossless stream description to
kAppleLosslessFormatFlag_16BitSourceData
than to do a separate float->int conversion. Brad's right; if you're
going to downsample to a crummy sample rate like 16 kHz, there's no
reason to prefer lossless to AAC.
Doug
On Dec 6, 2005, at 14:14, 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) 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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--
Doug Wyatt
Core Audio, Apple
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