[Om2008.9] Using Freerunner as a digital audio player

Alastair Johnson alastair at truebox.co.uk
Mon Oct 13 17:17:41 CEST 2008

Sam Kuper wrote:
> 2008/10/13 <GNUtoo at no-log.org <mailto:GNUtoo at no-log.org>>
>     I was *very* disapointed when I discovered the audio quality via
>     headphones:in IRC they told me that it was because the capacitor(that is
>     between headphone out and the sound card)'s value was too low and
>     acted as
>     a filter...
>     As I am too afraid of breaking the openmoko i won't change the capacitor
>     myself...but what could I do? build/buy an external amplifier(will
>     it work
>     fine?) or find a place where they could remplace me the capacitor?
> I wasn't aware of this problem, but if what you are saying is that the 
> headphone out socket of the phone has a capacitor connected across it 
> that is attenuating the high frequencies, then an external amplifier 
> connected to that socket will not be able to restore those frequencies.

The cap is 1uF in series - see the schematics at 
http://downloads.openmoko.org/schematics/GTA02/ and check the archives 
for a thorough analysis by Joerg:

An external amp will provide a high impedence load, so things will be 
better, but unless there was an unannounced change in the componenet 
values for the later boards you will be stuck with bass rolling off 
below 160Hz.

On the hardware list you can find discussion of the modification options.

> A well-designed amplifier that takes a digital audio feed from the USB 
> socket - a USB soundcard, essentially, would work in principle, but in 
> practice may be expensive/time-consuming to implement.

There are any number of usb headsets out there. Many would be trivial to 
modify for use with other headphones. If you're serious about your audio 
there's always the HeadRoom BitHead.

> My advice would be to try listening to music on your phone. If it sounds 
> fine, don't worry about it further. If it isn't fine, try using a distro 
> that is known to have good sound capabilities (someone else mentioned 
> that Debian gives high playback quality). Only if this fails to give you 
> adequate quality might it be worth investigating hardware 
> replacements/augmentation.
> Regards,
> Sam
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