remove delay clicks

Hi Florian,

This is a problem that affects anybody processing audio signals and has only one of two solutions. Audio signals swing up and down above (+volts) and below (-volts) a central baseline at 0 volts. Whenever the audio stream is suddenly cut off, the signal will jump from wherever it is to 0 volts in an instant, determined by the sampling rate. This will produce a sudden "click" in the audio stream that will vary from a tiny sound to a loud bang depending on how far away from the central baseline the signal voltage was at the time. This can be compounded if the signal is suddenly spliced into something else that creates a net swing from positive to negative or vice versa.

One method of eliminating this is to monitor the signal and do the switch when it crosses the zero volt line. You also have to be able to predict when the incoming signal will also cross the baseline and, given that any digital signal jumps in voltage from one sample to next, even that can result in a small click.

The clever way to solve this is to blend the signals during the transition, decreasing the outgoing signal to zero over a number of samples whilst simultaneously adding in the new signal at the same rate. You'd be surprised as to how few samples you need to use to do this (ten samples would be a good starting point) but the more samples you can handle the smoother the transition will be. My suggestion is that you create two audio buffers for the original and new signal. When the delay bang occurs, progressively divide the original signal by the number of samples chosen so that in that period the signal diminishes to zero. At the same time, progressively multiply the new signal in steps to reach its full amplitude as the original signal reaches zero. By adding the two signals together, a smooth, and hopefully inaudible, transition will occur. I appreciate that this will not be easy but the effort will be worthwhile.

If you can't buffer the signals and overlap them then you can almost achieve the same thing by diminishing the original signal to zero and then amplify the new signal from zero to its full amplitude. This might cause a slight "gap" in the signal but if you keep the transition short enough you will hardly notice it.

You can't use "digital" mutes for this as it will essentially leave you with the same unpleasant clicks. Most good mixing desks have voltage controlled amps that slowly decrease the gain to zero when the mute button is pressed, essentially achieving the same gradual transition from sound to silence as I have described above. Unmuting, ramps the gain back up from zero to full amplitude.

Does any of this help/make sense?

Andy.