Confused about 'default' device

[Timur Tabi <timur@freescale.com>]

I'm having a hard time understand when ALSA does sample rate and format 
conversion, and when it doesn't.

When I issue the following command:

	mplayer -ao alsa filename.mov -v

I get this output:

Building audio filter chain for 48000Hz/2ch/s24le -> 0Hz/0ch/s8...
[libaf] Adding filter dummy
[dummy] Was reinitialized: 48000Hz/2ch/s24le
[dummy] Was reinitialized: 48000Hz/2ch/s24le
alsa-init: requested format: 48000 Hz, 2 channels, 11
alsa-init: using ALSA 1.0.8
alsa-init: setup for 1/2 channel(s)
alsa-init: using device default
alsa-init: pcm opend in blocking mode
alsa-init: format s24le are not supported by hardware, trying default
alsa-lib: pcm_plug.c:729:(snd_pcm_plug_hw_refine_schange) Unable to find
an usable access for 'default'
alsa-init: unable to set format: Invalid argument

Sure enough, my device can't handle s24le.  It can only handle s16be.

I was under the impression that ALSA can do format and sample rate conversion if 
necessary, but I guess that's not always the case.  I know ALSA has a bunch of 
plug-ins for that, but I guess they're not used by default.

What do I need to do to tell ALSA to convert s24le to s16be?