[sdiy] Wave table interpolation

[paula at synth.net]

<cough>

256 Kbyte of SRAM

http://www.st.com/en/microcontrollers/stm32f412ve.html


Still, that's quite a chunk of RAM.

Paula


On 2017-11-13 15:08, Tom Wiltshire wrote:
> 100MHz?! 256mB RAM?! Easy life!! That’s no limitations at all. <insert
> Monty Python's Yorkshireman paraphrasing here>
> 
> Yes, you could certainly make linear interp work well in such a 
> situation.
> 
> You’ve got multiply instructions and probably multiply-accumulate too,
> so linear interp is only a couple of instructions and you don’t have
> to worry about whether it’s powers of 2 or not (although that does
> always seem to make life easier…)
> 
> Tom
> 
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
> 
>> On 12 Nov 2017, at 22:50, Tim Ressel <timr at circuitabbey.com> 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!
>>> 
>> 
>> --
>> --Tim Ressel
>> Circuit Abbey
>> timr at circuitabbey.com
>> 
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy