// Produce a sinusoid of maximum amplitude.
        let mut data_source : Vec<f32> = (0u64..).map(move |t| t as f32 * 900.0 * 2.0 * 3.141592 / SAMPLES_RATE as f32)     // 440 Hz
                                    .map(move |t| t.sin()).take(100000).collect();