So, I had a friend who has a load of recordings from about 10 years ago which were done on a weird dictophone. The files had the extension .FC4 which according to the internet is a legacy Amiga audio format with no more support. Great.

First thing was to run file on it:

$ file t.FC4 
t.FC4: data

Great. Let’s see if we can do a better job looking at a hex dump (with xxd):

0000000: 4649 4c45 0103 0101 0333 0fff ffff ffff  FILE.....3......
0000010: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000020: aa10 1f40 01ff ffff ffff ffff ffff ffff  ...@............
0000030: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000040: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000050: 4d49 2d53 4334 ffff ffff ffff ffff ffff  MI-SC4..........
0000060: 4456 522d 3030 37ff ffff ffff ffff ffff  DVR-007.........
0000070: 4130 322d 3033 3031 3031 3033 3531 3135  A02-030101035115
0000080: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000090: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000a0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000b0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000c0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000d0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000e0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000f0: 4643 34ff ffff ffff ffff ffff ffff ffff  FC4.............
0000100: 5249 4646 501f 0000 5741 5645 666d 7420  RIFFP...WAVEfmt 
0000110: 1400 0000 5003 0100 401f 0000 2910 0000  ....P...@...)...
0000120: 1e00 0000 0200 3a00 6461 7461 0000 0000  ......:.data....
0000130: 00fe ffff feff fffe ffff feff ffef 55ff  ..............U.
0000140: feff feff feff feff feff effe feff efef  ................
0000150: efef efef efef efef efef efef 55ef efef  ............U...
0000160: effe feff efef feff effe ffff ffff ffff  ................
0000170: ffff ffff ffff ffff ffff 55ff ffff ffff  ..........U.....
0000180: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000190: ffff ffff ffff ffff 55ff ffff ffff ffff  ........U.......
00001a0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00001b0: ffff ffff ffff 55ff ffff ffff ffff ffff  ......U.........
00001c0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00001d0: ffff ffff 55ff ffff ffff ffff ffff ffff  ....U...........
00001e0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00001f0: ffff 55ff ffff ffff ffff ffff ffff ffff  ..U.............
0000200: ffff ffff ffff ffff ffff ffff ffff ffff  ................
0000210: 55ff ffff ffff ffff ffff ffff ffff ffff  U...............
...
00006e0: ffff ffff fd88 3fe1 e1e1 e1ef d21e 1e1e  ......?.........
00006f0: 1fff ffff c821 e1e1 bb32 f2d1 55f1 e1e1  .....!...2..U...
0000700: dbac f61f ffff ac15 fe2e 1e1d 8f2f 1e1e  ............./..
0000710: dc2d 4fe1 d9d4 f3ef ed31 55b2 e219 b4fb  .-O......1U.....

So, looks like at offset 0x100 (256) we have something that is a RIFF/WAV file, then the stuff that shows as U is probably a chunk-size block or somesuch. Given the blocks of data afterwards it could probably be 16-bit single channel at a guess. Perhaps something can read that if we cut the initial header off and re-save:

$ xxd -s -256 -r t out.wav
$ file out.wav 
out.wav: RIFF (little-endian) data, WAVE audio, mono 8000 Hz

Ah-ha looks like file has a clue now. Let’s try to play it:

$ mplayer out.wav
...
Requested audio codec family [sc4] (afm=acm) not available.
Enable it at compilation.
Cannot find codec for audio format 0x350.
...

D’oh. Opening as a raw file in audacity shows pretty much white-noise (whereas you’d have expected it to be something vaguely like speach but with blips in every so often if it was any sort of valid PCM or wave type encoding).

After searching around for a long time I discovered this post which talked about a very similar looking header and especially WAV encoding 0x350. This linked to an mplayer plugin with an acm and inf file however the ubuntu version of mplayer doesn’t support w32codecs. I tried installing this in several different ways in a windows 7 vm but couldn’t get it to work.

I then tried compiling mplayer from source only to be greeted with:

cc -MMD -MP -Wundef -Wall -Wno-switch -Wno-parentheses -Wpointer-arith -Wredundant-decls -Werror=format-security -Wstrict-prototypes -Wmissing-prototypes -Wdisabled-optimization -Wno-pointer-sign -Wdeclaration-after-statement -std=gnu99 -Werror-implicit-function-declaration -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -D_ISOC99_SOURCE -I. -Iffmpeg -O4 -march=native -mtune=native -pipe -ffast-math -fomit-frame-pointer -fno-tree-vectorize -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE  -fpie -DPIC -D_REENTRANT  -I/usr/include/freetype2 -DZLIB_CONST -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -c -o loader/wrapper.o loader/wrapper.S
loader/wrapper.S: Assembler messages:
loader/wrapper.S:31: Error: `pusha' is not supported in 64-bit mode
loader/wrapper.S:34: Error: operand type mismatch for `push'
loader/wrapper.S:38: Error: operand type mismatch for `push'
loader/wrapper.S:40: Error: operand type mismatch for `push'
loader/wrapper.S:45: Error: operand type mismatch for `push'
loader/wrapper.S:46: Error: operand type mismatch for `push'

D’oh. Rather than mess around with trying a 32-bit compile or hacking the assembly I remembered I had a 10-year old laptop lying around with a very old 32-bit install of gentoo. Power it up, install the codec files and it plays them!

I then try to extract some proper PCM WAV file from the FC4 file using mencoder. But mencoder doesn’t support audio-only. I also try using the -dumpstream option in mplayer but that just dumps the encoded audio. So finally I come across the -ao pcm option which puts out a nice plain wav file that I can encode into mp3 or any other format.