Audio codecs are very good at retaining signal phase as the human ear is very sensitive to errors in this. Whilst I'm not saying it is impossible, I think it would be very unlikely that the interval change is due to transcoding.
Also, as it is a moving receiver this effect is almost expected.
Sadly the towed pinger locator (TPL-25) is omni directional.
But by analysing the inter-ping interval you can determine a CPA point. Something below I wrote on reddit about this technique:
Due to the pulse length (10ms) there is an inherent 100Hz bandwidth in the signal, which is equivalent to the frequency shift you would expect if moving at 4m/s. In other words, Doppler measurements of a single pulse is NOT going to be useful in attempting to localise.
However, because the pinger has a relatively stable repetition rate (about 1.106s from what I could see) you can do a cool trick to localise it (assuming the source is stationary):
- Perform a pass over the area of interest (ie where the pinger is)
- measure the ping times of arrival (either manually with waveform inspection or write an envelope detector)
- plot the ping times of arrival versus the time of arrival modulo ping interval. ie in MATLAB: plot(toa, mod(toa, 1.106), '.');
- The plot indicates the relative distance from the source / receiver. Hopefully the plot should contain a local minimum representing a closest point of approach (ie like http://imgur.com/v1ZclaX ). This means that the source lies in a line perpendicular to the vessels course centred at the location of the receiver (ie TPL-25) at CPA time
- Perform another pass over the area of interest on a course perpendicular to the first
- measure the ping times of arrival and look for CPA as before
- You should now have two intersecting lines. Guess what should be at the intersection? :)
This relies on having a stable ping repetition rate.
BTW this is the author of the linked post here :)
Rodney
Edit: @ISL has hit the nail on the head there. Hyperbolic trilateration using multiple simultaneous receivers is the way to go. RAAF will be trying this with sonobuoys deployed from P3-C Orions
In fact they explicitly have not launched the sidescan capable Bluefin21 AUV because of fears of acoustic contamination of the towed pinger audio.