On Sat, May 09, 2015 at 06:40:54PM +0100, Russell King - ARM Linux wrote:
> Even VLC _doesn't_ if it's outputting to a standard audio - in other
> words, if you don't tick the SPDIF direct output option which defaults
> to disabled (which, when enabled, opens the device passing the AES
> bits _and_ permits it to send a compressed audio stream.) I've looked
> at this in VLC many times...
FYI, here's the code:
vlc_fourcc_t fourcc = aout->format.i_format;
bool spdif = false;
switch (fourcc)
{
... other linear float/integer codec IDs ...
case VLC_CODEC_U16B:
pcm_format = SND_PCM_FORMAT_U16_BE;
break;
case VLC_CODEC_U16L:
pcm_format = SND_PCM_FORMAT_U16_LE;
break;
...
default:
if (AOUT_FMT_SPDIF(&aout->format))
spdif = var_InheritBool (aout, "spdif");
if (spdif)
{
fourcc = VLC_CODEC_SPDIFL;
pcm_format = SND_PCM_FORMAT_S16;
}
else
if (HAVE_FPU)
{
fourcc = VLC_CODEC_FL32;
pcm_format = SND_PCM_FORMAT_FLOAT;
}
else
{
fourcc = VLC_CODEC_S16N;
pcm_format = SND_PCM_FORMAT_S16;
}
}
/* Choose the IEC device for S/PDIF output:
if the device is overridden by the user then it will be the one.
Otherwise we compute the default device based on the output format. */
if (spdif && !strcmp (device, "default"))
{
unsigned aes3;
switch (aout->format.i_rate)
{
#define FS(freq) \
case freq: aes3 = IEC958_AES3_CON_FS_ ## freq; break;
FS( 44100) /* def. */ FS( 48000) FS( 32000)
FS( 22050) FS( 24000)
FS( 88200) FS(768000) FS( 96000)
FS(176400) FS(192000)
#undef FS
default:
aes3 = IEC958_AES3_CON_FS_NOTID;
break;
}
free (device);
if (asprintf (&device,
"iec958:AES0=0x%x,AES1=0x%x,AES2=0x%x,AES3=0x%x",
IEC958_AES0_CON_EMPHASIS_NONE | IEC958_AES0_NONAUDIO,
IEC958_AES1_CON_ORIGINAL | IEC958_AES1_CON_PCM_CODER,
0, aes3) == -1)
return VLC_ENOMEM;
}
...
/* VLC always has a resampler. No need for ALSA's. */
const int mode = SND_PCM_NO_AUTO_RESAMPLE;
int val = snd_pcm_open (&pcm, device, SND_PCM_STREAM_PLAYBACK, mode);
So, the result is:
* VLC opens the ALSA device _without_ AES information if the fourcc
being passed if the format is a linear PCM type.
* If the format is not a linear PCM type _and_ it is a format which
satisfies AOUT_FMT_SPDIF(), _and_ the SPDIF passthrough option is
enabled, _then_ we open the IEC958 audio output device with the
first four AES data bytes specified with SND_PCM_FORMAT_S16.
Moreover, the first data byte always indicates that the stream is
non-audio.
So, in the case of linear PCM, the AES data bytes are *not* specified
by VLC.
So, if we don't have the kernel driver specifying the IEC958 information
for HDMI/SPDIF outputs, and users of ALSA's userspace APIs don't generate
them for linear PCM, how are the required AES data bytes generated?
--
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.
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