[sdiy] Wave table interpolation

[Richie Burnett]

Linear interpolation isn't particularly clean (it isn't band-limited) but 
can work adequately if you oversample the data in the wave tables first, 
and/or use a sufficiently high playback sample-rate like 96kHz.  In the case 
of the former, if the data in the wavetables is already oversampled 
considerably, then linear interpolation between the points isn't such a bad 
approximation.  And the latter gives space for the images from the imperfect 
interpolation to go, before they alias back into the audio band where you 
will hear them...

If you run your playback at 48kHz, then the demands on the interpolator are 
very stringent.  It should be flat out to 20kHz or thereabouts, and then be 
something like 80dB or more down by 28kHz if you aren't to hear aliasing 
back into the audio band below 20kHz.  That's only 8kHz of transition band 
for the interpolating filter to work it's magic.  However, doubling the 
playback sample rate to 96kHz, means that you've got all the space between 
20kHz and 76 kHz for the interpolating filter's transition band.  That's 7 
times as wide, for a mere x2 oversampling during playback, so the demands on 
the interpolating filter are greatly reduced!

-Richie,

-----Original Message----- 
From: Tim Ressel
Sent: Sunday, November 12, 2017 6:56 PM
To: SYNTH DIY
Subject: [sdiy] Wave table interpolation

Hi,

Another fun one. I've been looking at the WaveEdit software. It looks
like it produces waves that are 256 samples by 16 bits. Obviously you'll
want to interpolate between the samples. Linear interpolation works and
is cheap but is brute force and causes lots of distortion. I've looked
at cubic interpolation. Is that a viable approach? My app needs 4 tables
live at a time so I can interpolate (linearly) between them, so this all
has to happen on the fly. Cubic looks a bit math intensive.

Thanks!

-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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