dsp.py - sync code omits a necessary negative sign

[kleydon]

According to SoX documentation, a low pass filter is called by the underlying sox library using a negative sign, e.g sinc -4k, however the code in dsp.py ommits this negative sign. As a result, attempts to create a low pass sinc filter result in a high pass sinc filter.

[carlthome]

Yikes, good catch! @kleydon or @hadware care to fix this?

6740898

[hadware]

Right. Can you assign me to the task so I don't forget?

[hadware]

Thanks, sorry I forgot that, I'll correct it ASAP.