Filtering out unwanted square wave (Radio: DCS/DPL signal) I've need to remove a DPL/DCS signal from a recording. Roughly speaking, a DCS signal is a low frequency (67.15Hz) square wave sent at the same time, over the same carrier, as speech. Because it is a square wave, it has many strong harmonics that overlap with speech. Obviously, the speech must be preserved.

On the surface, this seems like a problem that might be solved the same way as removing 60Hz power-line noise: lots of notch filters. However, power-line noise tends to be weaker and comes from a source that is much closer to a sine wave. 

So, my question is: is there a better way to do this?

This link contains more info about DCS: http://onfreq.com/syntorx/dcs.html
It mentions "Since DCS creates audio harmonics well above 300 Hz (i.e. into the audible portion of the band from 300 to 3000 Hz), radios must have good filters to remove the unwanted DCS noise." Ha!

EDIT: DCS is not a "square wave". I think the correct way to explain it is to say it's a NRZ binary code clocked at 67.15 Hz.
If you know that it is a square wave, and the frequency is roughly constant, I would think about synthesizing an inverted copy of it and adding it to the signal.  The remainder would be your speech, plus some residual harmonics of the square wave frequency, which might be dealt with by a 1-channel adaptive filter (see Sec. IIC of this link: http://www.cs.cmu.edu/~aarti/pubs/ANC.pdf).  It's possible that the adaptive filter will do it all by itself--I have used them with good results on periodic interfering signals with attention to the delay setting involved.
In order to get the parameters of the inverted signal, you would do one of two things:
1. Fit a two-parameter square wave to the signal using least squares minimization (two parameters being the initial time delay and the amplitude, or add other parameters as you see fit).
2. Use an FFT to estimate phase and amplitude of the fundamental sine wave, then use the theoretical ratio of the amplitude of a square wave to its fundamental (isn't it 2/pi?) to reconstruct the square wave from that.

Which of these two approaches you pick depends on the bandwidth of the speech and its amplitude relative to your DCS signal.  Good Luck!