Turtle art, reinstate sensor input

[tonyforster]

See tickets 343 and 155
Sensor input is disabled in TA V42
1)There are issues with alsaaudio on non XO architecture which need resolving.
2)Voltage should convert as avg, not rms (as does resistance)
3)Alsaaudio appears to be returning an undersize buffer occasionally in TAP V18 which crashes pitch ( unpack() is given an odd length string ) and I think produces occasional zero values in other modes

[erikos]

Moving it to 0.86 as it seems an enhancement.

[walter]

Given the recent discussion about doing physical experiments, this is of renewed interest/priority. But I don't think I could get it done before string freeze... deferring to 0.88.

[walter]

Closer, as we have the 0Install approach to bundles. Sascha also recommended I look into gstreamer as an end-run around the problem... will aim for 0.90.

[walter]

Based upon Measure code. Tested on XO 1.5. Need to add fft for pitch and test more extensively.

[walter]

I've tested on an XO 1.5 running 303 (Sugar 0.84) and an XO 1 running 703. Obviously we need more testing... but getting closer.

[tonyforster]

test results on xo1 gain and offset

[tonyforster]

Test results uploaded for 3 x XO1 - sensor gain.xls
resistance accuracy +-5% wrt full scale raw data
voltage +-3% of full scale
Hz accuracy +-46000/len(buf)

Hz = 46000/len(buf) x index
Volts = 0.00002225*float(_avg(buf))+1.140
Ohms = 2.718**(0.000045788*float(_avg(buf))+8.0531)

[walter]

update audio mode every time

[walter]

I released v103. It may still need some fine-tuning, but it is seeming working well enough to put it out into the world. We should open a new ticket for bugs associated with sensors.

[tonyforster]

See #2527 for issues relating to XO-1.5 sensor initialisation

[tonyforster]

see #3258 Turtleart XO-1.75 sensor calibration