In this paper, we present a variable bit rate control method for speech/audio coding, under the constraint that the total bit rate of a super-frame to be a constant. The proposed method uses a trellis diagram for optimizing the overall quality of the super-frame. In order to reduce the computational complexity, the trellis diagram uses approximation by ignoring the encoder memory state between different paths. Simulations on the AMR Wideband show that the proposed variable bit rate control achieves up to 4.3 dB improvements to the constant rate coding in the perceptual weighted SNR.