[mythtv-users] AC3 passthrough not working

Richard Shaw hobbes1069 at gmail.com
Sun Apr 12 01:44:00 UTC 2009


On Sat, Apr 11, 2009 at 8:31 PM, Allen Edwards
<allen.p.edwards at gmail.com> wrote:
>
>
> On Fri, Apr 10, 2009 at 9:09 AM, Glenn Sommer <glemsom at gmail.com> wrote:
>>
>> Hey
>>
>> I'm trying to get AC3 / DTS passthrough to work in MythTV 0.21-fixes.
>>
>> This is what I've done sofar.
>>
>> 1. Tested the SPDIF output works using mplayer (mplayer -ac
>> hwa3c,hwdts -ao alsa:device=hw=0.1)
>>
>> 2. Configured MythtTV:
>> Audio output device = ALSA:default
>> Passthrough output device = ALSA:iec958( AES0 0x02 )
>> Max Audio Channels = Stereo
>> Upmix = Passive
>> Enable AC3 to SPDIF passthrough = checked box
>> Enable DTS to SPDIF passthrough = Checked box
>> Aggressive Sound Card = Not checked box
>> Use internal volume control = Not checked box
>>
>> (All other channels NOT using AC3 or DTS are working fine over spdif)
>>
>> aplay -L says:
>> mythtv at mediecenter ~ $ aplay -L
>> default:CARD=Intel
>>     HDA Intel, ALC888 Analog
>>     Default Audio Device
>> front:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     Front speakers
>> surround40:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     4.0 Surround output to Front and Rear speakers
>> surround41:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     4.1 Surround output to Front, Rear and Subwoofer speakers
>> surround50:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     5.0 Surround output to Front, Center and Rear speakers
>> surround51:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     5.1 Surround output to Front, Center, Rear and Subwoofer speakers
>> surround71:CARD=Intel,DEV=0
>>     HDA Intel, ALC888 Analog
>>     7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
>> null
>>     Discard all samples (playback) or generate zero samples (capture)
>>
>> aplay -l says:
>> mythtv at mediecenter ~ $ aplay -l
>> **** List of PLAYBACK Hardware Devices ****
>> card 0: Intel [HDA Intel], device 0: ALC888 Analog [ALC888 Analog]
>>   Subdevices: 1/1
>>   Subdevice #0: subdevice #0
>> card 0: Intel [HDA Intel], device 1: ALC888 Digital [ALC888 Digital]
>>   Subdevices: 1/1
>>   Subdevice #0: subdevice #0
>>
>> 3. I've ensured no /etc/asound.conf or ~/.asound.conf files exist!
>>
>> When trying to watch something containing AC3, I shortly see my
>> receiver says Dolby Digital... By looking at my receiver - it seems
>> that it knows an AC3 stream is there, but it contains 0 channels (my
>> receiver shows the channels in use, and all channels are off when
>> using MythTV).
>>
>> When using mplayer everything works as expected.
>>
>> Clearly I must have missed something obivios here - I just cannot
>> figure out what?
>
> Why on earth would you want to resample.  The goal should bit true digital
> output.  Pass the bits to the receiver and let it do a DAC.  For best
> quality, don't decode, encode, decode just decode.
> Allen

He may not have a choice. Some sound cards will not output anything
other than 48kHz. My nForce3 based MB didn't seem to want to but then
again, I haven't tried now that I've gone to an AMD chipset MB.

Richard

Richard


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