Re: [Qemu-devel] [PATCH] PC speaker emulation (fixed point)

[Fabrice Bellard <fabrice@bellard.org>]

Sebastian Kaliszewski wrote:
> Joachim Henke wrote:
> 
>> Ok, these are really strong arguments. Thanks a lot for your  
>> interesting statements! I'll do some testing on square waves and will  
>> post an updated patch, as I am also not totally satisfied with the  
>> current sound myself.
> 
> 
> One little suggestion...
> 
> Real PC-speaker is rather poor source of sound, and I also noticed, that 
> sound cards which took PC-speaker sound for themselves (some SB-clones 
> did route PC-speaker sound into thier own output) liked to low-pass 
> filter the resulting audio. So to make things as real as feasible, use 
> your wave table to store something like square wave with rounded corners 
> -- maybe sth like pow(sin(x), 0.2) (i.e. sinus rooted to 5th degree) 
> will sound pleasant enough.

If you want to model the real PC speaker, the best to do is to generate 
a square signal and to pass it thru a low pass filter with a cut off 
frequency of a few kHz. Then you could even be able to play samples thru 
the simulated PC speaker using the tricks used in old MSDOS programs, 
provided QEMU implements a precise cycle counter (it will come someday !).

Fabrice.