This paper proposes a reference signal for use in evaluating the speech quality of low bit-rate coded signals. The proposed signal is generated by adding a simulated low bit-rate coding distortion signal to the original signal. This low bit-rate coding distortion signal is modeled with both spectrum envelope distortion and additive white noise, so that the simulated coding distortion signal is synthesized using both Linear Predictive Coding (LPC) parameters derived from a signal who as spectrum envelope is distorted by spectrum-shaped noise, and by a LPC prediction error signal with added white noise. The simulated-signal parameters, the amount of spectrum-shaped noise and white noise, approximate the average characteristics of low bit-rate coding distortion signals. Experimental results show that the proposed reference signal suppresses variability in evaluation scores and improves their reproducibility in repeated assessment tests.

Keywords: speech quality, low bit-rate coding, reference signal, modulated noise reference signal.