| 摘要 |
There is no room for a large speaker in miniature stereo players or a flat-screen TV, so the speaker itself tends to have a poor low frequency response. To provide an improved impression of low frequency reproduction the LF audio signal is isolated 22 and its frequency is multiplied 10 before it is recombined 25 with the HF audio signal. By a psychoacoustic effect the listener perceives the existence of a low frequency fundamental sound component from the presence of the overtones. Frequency multiplication is effected by storing each cycle between positive-going zero-crossings of the digital signal and then reading it out from memory twice, at twice the storage rate (figures 3 and 5). By switching waveforms at zero-crossings there are no discontinuities in the output. A zero-crossing threshold may have a small offset (figure 6), to limit the shortness of the repeated waveforms. The repeated waveform may span several cycles (figure 7). The waveform of very long cycles may not be repeated (figures 8-9). |
