[pvrusb2] Noise

[isely at isely.net]

Jan:

That's an odd symptom.  Getting audio from an RF signal actually has a 
longer processing path than audio from the line-in port.  In fact, 
there should be very little (if anything) involved with the line-in port 
that isn't also involved with processing audio from an RF signal.  In 
either of those two cases, the audio is processed by a separate chip 
(cx25840 IIRC) and the result of that is what is fed into the mpeg 
encoder chip.  Once past the cx25840, there's no difference in the 
datapath.

There's a separate driver for the cx25840, which the pvrusb2 driver 
employs, which is part of the v4l2 subsystem as a whole, so any problem 
involving audio like you describe should also be likely with any other 
video capture driver that also happens to use a cx25840.  I know this 
isn't being very helpful, but it does suggest that looking for other 
online chatter involving that part might reveal another clue.

  -Mike


On Sun, 21 Apr 2019, Jan Ceuleers wrote:

> Dear list,
> 
> I am seeking some help with an audio defect that afflicts some
> recordings made using Hauppauge PVR 1950 devices.
> 
> I have two mythtv backends:
> 
> - the slave backend has three 1950s that record from analogue cable.
> Audio recorded from these tuners is fine.
> 
> - the master backend has two 1950 PVRs that record from set-top boxes
> (i.e. from the composite video input and line audio inputs). All
> recordings made using these two tuners suffer from an audio defect as
> described below.
> 
> The audio defect is a clearly audible repetitive "whistling" noise, with
> a periodicity of about 1 second. This noise is not present on the audio
> signal that goes into the PVR.
> 
> I have tried many variations of v4l2-ctl -d$device -f $freq without
> effect (and have settled on 450 MHz as the frequency). The point of
> doing this is that I want to exclude noise from the channel the analogue
> tuner happens to be tuned to.
> 
> Many moons ago a pvrusb2 device had multiple audio inputs from which one
> could select the right-one using v4l2-ctl -d$device
> --set-audio-input=$audiodev , with valid values for audiodev being 0, 1
> and 2. The correct setting for my use case was 1. This is no longer the
> case: the device presents only one audio input which is numbered 0.
> 
> I'd be grateful for any hints.
> 
> Thanks, Jan
> 
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-- 

Mike Isely
isely @ isely (dot) net
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