[sdiy] Wave table interpolation

[rburnett at richieburnett.co.uk]

Using a high sample rate like 192kHz certainly gives you a lot more 
"wiggle room".

-Richie,

On 2017-11-12 22:50, Tim Ressel wrote:
> I meant clean code-wise  :-)
> 
> Target sample rate is 192KHz. TI makes a super cool 8 channel audio
> dac PCM1681. Target proc is the STM32F412VET6, cortex M4, 256 mB ram,
> 100MHz.
> 
> So you think I can get away with linear?  That would make the interop
> code simple. Since I am filling in 256 points in between samples the
> math is all powers of 2.
> 
> -t-i-m-b-o-
> 
> On 11/12/2017 11:28 AM, Richie Burnett wrote:
>> 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!
>>