From: kim@Lise.Unit.NO (Kim Gunnar St|vring yhus)
Newsgroups: comp.sys.acorn
Subject: Sound ( Was Re: New machine )
Date: 8 Apr 92 13:22:56 GMT
Organization: Norwegian Institute of Technology

>>> banks_p@kosmos.wcc.govt.nz
>> Henrik Sundt 
> Malcolm lithgow@uslp.UUCP
Me: Kim 0yhus

>>>   The Arc is capable, now, of up 160kHz sound (mono) and 80kHz stereo
>>>sound. What more do you want?
>>More bits !

>>High sample rates does not help as long as the bit resolution is only
>>8 bit (logaritmic!), which gives a s/n ratio of 48 dB [36dB:Kim] (worse than
>>a bad music cassette). To get high sound quality it is necessary with
>>at least 12 bit, preferably 16 (as CD). Then no higher rate than 40kHz
>>(or 44.1 due to standard) is useable, since the human ear
>>cannot hear frequencies higher than 20kHz.

>Yes, the Arch could definitely do with more than 8 bit logarithmic, but
>doesn't 8 bit logarithmic give the same dynamic range as 12 bit linear?
>(I know it doesn't give the same precision, but who gives a damn, this is
>a computer we're talking about.) Asking for 16 bits is a bit silly, only
>the most expensive stereo systems can do justice to this sort of sound
>quality, and most people only use their internal speaker! Of course, if I
>can get 16 bits for the same price, I'll take it (so long as it's
>connected to a decent computer, not a dog like the Amiga, IBM PC, or
>Macintosh).

Precision is the same as dynamic range. The reason for this, is that sounds jump
all over the place, so it needs the same precision everywhere. For example a pure
sine tone spend more of its time at the bottom and top of its curve. The dynamic
range is thus restricted by the lowest precision, which is 1/64 of sound level.
The logarithmic sound format uses 4 bits to represent the levels from <1,2>, that
is 16 levels, and 1 sign bit, that gives 32 levels, and the levels <-1,0> & <0,1>
have higher resolution, but are far to few, makes 64 EQUALLY SPACED levels, 6 bit
precision, 36dB signal/noise.
The fine thing about logarithmic sound scales, is that it is 36dB also for low
sounds, because of scale invariance, while linear systems lose the top bits with
downscaling.
(By the way, Amiga is a computer, not a dog. :-)

>>As many of you may have noticed, the Amiga has a higher sound quality
>>(better dynamics) than the Archimedes. This is mainly because in
>>addition to the linear 8 bit resolution it has a 5 bit volume control,
>>i.e. a total of 13 bit resolution, as the volume can be changed
>>at a high rate.

>Which would make it marginally better than the 8 bit logarithmic, with
>the disadvantage that you could only play one channel at a time! Wow, I'm
>impressed.

The amiga use 8 bits, which gives 48dB s/n, hearably better than 36dB.
(Less noise)
In addition it has 5 bits for scaling. As logarithmic scale, this would equal
8+5=13 bits.
The Amiga has 4 channels sound dma, always. Actually, it is the Archimedes that
emulates 8 channels, by rapidly switching one channel around!

>>Something similar can probably be done with the Archimedes,
>>by using two channels as high and low byte of a sample.

>I doubt it. If your A/D converters only have a certain range, theirs not
>much you can do without extra electronics.

The Archimedes DO have extra electronics, namely the 7kHz 1'st order filter. This
filter removes the switching noise, and unfortunately, some of the sound. Much of
Amigas better sound, come from turning off its equivalent filter.

I am trying to make a program that tricks the Archimedes into using 12-13 bits,
by spreading the bits over 2 channels per loudspeaker, some dithering, and
compensating for the 7kHz filter digitally.

Expect to hear more from me, through your own Archimedes :-)

                     Kim 0