Terrible audio distortion on iMac

danchr · n00b Joined: 01 Apr 2003 Posts: 1

Hi,

I recently installed Gentoo 1.4 on my 400MHz iMac DV, and I am pretty satisfied

The only major issue I have it that there is terrible audio clipping, no matter if I use ALSA or OSS, XMMS or MPlayer, or different sound output plugins in XMMS. Turning the volume down seems to help a bit, but not much. My iMac is 4 years old, and I have never experienced clipping to this extent running Mac OS X.

I really have no idea what's wrong, but it's the main thing keeping me from using Linux. Does anyone have any ideas how I could troubleshoot the issue, or even where in the sources I could look to see if something fundamental is wrong?

I used the GPR CDs extensively during installation, and what I compiled myself used CFLAGS="-O3 -pipe -mtune=750 -mcpu=750 -mpowerpc-gfxopt".

Any help would be greatly appreciated

WhackAMole · n00b Joined: 27 Jul 2003 Posts: 42 Location: New York

I am not entirely sure what you mean by clipping (I'm not a sound guy), but I used to use Gentoo on an identical machine. My sound was almost painful to use until I used gnome's mixer to turn down one of the volumes that seemed to dump out sounds based on what was on the screen. I don't remember which it was. I would use alsamixer and mess around with the various bars--especially the stuff to the far right. Hope this helps.

drspin · Posted: Sat Sep 27, 2003 7:17 am Post subject: Something to try

Visit the ALSA site... There's a command line mixer that you can use - it will give you access to channels that are unavailable under GNOME MiXER and KMIX... Look for something related to boost, or output, if you tell me all the channles that it gives you acces to, I could reccomend one to try... Turn this down - I had to do this on my x86 to get my EMU10K1 sounding decent... the hardware PCM volume was sending full blast causing the buil-in amp to clip out... Clipping is when the amp is trying to put out more power than it can so it shuts down temporarily (I THINK)

turned it down - saved the settings and VIOLA - clean sound again

ThreeFarthingStone · n00b Joined: 25 Feb 2003 Posts: 40 Location: USA

I've had the same mystery sound on an iMac DV 400mhz. I can sometimes hear a fuzz for tabs switching, menus popping, windows scrolling, insertion point moving, console text moving during compiles. It's not as loud as normal sound (beeps, music) though, so I can barely here it, because I don't play normal sounds very loudly.

The mystery sound has occurred under Linux with OSS (kernel sound driver) and OpenBSD. On OSS I always had to unmute a certain sound channel with KMix. On OpenBSD the sound is almost inaudible. When I switched Linux to ALSA, I never had the mystery sound, I guess because ALSA is by default muted and I didn't unmute that channel. (Muting/channels is adjusted with the command amixer.)

The sound seems to get softer over time. I used to have a faint sound on Mac OS 9 (I could hear a fuzz while moving between menus) but now I don't.

I just don't know why any hardware would support the screen as a SOUND device! I guess its just another strageness on this iMac, like the hard drive crashing the OS or the screen being too far left in 24-bit color mode unless its adjusted.
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peaceful · Apprentice Joined: 06 Jun 2003 Posts: 287 Location: Utah

How did you get sound working?

I have a 333MHz original iMac, and I can't get sound working -- period. Even more annoying, if I grab a PPC livecd and pop it in, the sound works. AARgh!

So, how did you get sound working in the first place? I read all the install docs, I've searched the forum -- nothing has ever worked.

WhackAMole · n00b Joined: 27 Jul 2003 Posts: 42 Location: New York

I am not sure if the ppc livecd uses alsa. I don't imagine it does as most people use the kernel option for powermac /dev/dsp to get sound. Try recompiling your kernel with that option--that is how I did it. If you use make menuconfig then it is an option under sound card support. Alternatively you may try the ALSA drivers. On the ALSA wiki I found this comment on it
"Use the very latest CVS, this driver is under development.
You need the module snd-powermac.

There is experimental support for auto-muting the speakers when headphones are plugged in on the PowerBooks? - it can be disabled by a mixer switch.

On the Pismo G3, I found I had to toggle "MIC-BOOST" in alsamixer once to get sound out of it - this small buglet is probably going to be squshed soon. "
Can't vouch for it as I ended up using OS X.2 only on my macs due to constant stability and other issues with gentoo 1.2 ppc (everything was harder to install generally than on my x86--where I love gentoo). Frightfully slow though it was, I found yellow dog to be a much better experience.

I'm not sure what the state of affairs is at this point on the ppc.