[Jack-Devel] How does --hwmix work?

David Kastrup dak at gnu.org
Wed May 3 08:05:04 CEST 2017

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

> On Tue, 02 May 2017 21:59:48 +0200, David Kastrup wrote:
>>I have an RME Hammerfall DSP, a card that I believe supports --hwmix on
>>jackd.  How does this work?  Does this require special support by the
>>application?  If so, which applications exist using it via Jack?
> Hi,
> I don't know what this option should do [1], maybe you are confusing
> it with hwmon. IIUC HWMIX is a feature to use a sound card without a
> sound server, but since jack is a sound server, this option might be
> irrelevant. I might be mistaken ;).
> However, if you want a hardware mixer, you don't need a jack option,
> instead try running   hdspmixer   . I suspect hdspmixer doesn't work
> with all RME cards.

hdspmixer works with the Hammerfall DSP but it writes all over the mixer
right on startup.  Not suitable for changing single settings: I am using
alsactl for that right now.

> [1]
> [rocketmouse at archlinux ~]$ man jackd | grep hwmix -A3
> [rocketmouse at archlinux ~]$ man jackd | grep hwmon -A3
>        -H, --hwmon
>               Enable hardware monitoring of capture ports.  This is a
> method for obtaining "zero latency" mon‐ itoring  of  audio  input.
> It requires support in hardware and from the underlying ALSA device
> driver. --
>               (M-Audio  Delta  series,  Terratec,  and  others) support
> --hwmon.  In the future, some consumer cards may also be supported by
> modifying their mixer settings.
>               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.

Sigh, you are right.  But the question remains:  What applications will
support this and what does it mean?  For example, the DSP Hammerfall has
about two dozen inputs and about two dozen outputs.  What does "hardware
monitoring of capture ports" mean then?  Where are they monitored to by
which application?

David Kastrup

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