[FFmpeg-user] direct stream copy AC3 to HDMI

[drwho]

ffmpeg (2.8.11-0ubuntu0.16.04.1, and current snapshot) is converting my 
ac3 file to pcm when I try to play it to hdmi.  Is there away to just 
demux it and send it "direct stream copy" over the hdmi so the Audio 
Video Receiver can decode it?  I am successful using aplay, but not ffmpeg.

// AVR shows Dolby Digital
aplay -v ac3_48k.ac3 -r48000 -c2 -fS16_LE -Dhdmi:CARD=PCH,DEV=2

// AVR shows PCM Stereo
ffmpeg -i ac3_48k.ac3 -f alsa hdmi:CARD=PCH,DEV=2

ffmpeg -i ac3_48k.ac3 -c:a copy -f alsa hdmi:CARD=PCH,DEV=2
   Could not write header for output file #0 (incorrect codec parameters 
?): Function not implemented

ffprobe -show_format ac3_48k.ac3

Input #0, spdif, from 'ac3_48k.ac3':
   Duration: 00:00:07.01, bitrate: 1536 kb/s
     Stream #0:0: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
[FORMAT]
filename=ac3_48k.ac3
nb_streams=1
nb_programs=0
format_name=spdif
format_long_name=IEC 61937 (compressed data in S/PDIF)
start_time=N/A
duration=7.008000
size=1345536
bit_rate=1536000
probe_score=100
[/FORMAT]

aplay -L

hdmi:CARD=PCH,DEV=2
     HDA Intel PCH, HDMI 2
     HDMI Audio Output

hw:CARD=PCH,DEV=8
     HDA Intel PCH, HDMI 2
     Direct hardware device without any conversions

Jon