# Linear interpolation¶

• Author or source: uh.etle.fni@yfoocs
• Type: Linear interpolators for oversampled audio
• Created: 2007-02-19 10:02:41
notes
```Simple, fast linear interpolators for upsampling a signal by a factor of 2,4,8,16 or 32.
Not very usable on their own since they introduce aliasing (but still better than zero
order hold). These are best used with already oversampled signals.

-- Peter Schoffhauzer
```
code
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120``` ```#ifndef __LIN_INTERPOLATOR_H_ #define __LIN_INTERPOLATOR_H_ /************************************************************************ * Linear interpolator class * ************************************************************************/ class interpolator_linear { public: interpolator_linear() { reset_hist(); } // reset history void reset_hist() { d1 = 0.f; } // 2x interpolator // out: pointer to float[2] inline void process2x(float const in, float *out) { out[0] = d1 + 0.5f*(in-d1); // interpolate out[1] = in; d1 = in; // store delay } // 4x interpolator // out: pointer to float[4] inline void process4x(float const in, float *out) { float y = in-d1; out[0] = d1 + 0.25f*y; // interpolate out[1] = d1 + 0.5f*y; out[2] = d1 + 0.75f*y; out[3] = in; d1 = in; // store delay } // 8x interpolator // out: pointer to float[8] inline void process8x(float const in, float *out) { float y = in-d1; out[0] = d1 + 0.125f*y; // interpolate out[1] = d1 + 0.25f*y; out[2] = d1 + 0.375f*y; out[3] = d1 + 0.5f*y; out[4] = d1 + 0.625f*y; out[5] = d1 + 0.75f*y; out[6] = d1 + 0.875f*y; out[7] = in; d1 = in; // store delay } // 16x interpolator // out: pointer to float[16] inline void process16x(float const in, float *out) { float y = in-d1; out[0] = d1 + (1.0f/16.0f)*y; // interpolate out[1] = d1 + (2.0f/16.0f)*y; out[2] = d1 + (3.0f/16.0f)*y; out[3] = d1 + (4.0f/16.0f)*y; out[4] = d1 + (5.0f/16.0f)*y; out[5] = d1 + (6.0f/16.0f)*y; out[6] = d1 + (7.0f/16.0f)*y; out[7] = d1 + (8.0f/16.0f)*y; out[8] = d1 + (9.0f/16.0f)*y; out[9] = d1 + (10.0f/16.0f)*y; out[10] = d1 + (11.0f/16.0f)*y; out[11] = d1 + (12.0f/16.0f)*y; out[12] = d1 + (13.0f/16.0f)*y; out[13] = d1 + (14.0f/16.0f)*y; out[14] = d1 + (15.0f/16.0f)*y; out[15] = in; d1 = in; // store delay } // 32x interpolator // out: pointer to float[32] inline void process32x(float const in, float *out) { float y = in-d1; out[0] = d1 + (1.0f/32.0f)*y; // interpolate out[1] = d1 + (2.0f/32.0f)*y; out[2] = d1 + (3.0f/32.0f)*y; out[3] = d1 + (4.0f/32.0f)*y; out[4] = d1 + (5.0f/32.0f)*y; out[5] = d1 + (6.0f/32.0f)*y; out[6] = d1 + (7.0f/32.0f)*y; out[7] = d1 + (8.0f/32.0f)*y; out[8] = d1 + (9.0f/32.0f)*y; out[9] = d1 + (10.0f/32.0f)*y; out[10] = d1 + (11.0f/32.0f)*y; out[11] = d1 + (12.0f/32.0f)*y; out[12] = d1 + (13.0f/32.0f)*y; out[13] = d1 + (14.0f/32.0f)*y; out[14] = d1 + (15.0f/32.0f)*y; out[15] = d1 + (16.0f/32.0f)*y; out[16] = d1 + (17.0f/32.0f)*y; out[17] = d1 + (18.0f/32.0f)*y; out[18] = d1 + (19.0f/32.0f)*y; out[19] = d1 + (20.0f/32.0f)*y; out[20] = d1 + (21.0f/32.0f)*y; out[21] = d1 + (22.0f/32.0f)*y; out[22] = d1 + (23.0f/32.0f)*y; out[23] = d1 + (24.0f/32.0f)*y; out[24] = d1 + (25.0f/32.0f)*y; out[25] = d1 + (26.0f/32.0f)*y; out[26] = d1 + (27.0f/32.0f)*y; out[27] = d1 + (28.0f/32.0f)*y; out[28] = d1 + (29.0f/32.0f)*y; out[29] = d1 + (30.0f/32.0f)*y; out[30] = d1 + (31.0f/32.0f)*y; out[31] = in; d1 = in; // store delay } private: float d1; // previous input }; #endif ```

```I incorporated the 32x interpolator with something along this

void process32x(float const in_l, float const in_r, float *out_l, float *out_r)
{
float y_l = in_l-f_d1_l;
out_l[0] = f_d1_l + (1.0f/32.0f)*y_l;    // interpolate
