[alsa-devel] IEC switch issues

Clemens Ladisch clemens at ladisch.de
Wed Jun 23 17:10:01 CEST 2010

pl bossart wrote:
>>> If I play my AC3 data on my HDAudio/SPDIF output using the hw:0,1
>>> device, I can use alsamixer/amixer to mute/unmute.
>>> Now if I use the iec958: plugin, I can't control the mute switch any longer:
>> Muting an AC-3 stream would require encoding a stream of silent PCM
>> samples (no data is not the same as silence); therefore, the iec958
>> plugin disallows muting.
> Actually the problem happens also with plain PCM rendered on the
> iec958 device.There's no way to use the IEC switch.
> So why do we have an S/PDIF switch in the first place?

Because the hardware has this switch; the driver just exposes all
hardware features.  

> If it cannot be used when you sent IEC-formatted data (be that PCM
> or AC3-formatted data), then why bother?

Some codecs allow looping back the ADC output to the SPDIF transmitter.
In these cases, disabling the SPDIF transmitter might be useful.


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