[op25-dev] P25 symbol timing recovery: tone detection method

P25 signals modulated using the CQPSK format throw off a loud tone at 4,800 Hz, as shown by a plot of the power spectrum of the signal's magnitude. C4FM-modulated P25 signals do not appear to possess this interesting property. The file in samples 'prwn-d256.dat' appears to be a CQPSK signal with distinctive LSM characteristics. I've uploaded a simple python app which can be used to plot a complex capture sample file - magsq-psd.py. The one non-obvious arg (--calibration) must be used if/as necessary to bring the signal to near zero IF. Note that hardware FM discriminator-tapped receivers cannot be used here since a copy of the signal prior to demodulation is required. A receiver software implementation using the tone detection method can be done using a low-complexity setup such as a DSP CIC filter. Another nice thing is that a tone detection loop works well even when there is some frequency mismatch... Max