[ubuntu-studio-users] "zero-latency" - Was: Sorry for repost, this is the finalised reply. With all the wrong spelling / erata removed.

Ralf Mardorf ralf.mardorf at alice-dsl.net
Thu Dec 5 18:28:48 UTC 2013


Hi :)

I try to correct the fallacy about latency and to explain it in a
non-exact, non-technically way.

On Thu, 2013-12-05 at 17:41 +0000, Alex Armani wrote:
> zero-latency kernel

Even the RT patch with full RT enabled is even not hard real-time
capable, let alone that zero-latency is possible in reality ;).

There even is latency before the light from your display touches your
eyes and before your perception is able to form a view of what is
displayed. There is no reality with zero-latency.

Now, important for audio engineering is to understand the differences
between different kinds of preemption and preemption rt.

Hard real-time e.g. is possible with stand alone gear and oldish
computers, such as the C64, because for such gear and old computers
there's direct access to the hardware, without any layers. For a PC this
is impossible, even not with a kernel-rt. The lowlatency kernel is
relatively far away from a kernel-rt. You will notice that humans are
able to handle latency, since everything in our live comes with latency,
for example analog music instruments have got latency. The issue caused
by PCs is jitter and the low latency kernel does still produce much MIDI
jitter. You can get rid of more, but nor all jitter, if you use full
real-time capabilities enabled, by a patched kernel. Clean audio
production sometimes can be done with a vanilla kernel, since audio
jitter belongs much to the kind of audio card and a vanilla kernel on a
fast PC sometimes does provide more than is enough to get less jitter,
sure you can lower the latency, the harder the kernel does enforce
preemption rt.

The more layers or regarding to the kind of protocol a layer does talk
to other layers, the higher the latency and the more jitter. I guess the
preemption rt processes within the kernel does optimize priorities, but
can't do much about issues caused by layers.

At least there definitively is no zero-latency possible, even if PC and
all the layers once should be more precise than old computers were.

Regards,
Ralf




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