From 0d6b297d4e0e983d39b6118f8f9f69bd5958aea4 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Robert=20J=C3=B6rdens?= Date: Mon, 1 Mar 2021 13:29:59 +0100 Subject: [PATCH] cossin: don't cancel -1 phase offset --- dsp/build.rs | 12 +++++++----- dsp/src/cossin.rs | 27 +++++++++++++++------------ 2 files changed, 22 insertions(+), 17 deletions(-) diff --git a/dsp/build.rs b/dsp/build.rs index 5e5fe1d..edd56b0 100644 --- a/dsp/build.rs +++ b/dsp/build.rs @@ -26,14 +26,16 @@ fn write_cossin_table() { const AMPLITUDE: f64 = u16::MAX as f64; for i in 0..(1 << DEPTH) { - // use midpoint samples to save one entry in the LUT - let phase = (PI / 4. / (1 << DEPTH) as f64) * (i as f64 + 0.5); - // add one bit accuracy to cos due to 0.5 < cos(z) <= 1 for |z| < pi/4 - let cos = ((phase.cos() - 0.5) * 2. * AMPLITUDE).round() as u32 - 1; - let sin = (phase.sin() * AMPLITUDE).round() as u32; if i % 4 == 0 { write!(file, "\n ").unwrap(); } + // Use midpoint samples to save one entry in the LUT + let (sin, cos) = + (PI / 4. * ((i as f64 + 0.5) / (1 << DEPTH) as f64)).sin_cos(); + // Add one bit accuracy to cos due to 0.5 < cos(z) <= 1 for |z| < pi/4 + // The -1 LSB is cancelled when unscaling with the biased half amplitude + let cos = ((cos * 2. - 1.) * AMPLITUDE - 1.).round() as u32; + let sin = (sin * AMPLITUDE).round() as u32; write!(file, " {},", cos + (sin << 16)).unwrap(); } writeln!(file, "\n];").unwrap(); diff --git a/dsp/src/cossin.rs b/dsp/src/cossin.rs index 9240d0d..5dae7c3 100644 --- a/dsp/src/cossin.rs +++ b/dsp/src/cossin.rs @@ -11,7 +11,7 @@ include!(concat!(env!("OUT_DIR"), "/cossin_table.rs")); /// /// # Returns /// The cos and sin values of the provided phase as a `(i32, i32)` -/// tuple. With a 7-bit deep LUT there is 9e-6 max and 6e-8 RMS error +/// tuple. With a 7-bit deep LUT there is 9e-6 max and 4e-6 RMS error /// in each quadrature over 20 bit phase. pub fn cossin(phase: i32) -> (i32, i32) { // Phase bits excluding the three highest MSB @@ -40,11 +40,14 @@ pub fn cossin(phase: i32) -> (i32, i32) { // The phase values used for the LUT are at midpoint for the truncated phase. // Interpolate relative to the LUT entry midpoint. - // Also cancel the -1 bias that was conditionally introduced above. - phase -= (1 << (ALIGN_MSB - 1)) - (octant & 1) as i32; + phase -= 1 << (ALIGN_MSB - 1); + + // Cancel the -1 bias that was conditionally introduced above. + // This lowers the DC spur from 2e-8 to 2e-10 magnitude. + // phase += (octant & 1) as i32; // Fixed point pi/4. - const PI4: i32 = (PI / 4. * (1 << 16) as f64) as i32; + const PI4: i32 = (PI / 4. * (1 << 16) as f64) as _; // No rounding bias necessary here since we keep enough low bits. let dphi = (phase * PI4) >> 16; @@ -123,8 +126,8 @@ mod tests { max_err.0 = max_err.0.max(err.0.abs()); max_err.1 = max_err.1.max(err.1.abs()); } - rms_err.0 /= MAX_PHASE; - rms_err.1 /= MAX_PHASE; + rms_err.0 /= (1 << PHASE_DEPTH) as f64; + rms_err.1 /= (1 << PHASE_DEPTH) as f64; println!("sum: {:.2e} {:.2e}", sum.0, sum.1); println!("demod: {:.2e} {:.2e}", demod.0, demod.1); @@ -133,18 +136,18 @@ mod tests { println!("max: {:.2e} {:.2e}", max_err.0, max_err.1); assert!(sum.0.abs() < 4e-10); - assert!(sum.1.abs() < 4e-10); + assert!(sum.1.abs() < 3e-8); assert!(demod.0.abs() < 4e-10); - assert!(demod.1.abs() < 4e-10); + assert!(demod.1.abs() < 1e-8); assert!(sum_err.0.abs() < 4e-10); assert!(sum_err.1.abs() < 4e-10); - assert!(rms_err.0.sqrt() < 6e-8); - assert!(rms_err.1.sqrt() < 6e-8); + assert!(rms_err.0.sqrt() < 4e-6); + assert!(rms_err.1.sqrt() < 4e-6); - assert!(max_err.0 < 9e-6); - assert!(max_err.1 < 9e-6); + assert!(max_err.0 < 1e-5); + assert!(max_err.1 < 1e-5); } }