Re: [Jack-Devel] low latency . what does it mean? how is it achieved?

On Wed, Apr 16, 2014 at 2:48 AM, Athanasios Silis <
[hidden]> wrote:

> Hi all,
> I find myself wondering again and again what makes a difference between a
> simple AC '97 audio chip and a professional sound card...
>
> I mean the chips are different, sure, but I always hear about
> "professional equipement, does not go through the normal application
> execution path", or something in those lines.
>

marketing BS from the old windows days when the only "normal" path for
sound added 100ms. this has not been true for a long time on windows, and
has never been relevant on linux.


>
> both ac97 and professional chips go through ALSA (in linux at least). what
> gives an edge to the professional chip then? Why can I not run an ac97 with
> a latency of 10msec or so (and a low latency kernel) ? I mean, the
> processing power of a core i7 should be enough to run audio, right?
>

latency rarely has anything to do with processing power. latency is about
the ability to meet deadlines. it is not dependent on processing power
(typically) but rather on the lack of operations that take a long time and
prevent sufficiently "on-time" processing of audio data.

the key features of "pro" audio interfaces are typically:

    * playback and capture streams run from the same sample clock and are
thus always synchronized
    * often have analog connections outside the radio-frequency-noisy
environment of the computer chassis
    * have the option to use an external sample clock, enabling correct
integration with multiple devices
    * lack of onboard DSP (though this varies by manufacturer)
    * ability to handle different sample rates in the hardware without
relying on resampling by the operating system device driver

the ac97 chipset itself isn't particularly bad, but it is typically mounted
on the motherboard along with analog connection points. doing
analog->digital and digital->analog conversions in this environment is not
ideal. in addition, the ac97 chipset and specification has a lot of stuff
associated with consumer/home "theater" requirements that are not of
interest for use in pro-audio or music creation scenarios.

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