[Jack-Devel] How does --hwmon work? (was: How does --hwmix work?)

David Kastrup dak at gnu.org
Thu May 4 10:21:02 CEST 2017

Ralf Mardorf <ralf.mardorf at alice-dsl.net> writes:

> On Wed, 03 May 2017 18:07:31 +0200, David Kastrup wrote:
>>Without --hwmon, port monitoring  requires JACK to read
>>		audio into system memory, then  copy it back out to the
>>		hardware again, imposing the  basic JACK system latency
>>		determined by the --period and --nperiods parameters.
> If you are using hdspmixer, then "Without --hwmon" routing the signal
> could be done without latency,

Can we focus on the question of what happens _with_ --hwmon to jackd
and/or the "via Audio Driver" monitoring option in Ardour?

I know what the mixer hardware can do in general and specifically when
using hdspmixer (or the equivalent amixer -d hw:DSP cset numid=5 ...).
But that's not relevant to Jack and wasn't my question.

> IOW you do _not_ need to use the "--hwmon" option. FWIW "zero latency"
> doesn't mean 100% "zero latency", it just means that it is "zero
> latency" for our perception and even for the perception of an autistic
> savant. You at least get the latency caused by the speed of light and
> delay by a few electronic components.

Sigh.  I even listed the latencies involved here already.  Since the
mixing is in the digital domain, you get buffering delays and the delays
for the down-/up-sampling filters of A/D-D/A conversion, roughly 2ms all
in all.

> There is no software that supports "--hwmon", but seemingly some audio
> interfaces allow hardware monitoring, when using this option.

That does not even make sense.  The audio interface needs to get passed
information by some software in order to do something specific.  It does
not have a button labelled "--hwmon" on its box.

> If you are using hdspmixer you don't need it

I was not talking about using hdspmixer.  The question was what effects
jackd --hwmon and/or the Ardour monitoring option "via Audio Driver"
(discovered after the initial question was already posed but very likely
related) have.

Also the question was where information about supported cards and the
used API to Jack were to be found since the manual page describing the
options represented the status as of 2003 AD.

> and if you are using a mixing console, you even don't need hdspmixer
> to do hardware monitoring.

And if I am using a tape drive, I don't even need a computer.  But let's
assume, for the sake of solving this question, that I am using a
computer, not a mixing console, an RME Hammerfall DSP (or some other
supported card, whatever it may be) and am not using hdspmixer in order
to set up a monitoring solution independent of Jack and/or Ardour.

David Kastrup

More information about the Jackaudio mailing list