[Alsa-user] Route input to output with minimal latency

[Robert Bielik]

Hi all,

I want to route input to output with minimal possible latency, this will run on a Raspberry Pi, and the latency should be < 1 ms.

I was thinking... if the ALSA capture and playback device is mmapped to the same buffer area, this should be dealt with automatically. Is this possible ?

Regards
/Robert

[Ralf Mardorf]

Post by Robert Bielik
I want to route input to output with minimal possible latency, this
will run on a Raspberry Pi, and the latency should be < 1 ms.
I was thinking... if the ALSA capture and playback device is mmapped
to the same buffer area, this should be dealt with automatically. Is
this possible ?

http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html

I wonder if you are mistaking apples for oranges. Do you want to
connect the audio device's hardware outputs with the audio device's
hardware inputs with cables or do you want to connect software outputs
with software inputs?

[Ralf Mardorf]

No, software input to output, i.e. capture data and send to playback
directly with minimal latency.
But I realize that mmapping is not possible since capture and playback
device will have separate buffers, so I still have to copy from
capture to playback buffers.

I don't know to what "zero-copy" in the description refers to, but even
if there should be the need to copy, I doubt that latency caused by
such copies would be an issue, at least not for a negligible amount of
IOs. I guess if latency isn't caused by audio hardware, than usually by
the signal processing of applications, but unlikely by the routing.
However, I never programmed using ALSA and my audio recording
experiences are from decades ago. I was just surprised about the
< 1 ms, since I would expect software side this shouldn't be an issue.

I might be mistaken.

[Clemens Ladisch]

Post by Robert Bielik
I want to route input to output with minimal possible latency, this
will run on a Raspberry Pi, and the latency should be < 1 ms.

This is easiest to do if the hardware already has this routing built in.

Otherwise, you have to do the capture and playback in software. See the
alsaloop tool. What latency you can reach depends on how much other
applications and drivers interfere with the scheduling; on the Pi,
typical culprits are WiFi, ethernet, or USB.
Also see <https://wiki.linuxaudio.org/wiki/raspberrypi>.

Post by Robert Bielik
if the ALSA capture and playback device is mmapped to the same buffer
area, this should be dealt with automatically. Is this possible ?

Only in theory. The current API has no mechanism for that.


Regards,
Clemens

[Gordon Scott]

Post by Clemens Ladisch
What latency you can reach depends on how much other
applications and drivers interfere with the scheduling; on the Pi,
typical culprits are WiFi, ethernet, or USB.

Indeed. Any serial and/or packetising interface will certainly add
_some_ latency. The drivers need to collect and slice up the data, put
it into those packets and send them over the medium. Then the packets
must be unpacked and the data stream put back together.

IIRC (and I may not, so feel free to correct), USB 2.0 and lower will
cause 1ms of latency all on it's own, just because of the time-slotting
used in the serial stream.

Ethernet can certainly be quicker, but can also be less predictable if
there's any other traffic or you use large packets.

I imagine, but do not know, that WiFi may be slower. A quick 'ping' test
here shown two to fifteen ms, but that may be caused by other factors.
That compares with a fairly steady 0.5ms over Ethernet.

If you are looking for total latency of 1ms, that it likely
over-optimistic, though it might _just_ be feasible. Just one lost
packet, though, will likely cause an audible click (those are fun on a
multi-killowatt PA).

Safer would be, say, 5ms.

3ms is probably OK as it gives enough time to re-send a missing packet
_and_ work towards recovering the lost time.

[Clemens Ladisch]

Post by Clemens Ladisch
What latency you can reach depends on how much other
applications and drivers interfere with the scheduling; on the Pi,
typical culprits are WiFi, ethernet, or USB.

Indeed. Any serial and/or packetising interface will certainly add _some_ latency.

No; I meant that these drivers will interfere with sound when they are
doing something else.


Regards,
Clemens

[Robert Bielik]

Hello Clemens,

Post by Clemens Ladisch
Otherwise, you have to do the capture and playback in software. See the
alsaloop tool. What latency you can reach depends on how much other
applications and drivers interfere with the scheduling; on the Pi, typical
culprits are WiFi, ethernet, or USB.
Also see <https://wiki.linuxaudio.org/wiki/raspberrypi>.

Thanks, I'll look at alsaloop tool. For further info, I am using an I2S sound card (http://www.audioinjector.net/rpi-hat) , and have setup an ALSA device to route playback.pcm through a LADSPA plugin, so all DSP will reside in the plugin, thus I only need to route ALSA input to output in the application 😊

Regards
/Robert