[pulseaudio-discuss] RTP makes high tone pitch

[Lars Stollenwerk]

Hi all,

I am using PulseAudio to implement a kind of intercom: In our ham radio club
house we want to install in every room a raspberry pi with PulseAudio running;
this is what I call a 'client'. Each client is connecet to a microphone and a
speaker using a USB sound card. To send and receive the audio signal over
network, on each client a set of PulseAudio modules is started using 'pactl
load-module ...'. The follwing modules are started:

Sending path
------------
Signal path: mic-source -> loopback -> null-sink -> rtp-send

module-null-sink
    sink_name=rtp-tx-sink
    rate=8000
    channels=1
module-loopback
    source=<mic-source>
    sink=rtp-tx-sink
    latency_msec=1
    max_latency_msec=10
    fast_adjust_threshold_msec=100
    adjust_time=1
    rate=8000
    channels=1
module-rtp-send
    rate=8000
    channels=1
    loop=false
    source=rtp-tx-sink.monitor
    destination=224.0.0.47

Receiving path
--------------
Signal path: rtp-recv -> null-sink -> loopback -> speaker-sink

module-null-sink
    sink_name=rtp-rx-sink
    rate=8000
    channels=1
module-loopback
    source=rtp-rx-sink.monitor
    sink=<speaker-sink>
    latency_msec=1
    max_latency_msec=10
    fast_adjust_threshold_msec=100
    adjust_time=1
    rate=8000
    channels=1
module-rtp-recv
    latency_msec=1
    sink=rtp-rx-sink
    sap_address=224.0.0.47

The null-sinks are used to mute the stream when necessary.

Generally, the set-up works. However, often the pitch of the voice is much too
high. I. e., the voice can be well understood but sounds more or less like a
Helium voice.

The values for latency are all chosen to be very small. The reason is that we
can accept short drop-outs, but not a big latency (few 100 ms is the maxium
acceptable).  I have tryed to increase max_latency_msec of the loopback
devices to 60 ms - without success.

My question to the community is: Does anyone know where the high pitch voice
comes from? And how to avoid this?

I am thankful for any help!

Best ragards
Lars
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