[alsa-devel] Speaker burnout

[Ricard Wanderlof]

On Tue, 7 Apr 2015, Eliot Blennerhassett wrote:

> At this setting, enabling the internal mic feedthrough with mic boost
> set to maximum,  feedback happens when mic playback gain is at -6dB
> (max=+12dB).
> The signal when recorded is 1325Hz rail to rail square wave.
> 
> My point is that in this case for normal usage the maximum output is
> fine, I'd even say it is required.  So limiting output would not be a
> good idea.
> Also when using the mic for speech capture (with headphones for output),
> a gain greater than the 'feedback inducing' one is likely to be useful.
> 
> So it is (only?) the pathological case of feeding mic direct to speakers
> that is problematic.
> 
> Given that we can't fix the hardware or wacky user behaviour,
> My suggestion would be to either limit, or hide completely the various
> "Mic Playback" controls that enable direct feed from input to output.
> What is the use case for this anyway? - karaoke?

Can it be used to provide a side tone (telephone system terminology for 
feeding back a portion of the microphone signal to the ear piece in a 
telephone; psychologically it helps avoid the impression that the 
telephone is not connected, so all telephone type equipment have it since 
the birth of telephone technology) in telephone applications?

> Given that this goes against the "expose everything in the hardware",
> maybe have a module option that can unhide these controls if anybody
> actually wants to use them.

This sounds like a useful suggestion to me. Many applications have an 
'advanced' option or button which enables things you normally don't need. 
If the 'mic to playback' function isn't really used normally it would seem 
a reasonable option but I suppose that needs to be researched first 
whether it really is of little use.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30