[plug]audio cds, was slowdown probs

[alan howard]

On Thu, 23 Aug 2001 20:42, Mike Holland wrote:
> On Thu, 23 Aug 2001, alan howard wrote:
> > if you have a signal generator , try changing from square wave to sine,
> > you can hear a difference at 9 khz.
>
> So you claim you can hear 27kHz ? Or is it just an imperfection in the
> signal generator?
>
> > also a square wave is made up of odd order harmonics, ie at 9khz it is
> > 9khz + 1/3 27khz  + 1/5 45khz  etc sine waves.
>
> OK. Then I'm even more skeptical.
>
>   I should have mentioned, that the 9kHz square wave (or any signal)
> should be put through a low pass filter before sampling, and so should
> sample identically to a 9kHz sine wave, with an ideal filter. Is that
> correct? If so, how does it make any sense talking about a 9kHz square
> wave on CD?  (even if the ear can detect 27kHz)

you shouldnt be able to hear a 27khz sine wave, but you can notice a slight 
difference in the sound of a 9khz square wave over a sine wave. i dont know 
why , but if someone with more knowledge about human hearing can fill us in 
on this , it would be appreciated. 
as i said previously , have a listen to something on scd and compare it to 
standard cd. 
also about low pass filters used before the a/d converter, these are there to 
stop mixing with the sample frequency. ie if a 25khz signal was sampled it 
could produce signals that are composed of the diference of 25khz and 44.1 
khz that would end up being audable.
the problem with normal cd is that it can not accuratly reproduce complex 
waves in the treble region.
rgds
alan howard