Re: [LAU] Intel-hda jack low latency howto

From: Jack O'Quin <jack.oquin@email-addr-hidden>
Date: Mon Nov 03 2008 - 18:01:58 EET

On Mon, Nov 3, 2008 at 2:36 AM, Matthew Smith <matt@email-addr-hidden> wrote:
> Quoth Arnold Krille at 2008-11-03 18:57...
>> One of the design decisions of the hda is to support 48 kHz for DVD. And to
>> make it simple they only support 48kHz in hardware. While you can open the
>> device with lower samplingrates this will get resampled to 48kHz either by
>> the driver or the chip. And as you don't know what quality that resampling
>> algorithm has (remember the only thing "high" in the hda is the name), you
>> will want to use 48kHz from start to end and only downsample to 41kHz
>> directly before creating the cd master. Which has the advantage that you can
>> use the fine libsamplerate instead...
>
> Interesting. I wonder, does it explain this?
>
> Whilst I use fully external ADC/DACs for music use, with the HDA card
> disabled, for everyday use - which means Skype since I don't do any
> other sort of everyday audio - I use a Zoom H2 as my microphone/input
> sound card and have headphones plugged into the on-board HDA.
>
> If I have the H2 set to sample at its default of 44.1ksps, playing back
> my voice on a Skype test call, I sound like someone who should be
> guarding a hareem ;-) Setting the H2 to sample at 48ksps, I sound my
> <cough/> "normal" self.
>
> Would it just be at the receiving end of the loop that my voice is
> pitch-shifting, or is there some funny re-sampling going on in the Skype
> software?
>
> I'm just so glad that I tested the sound before calling a client!

That can happen if you set the JACK sample rate to -r44100. JACK's
ALSA backend requests a sample rate "near" the requested value. ALSA
responds with 48000 because that is all the HDA device actually
supports. Since JACK does not resample, the pitch effectively gets
shifted up a step or so with the tempo gets speeded up as well.

This is not good. We should probably add a specific test for this
too-frequent gotcha in the ALSA backend. If it requests 41000 and
gets 48000 it should at least complain, and perhaps even fail to start
in that case.

-- 
 joq
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Received on Mon Nov 3 20:15:01 2008

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