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

[Joachim Henke]

I still prefer using a sine wave, it sounds more smooth and won't  
hurt our ears (and speakers) too much. I don't think that people want  
to play square waves, when they have their PC connected to their hifi  
system (c: The main purpose of the PC speaker is to generate tones  
and many programmers used it to play melodies. Why shouldn't we take  
this chance to make it sound as good as we can manage it?

I rewrote the wave form generation routine to fit into 32 bit  
integer, with the highest fixed point precision for the range of all  
possible parameters. To retrieve the sine values, we use a small  
lookup table with a resolution of 1024 entries (we need to store the  
first quarter only, since the rest is symmetric).

I extensively tested these routines (on iMac G5 / AMD K6, gcc 3 / gcc  
4) to ensure, that the final sample values differ at most by +/-1  
from their double float calculated counterparts. This difference  
doesn't seem to be much, but when comparing directly, you can hear a  
minor noise for frequencies below 200 Hz. Maybe it also depends a bit  
on the speakers used.

However, this approach should still be sufficient for our purpose.  
Most programs don't use too low frequencies, and if they do, people  
probably won't notice.


Just test the attached patch yourselves

Jo.


Sebastian Kaliszewski wrote:
> One more note about that. PC-speaker generates just plain square  
> wave not sine wave, so this would be more realistic. Then fixed  
> point calculation (16 bit integer part and 16 bit fractional) is  
> easy without all those sin calculation.

--
Joachim Henke
http://he-jo.net/

pc_speaker.diff.gz
Description: GNU Zip compressed data