The Ultra Low Delay (ULD) codec developed at the Fraunhofer IDMT is based on a versatile perceptual audio coding method that achieves very low encoding/decoding delay and is nevertheless capable of high compression ratios. Utilizing a perceptual model for irrelevance reduction, the ULD codec is in principle a variable bit rate codec. To achieve coding with constant bit rate, the use of bit reservoir techniques would result in additional coding delay. This paper presents a rate loop which ensures constant bit rate coding without increasing coding delay. It is shown that this technique does not decrease the decoded audio quality significantly.
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