out_l[1] = f_d1_l + (2.0f/32.0f)*y_l;
out_l[2] = f_d1_l + (3.0f/32.0f)*y_l;
out_l[3] = f_d1_l + (4.0f/32.0f)*y_l;
out_l[4] = f_d1_l + (5.0f/32.0f)*y_l;
out_l[5] = f_d1_l + (6.0f/32.0f)*y_l;
out_l[6] = f_d1_l + (7.0f/32.0f)*y_l;
out_l[7] = f_d1_l + (8.0f/32.0f)*y_l;
out_l[8] = f_d1_l + (9.0f/32.0f)*y_l;
out_l[9] = f_d1_l + (10.0f/32.0f)*y_l;
out_l[10] = f_d1_l + (11.0f/32.0f)*y_l;
out_l[11] = f_d1_l + (12.0f/32.0f)*y_l;
out_l[12] = f_d1_l + (13.0f/32.0f)*y_l;
out_l[13] = f_d1_l + (14.0f/32.0f)*y_l;
out_l[14] = f_d1_l + (15.0f/32.0f)*y_l;
out_l[15] = f_d1_l + (16.0f/32.0f)*y_l;
out_l[16] = f_d1_l + (17.0f/32.0f)*y_l;
out_l[17] = f_d1_l + (18.0f/32.0f)*y_l;
out_l[18] = f_d1_l + (19.0f/32.0f)*y_l;
out_l[19] = f_d1_l + (20.0f/32.0f)*y_l;
out_l[20] = f_d1_l + (21.0f/32.0f)*y_l;
out_l[21] = f_d1_l + (22.0f/32.0f)*y_l;
out_l[22] = f_d1_l + (23.0f/32.0f)*y_l;
out_l[23] = f_d1_l + (24.0f/32.0f)*y_l;
out_l[24] = f_d1_l + (25.0f/32.0f)*y_l;
out_l[25] = f_d1_l + (26.0f/32.0f)*y_l;
out_l[26] = f_d1_l + (27.0f/32.0f)*y_l;
out_l[27] = f_d1_l + (28.0f/32.0f)*y_l;
out_l[28] = f_d1_l + (29.0f/32.0f)*y_l;
out_l[29] = f_d1_l + (30.0f/32.0f)*y_l;
out_l[30] = f_d1_l + (31.0f/32.0f)*y_l;
out_l[31] = in_l;
f_d1_l = in_l; // store delay_l

float y_r = in_r-f_d1_r;
out_r[0] = f_d1_r + (1.0f/32.0f)*y_r;    // inrterpolate
out_r[1] = f_d1_r + (2.0f/32.0f)*y_r;
out_r[2] = f_d1_r + (3.0f/32.0f)*y_r;
out_r[3] = f_d1_r + (4.0f/32.0f)*y_r;
out_r[4] = f_d1_r + (5.0f/32.0f)*y_r;
out_r[5] = f_d1_r + (6.0f/32.0f)*y_r;
out_r[6] = f_d1_r + (7.0f/32.0f)*y_r;
out_r[7] = f_d1_r + (8.0f/32.0f)*y_r;
out_r[8] = f_d1_r + (9.0f/32.0f)*y_r;
out_r[9] = f_d1_r + (10.0f/32.0f)*y_r;
out_r[10] = f_d1_r + (11.0f/32.0f)*y_r;
out_r[11] = f_d1_r + (12.0f/32.0f)*y_r;
out_r[12] = f_d1_r + (13.0f/32.0f)*y_r;
out_r[13] = f_d1_r + (14.0f/32.0f)*y_r;
out_r[14] = f_d1_r + (15.0f/32.0f)*y_r;
out_r[15] = f_d1_r + (16.0f/32.0f)*y_r;
out_r[16] = f_d1_r + (17.0f/32.0f)*y_r;
out_r[17] = f_d1_r + (18.0f/32.0f)*y_r;
out_r[18] = f_d1_r + (19.0f/32.0f)*y_r;
out_r[19] = f_d1_r + (20.0f/32.0f)*y_r;
out_r[20] = f_d1_r + (21.0f/32.0f)*y_r;
out_r[21] = f_d1_r + (22.0f/32.0f)*y_r;
out_r[22] = f_d1_r + (23.0f/32.0f)*y_r;
out_r[23] = f_d1_r + (24.0f/32.0f)*y_r;
out_r[24] = f_d1_r + (25.0f/32.0f)*y_r;
out_r[25] = f_d1_r + (26.0f/32.0f)*y_r;
out_r[26] = f_d1_r + (27.0f/32.0f)*y_r;
out_r[27] = f_d1_r + (28.0f/32.0f)*y_r;
out_r[28] = f_d1_r + (29.0f/32.0f)*y_r;
out_r[29] = f_d1_r + (30.0f/32.0f)*y_r;
out_r[30] = f_d1_r + (31.0f/32.0f)*y_r;
out_r[31] = in_r;
f_d1_r = in_r; // store delay_r
}

Unfortunately, I am doing something crazy wrong. When I close my plug-in, my DAW freezes. I'm fairly new to programming and am not sure what I'm doing wrong.

I'm using your function to write to an audio buffer which is being set to a delay. This is what the call looks like.
process32x((fLeftAudioBuffer[i] * decay), (fRightAudioBuffer[i] * decay),
&fCircularLeftAudioBuffer[(fCircularBufferPosition + delayFrames)%kCircularBufferSize],
&fCircularRightAudioBuffer[(fCircularBufferPosition + delayFrames)%kCircularBufferSize]);

I would love to use this function because it's one line less than how I usually do it. It works great with no problems.
fCircularLeftAudioBuffer[(fCircularBufferPosition + delayFrames)%kCircularBufferSize] = fLeftAudioBuffer[jh] * decay;
fCircularRightAudioBuffer[(fCircularBufferPosition + delayFrames)%kCircularBufferSize] = fRightAudioBuffer[jh] * decay;

Everything seems simple, but I'm puzzled as to what may be wrong. Thanks.
```
```Never mind. I was calling the function from the wrong place. Works like a charm.

Thank you.
```