SPDIF rate setting.

SPDIF rate setting.

[James Courtier-Dutton]

It seems that more and more people are using SPDIF digital output at 
44.1khz. E.g. DTS tracks on CDs.
Currently, people are having to manually change the SPDIF rate setting 
using iecset or the application is having to play with AES3 in the 
"iec958:AES..." at device open.
How about, when someone opened the device "iec958" and then uses 
snd_pcm_hw_params_set_rate(), it should automatically set the iecset 
rate to equal the rate set by the set_rate function call.

Any comments?

James


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Re: SPDIF rate setting.

[Manuel Jander]

Hi James,

On Fri, 2004-09-17 at 16:00, James Courtier-Dutton wrote:
> It seems that more and more people are using SPDIF digital output at 
> 44.1khz. E.g. DTS tracks on CDs.
> Currently, people are having to manually change the SPDIF rate setting 
> using iecset or the application is having to play with AES3 in the 
> "iec958:AES..." at device open.
> How about, when someone opened the device "iec958" and then uses 
> snd_pcm_hw_params_set_rate(), it should automatically set the iecset 
> rate to equal the rate set by the set_rate function call.
> 
> Any comments?

There is a problem with that, because you are confusing output
samplerate with the application data samplerate. The
snd_pcm_hw_params_set_rate() is set according to the intended sample
rate an application writes to a PCM device, but the samplerate of the
SPDIF stream must be accordning to the external decoder you are using.
If both are different, a soundcard may use some hardware samplerate
converter to cope with that, or just set restriction which allow only
the samplerate suitable to the decoder.

As i have mentioned some tiome ago, i thing there should be some sort of
output device configuration, as an instance to collect and manage the
information of what the Hell is conected to the output jacks of the
users soundcard, and do some software management automatically according
to that. That would allow us to do automatic 5.1 to stereo downmix,
stereo to 5.1 upmix, format output correctly in case of SPDIF output,
arrange channels according to the speaker layout, etc.

Best Regards

Manuel Jander




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Re: SPDIF rate setting.

[James Courtier-Dutton]

Manuel Jander wrote:
> Hi James,
> 
> On Fri, 2004-09-17 at 16:00, James Courtier-Dutton wrote:
> 
>>It seems that more and more people are using SPDIF digital output at 
>>44.1khz. E.g. DTS tracks on CDs.
>>Currently, people are having to manually change the SPDIF rate setting 
>>using iecset or the application is having to play with AES3 in the 
>>"iec958:AES..." at device open.
>>How about, when someone opened the device "iec958" and then uses 
>>snd_pcm_hw_params_set_rate(), it should automatically set the iecset 
>>rate to equal the rate set by the set_rate function call.
>>
>>Any comments?
> 
> 
> There is a problem with that, because you are confusing output
> samplerate with the application data samplerate. The
> snd_pcm_hw_params_set_rate() is set according to the intended sample
> rate an application writes to a PCM device, but the samplerate of the
> SPDIF stream must be accordning to the external decoder you are using.
> If both are different, a soundcard may use some hardware samplerate
> converter to cope with that, or just set restriction which allow only
> the samplerate suitable to the decoder.
> 
> As i have mentioned some tiome ago, i thing there should be some sort of
> output device configuration, as an instance to collect and manage the
> information of what the Hell is conected to the output jacks of the
> users soundcard, and do some software management automatically according
> to that. That would allow us to do automatic 5.1 to stereo downmix,
> stereo to 5.1 upmix, format output correctly in case of SPDIF output,
> arrange channels according to the speaker layout, etc.
> 
> Best Regards
> 
> Manuel Jander
> 

I think it is you who are confused. If a device gets opened with device 
name "iec958:AES...", sample rate converters are disallowed, so the 
application rate will equal the hardware spdif rate.
If the device name is "front" or "default", sample rate converters are 
allowed.
Also, the sound card does not have to match the external decoder, it is 
the other way round, the sound cards sets a rate, and the external 
decoder just matches it. All external decoders can auto sense 44.1 kHz 
and 48 kHz.

James



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Re: SPDIF rate setting.

[Takashi Iwai]

At Fri, 17 Sep 2004 21:00:05 +0100,
James Courtier-Dutton wrote:
> 
> It seems that more and more people are using SPDIF digital output at 
> 44.1khz. E.g. DTS tracks on CDs.
> Currently, people are having to manually change the SPDIF rate setting 
> using iecset or the application is having to play with AES3 in the 
> "iec958:AES..." at device open.
> How about, when someone opened the device "iec958" and then uses 
> snd_pcm_hw_params_set_rate(), it should automatically set the iecset 
> rate to equal the rate set by the set_rate function call.

In some drivers, the IEC958 status bits are automatically updated
according to the sample rate of the PCM.  So, the rate setting isn't a
big problem if the driver is implemented well...


Takashi


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