[linux-audio-user] A most bizarre problem.

Joey Reid joe at joeyreid.com
Thu May 13 09:02:13 EDT 2004

in alsamixer, look for a "Rec." toggle on the line-in channel. I had 
this very problem with my i810 compatible (nforce2) onboards sound.

On May 13, 2004, at 9:03 AM, Malcolm Baldridge wrote:

> A few weeks into my admittedly bizarre Jack-based telephone logging +
> music-on-hold system, I ran into an interesting "bug".
> But first, the hardware in play:
> Motherboard: ECS P4IBMS (Pentium 4, i845 chipset)
> 00:1f.5 Multimedia audio controller: Intel Corp. 82801BA/BAM AC'97 
> Audio
> Card: Intel 82801BA-ICH2
> Chip: Realtek ALC200/200P rev 0
> ALSA-1.0.3b w/1.0.4 drivers + libraries, jackd is from late April 2004 
> CVS.
> My jackd invocation line is as follows:
>    jackd -R -d alsa -S -C -p 4096 -M -n 2 -r 11025
> I have an AT&T MERLIN phone system in my closet along with my main home
> server (see above hardware description).  I have my PCM (Line out) 
> plugged
> into the MERLIN system's music-on-hold port, that works well.
> I have two phone lines hooked up to JD Audio Phone Patch boxes to 
> provide
> local + remote side level correction (trans-hybrid technology), which 
> feeds
> into my Line In jack [each patch box provides a mixed in/out audio 
> source on
> a mono output, so I have Line 1 feeding Left, and Line 2 feeding 
> Right].
> This too works beautifully with my vox-activated audio logging program 
> which
> I'd adapted from one of the jackd sample clients. [It's ended up quite
> something else, but not too bad if I say so myself.]
> PROBLEM: When I have someone on hold on Line 2, say, and someone else 
> is
> talking on Line 1, the person on Line 2 hears the music-on-hold *AND* 
> the
> conversation of Line 1!!
> It gets better.  If I dial out on Line 3 [lines 3+4 are outbound 
> only], put
> that call on hold, and there are people talking on Lines 1+2, the 
> caller at
> the other side of Line 3 hears not only the music-on-hold, but BOTH 
> Line 1 +
> 2's conversations!!!
> YIKES!!!  This was a big shocker.  Naturally, I never did this sort of
> testing, being more concerned with DC bias, filtering curves and 
> bandpass
> settings, and which compressors to use for the online-browsable 
> phonelogs as
> well as the losslessly-compressed (FLAC of course) data archival logs.
> Obviously the PCM output is being mixed into whatever feeds into the 
> Line
> input of the AC97 codec.  This is annoying, and I can see no way of
> switching it off.  I've explored every Mute/on/off option in the 
> alsamixer,
> alas the dark fire did not avail me.
> I see nothing about "hardware monitoring" as an option for this 
> chipset,
> however Hammerfall and others are mentioned as supporting this feature;
> nothing in the intel-8x0 driver suggest this is a supported feature.
> I'm relatively new to PC audio hardware, but has the LINE-IN been fed 
> into
> the PCM output mixer from the beginning?  Is this a funky chipset bug 
> or
> weird issue or interaction?  I know that the output mixer is fed audio 
> from
> the CD and the WAVE device, but it seems a bit strange that the 
> LINE-IN port
> is also routed through in this way.  I can't try this under Windows, 
> as this
> machine has been kept clean from the Dark Software of Udûn since its 
> deployment.
> I obviously can't mute the PCM output, since that's the music-on-hold.
> After much experimentation, I figured I'd try the Microphone input 
> instead.
>  It seems noisier, even with the +20dB turned off.  The impedance is 
> quite a
> bit different, though fortunately I can get by with the gain set to 
> the bare
> minimum [3 in the ALSA mixer, which is the smallest non-zero value I'm
> allowed].  But the cross-surveillance bonus to the music-on-hold 
> problem is
> gone.
> Maybe I can do something tricksy like emit the music-on-hold on the 
> right
> channel only, and hook up Line 2's audio feed into the left channel of 
> the
> LINE-IN.  This is a cruel hack which depends on the mic+mix functions 
> in the
> codec doing their job properly, which is a huge ask.  Ah well.
> I'm back in action, only now I have only one channel of recording
> capability. :( :(
> Is there a mixer option/setting to disable this "pass-through" routing 
> of
> LINE-IN to the LINE-OUT sound port?
> In Mordor where the PC platform lies,
> =MB=
> -- 
> A focus on Quality.

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