The out of band noises of Eureka 147 DAB (digital audio broadcasting) signals produced by HPA (high power amplifier) nonlinearities were analyzed to reduce the interference to adjacent channels. The analysis used a typical TWTA (traveling wave tube amplifier) model with B (output backoff) to describe its nonlinearities. The results show that the linear increase of the B results in a linear decrease of the out of band noise power. Without a RF filter, B should be more than 9dB so that the out of band noises near the central frequency can achieve the DAB transmitting signal criterion. This result provides a necessary improvement for the design of DAB transmitting systems.