Date   : Wed, 19 May 2004 12:26:49 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: Sound wave representation

Matt Callow wrote:

> --- Charles Ripperton <charlesripperton@...> wrote: > Hi
>Beeb fans,
>  
>
>>I've got some queries about BBC Micro sound / playing samples.
>>My understanding is that the SN76489 is capable of 4-bit sound
>>sample playback, and that the nybbles used are unsigned. In
>>other terms, the value 15 means the highest positive wave
>>amplitude, the value 0 means the lowest negative wave
>>amplitude
>>and somewhere between the values 8 and 7 is the
>>'zero displacement'. 
>>
>>Is this accurate? If I'm incorrect, then please explain what
>>the true situation is. Cheers.
>>
>>Charles.
>>    
>>
>
>[snip]
>
>I don't  think that the sound ship can produce an output like
>and ADC for example.  I was reading an article on Superior
>Software's "Speech!" program the other day, which explains how
>it was done. 
>Take a look at:
>  
>
You're right there. If you put the waveform thru an oscilliscope you'll 
get something resembling Radio 4's AM transmission :P

The clever bit is though, the human ear isn't able to distinguish the 
effective "carrier" and instead demodulates it and you hear the 
internded waveform. The multi-channel trick is simply used to fiddle 
amplitude range to get a finer control. 6 bits is still not brilliant 
though.

<pedant>Psst.. DAC is digital->analogue :)</pedant>

-- Richard