Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!mentor.cc.purdue.edu!pur-ee!pur-phy!murphy
From: murphy@pur-phy (William J. Murphy)
Newsgroups: comp.sys.amiga
Subject: Re: Audio Improvements
Message-ID: <2450@pur-phy>
Date: 21 Aug 89 14:20:10 GMT
References: <3114@bucsb.UUCP> <1542@convex.UUCP> <9051@attctc.Dallas.TX.US>
Reply-To: murphy@newton.physics.purdue.edu.UUCP (William J. Murphy)
Distribution: na
Organization: Purdue Univ. Physics Dept., W. Lafayette, IN
Lines: 37

In article <9051@attctc.Dallas.TX.US> ltf@attctc.Dallas.TX.US (Lance Franklin) writes:
>Well, how about adding a "Ham-Mode" for the audio channel...
>
>Let's say, perhaps, that we have a 12 (or more) bit D-A on each channel.
>Each byte, when the channel is in this mode, is interpreted thusly:
> 
>     Bit 7 : Mode bit...when 0, bits 6-0 will be loaded directly into the
>             high order 7 bits, with all low-order bits set to zero.
>             When set to 1, bits 6-0 are interpreted as a signed 7 bit
>             number which is added to the current value of the D-A 
>
>
>| Lance T Franklin        | | "And all who heard should see them there,
>| ltf@attctc.DALLAS.TX.US | |  And all should cry, Beware!  Beware!
I don't see the purpose in doing what you suggest.  If you have a 12-bit
DAC, then with the 16-bit DMA we already have, why do you need to do
the audio-HAM mode?  I would think that it would just be easier to do 
12-bit audio directly.  Besides, the Audio-HAM mode would not reduce the
storage requirements of a raw audio sample. Rather, what you suggest 
increases the required space by a bit or two per sample.

I do see where if you had a "low-volume" passage of sound (where only the
first six bits are used for a period of time), this might save in storage
of the sample.  The hardware could assume that the bit 7 being set implies
that if the immediately following byte were bit 7 off, it would be loaded
in the lower 6 bits.

byte 1 bit 7    byte 2 bit7    byte 1 loaded as   byte 2 loaded as
on              on              high 6bits       next sample high 6bits
on              off             high 6bits       this sample low 6bits
off             on              low 6bits        next sample high 6bits
off             off             low 6bits        next sample low 6bits

Is this what you were thinking of Lance?

William J. Murphy
murphy@newton.physics.purdue.edu