[sdiy] OT - Linear interpolation in C

[Mikko Helin]

I think Microchip C supports 1.15 in _Q15 data type, at least their
Fixed Point Math Librarty uses this and Q16 (which is 15.16
fractional) data types.

See:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2680&dDocName=en552208

- Mikko

On Sun, Jan 30, 2011 at 4:10 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> Sorry to post off-topic (sort of - this is an LFO I'm working on). Perhaps any replies would be better to me privately if you think C programming isn't sufficiently synthy. There *does* seem to have been a lot of it about recently.
>
> The problem I'm having is in converting the following assembly into C.
> I've got a 32-bit phase accumulator PHASE_HI and PHASE_LO. I've got 256-entry waveform tables, and the linear interpolation comes in to make straight line sections between those points.
>
> Now, what's below is not running debugged code, just something I've knocked up to demonstrate how I normally go about this kind of thing. The plan is to use the top 8 bits as an index into the waveform table, whilst the bottom 8 bits indicate the 'sub sample' position between two table entries.
>
> The first section (doing the table lookups) is easy, and I've got that working fine in C. It's the second section I'm struggling with. Specifically, I can't see how the C variable types I've got relate to the 1.15 format accumulator usage that I'm used to. I've got a 1.15 integer type in C (signed int), but it is treated as a full range integer (eg 32767 to -32768) rather than as a fractional number from -1 to nearly 1, which is how I use it with the accumlator.
> I could "upgrade" my variables to "float", but that's unnecessary since I already know it can be done efficiently with 16-bit variables, and that I can avoid division by using shifts.
>
> So how do I get C to treat a signed int as if it runs from -1 to 1 rather than -32768 to 32767? Or is there a better way?
>
> Thanks,
> Tom
>
>
> LinearInterp:
> ; Work out the linear interpolated value
> ; Value = (THIS * 1-index) + (NEXT * index)
>
>        ; Deal with waveform look-up
>        mov PHASE_HI, W2                        ; Get high word of phase
>        lsr     W2, #8, W2                              ; Shift down so I only have the top 8 bits
>        sl      W2, W2                                  ; Shift up to make an index (2 bytes per entry)
>        mov     #tblpage(LFO_SineTable), W1             ; Set up TBLPAG
>        mov     W1, TBLPAG
>        mov     #tbloffset(LFO_SineTable), W1           ; Get base position
>        add     W1, W2, W1                                              ; Add offset
>        ; Get the first lookup result
>        tblrdl  [W1++], W6                                      ; Get the data(THIS into W6)
>        ; Get the second lookup result
>        tblrdl  [W1], W7                                        ; Get the data (NEXT into W7)
>
>        ; Use the 8 lower bits of the high word for the linear interp
>        mov PHASE_HI, W4                ; Index
>        and     W4, #255, W4            ; Mask off the top word
>        sl      W4, #7, W4                      ; Shift up to give range 0-1
>        com     W4, W5                          ; 1-Index
>        bclr    W5, #15                         ; Clear the sign bit again
>        mpy     W5*W6, A                        ; (THIS * 1-index)
>        mac     W4*W7, A                        ; (NEXT * index)
>        sac.r   A, W0                   ; Get rounded result into W0
>
> (code is dsPIC ASM)
>
>
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>