[alsa-devel] iec958 and 24-bit audio

When I try playing 24-bit audio files on my SPDIF output, I get this

[alsa-lib]$ aplay -Diec958 file24bits.wav aplay: set_params:1059: Sample format non available Available formats: - S16_LE - S32_LE

Is there an built-in assumption that the iec958 device can only take as input what the slave hardware device supports? The IEC subframes can convey 24 bit audio, and it's possible to insert the 24 bits starting from bit 4 when adding the preambles and status bits. What am I missing? Thanks, -Poerre