Distorted audio output when using JACK and all interface output channels

[stefano-tronci]

Hi there! I found some odd behaviour while using this package on Arch Linux.

Here the description:

using PortAudio

# PortAudio.list_devices()
#0: HDA Intel HDMI: 0 (hw:0,3) [ALSA], [0/8]
#1: HDA Intel HDMI: 1 (hw:0,7) [ALSA], [0/8]
#2: HDA Intel HDMI: 2 (hw:0,8) [ALSA], [0/8]
#3: HDA Intel PCH: ALC269VB Analog (hw:1,0) [ALSA], [2/2]
#4: hdmi [ALSA], [0/8]
#5: pulse [ALSA], [32/32]
#6: default [ALSA], [32/32]
#7: system [JACK Audio Connection Kit], [2/4]

devID = 7

# The device is a Scarlett 2i4, 2 inputs and 4 outputs.

# This seems to work fine:
stream = open(devID, (0, 2), 48000)
x = 0.1 * randn(10 * 48000, 2)
write(stream, x)
close(stream)

# This produces weird distorted output:
stream = open(devID, (0, 4), 48000)
x = 0.1 * randn(10 * 48000, 4)
write(stream, x)
close(stream)

So, if I try to route audio to all the available outputs I get some weird sound.

It feels like a bug. Let me know if you need more info.

Linux arch 4.6.2-1-ARCH #1 SMP PREEMPT Wed Jun 8 08:40:59 CEST 2016 x86_64 GNU/Linux
jack2 1.9.10
julia 0.4.5

[seebk]

I can reproduce the bug with the Scarlet 2i4 with Jack on Debian Jessie. It works fine with the ASIO driver on Windows so I think it may be a Jack specific problem. But on first glance I could not find any obvious bug or explanation for this behaviour on Linux...

[stefano-tronci]

I see. Audio programming is well beyond my capabilities... so I totally have no idea what could be the problem. Do you think that linking this page on LinuxMusicians could help? There are developers that read the forums, maybe they could help spotting the problem...

[stefano-tronci]

Hi! My latest findings just in case they could be useful to you:

using PortAudio

# PortAudio.list_devices()
# 0: HDA Intel HDMI: 0 (hw:0,3) [ALSA], [0/8]
# 1: HDA Intel HDMI: 1 (hw:0,7) [ALSA], [0/8]
# 2: HDA Intel HDMI: 2 (hw:0,8) [ALSA], [0/8]
# 3: HDA Intel PCH: ALC269VB Analog (hw:1,0) [ALSA], [2/2]
# 4: hdmi [ALSA], [0/8]
# 5: pulse [ALSA], [32/32]
# 6: default [ALSA], [32/32]
# 7: system [JACK Audio Connection Kit], [2/4]

devID = 7
fs = PortAudio.get_devices()[devID + 1].default_sample_rate

# Still using the Scarlett 2i4

# Now this produces distorted output as well:
stream = open(devID, (0, 2), fs)
x = 0.1 * randn(10 * fs, 2)
tic()
write(stream, x)
toc()
# Elapsed time is around 20 s, 2X the expected time
close(stream)

# The smaller the buf_size the better the playback:
stream = open(devID, (0, 2), fs, 1)
x = 0.1 * randn(10 * fs, 2)
tic()
write(stream, x)
toc()
# Elapsed time is around 10 s, the output sounds natural
close(stream)

# Similar things happen when trying to use all 4 channels, but 2 of them (3 and 4) 
# stay muted for some reason. They appear correctly connected to jack Clients.

PuseAudio.terminate()

Hope it helps!