FFT bin based signal limiting
Volume extension includes limiting the magnitude of Fast Fourier Transform (FFT) frequency bins which allows increases to the perceived level of audio content without causing distortion. A soft limit and smoothing is applied to each FFT bin is to prevent or reduce distortion while maximizing output volume. Frequency resolution is significantly improved compared to volume extension methods utilizing filterbanks and hard limiting, and distortion is reduced because no hard limiting occurs.
1. A method for signal processing audio signals, the method comprising: receiving a time domain input signal; windowing the input signal; performing an FFT on the windowed signal to obtain complex FFT bin data; converting the complex FFT bin data into magnitude and phase FFT bin data; limiting the magnitude of the FFT bin data, wherein limiting the FFT bin data comprises selecting a limit value and setting all values above the limit value to the limit value; converting the limited magnitude and phase FFT bin data into limited complex FFT bin data; performing an inverse FFT on the limited complex FFT bin data to obtain limited time domain data; assembling a limited signal from the limited time domain data; and driving an acoustic transducer with the limited signal to generate sound waves.
2. A method for signal processing audio signals, the method comprising:
receiving a time domain input signal;
windowing the input signal using a Hanning window and 75 percent overlap;
performing an FFT on the windowed signal to obtain complex FFT bin data;
converting the complex FFT bin data into magnitude and phase FFT bin data;
limiting the magnitude of the FFT bin data using soft smoothing;
smoothing the limited magnitude of the FFT bin data, converting the limited and smoothed magnitude and phase FFT bin data into limited and smoothed complex FFT bin data;
performing an inverse FFT on the limited and smoothed complex FFT bin data to obtain limited time domain data;
applying the Hanning window to the limited time domain data;
adding the windowed limited time domain data to an accumulator;
taking the left most N/4 values in the accumulator to obtain N/4 samples of a limited signal;
shifting the values in the accumulator N/4 paces to the left;
setting the N/4 right most values in the accumulator to zero;
converting the limited signal to a limited analog signal;
amplifying the limited analog signal; and
driving an acoustic transducer with the amplified limited signal to generate sound waves for a listener.
3. The method of claim 1 , wherein:
the limited time domain data comprises a frame of inverse FFT data of size N; and
assembling a limited signal from the limited time domain data comprises:
shifting values in an accumulator of size N, M samples to the left;
setting the M right most values in the accumulator to zero;
adding the limited time domain data to the accumulator;
taking the left most M values in the accumulator to obtain M samples of the limited signal; and
repeating the shifting, setting, adding, and taking to obtain another M samples of the limited signal.
4. The method of claim 3 , wherein M is equal to N/4.
5. The method of claim 4 , wherein driving an acoustic transducer with the limited signal to generate sound waves comprises:
converting the limited signal to a limited analog signal; and
driving an acoustic transducer with the limited signal to generate sound waves for a listener.