[time-nuts] GPS Talking Clock

Nick Sayer nsayer at kfu.com
Sat Feb 17 23:01:58 EST 2018

It’s been a while since I’ve posted here, but I’ve had a bunch of irons in the fire. I’m working on adapting my GPSDO to the ECS ECOC-2522, which the manufacturer claims has a short term ADEV in the low -12s, but I haven’t gotten it doing that well yet.

But one thing that is ready (well, electronically it is - I’m still working on the laser cut case for it) is my GPS Talking Clock.

The story is that I called the USNO time number at midnight on New Year’s Day, but the wife noted that it was the wrong time zone. That got me thinking, and I wound up designing a GPS driven simulacrum.

It’s an ATXmega32E5 with the usual Venus838 timing module and a µSD card slot. The card is loaded with audio samples that the 32E5 plays back through its DAC. I got double-buffered DMA to work to feed the DAC, so audio playback is a largely background task. I just have to fill the buffer with the next block from the file every ~30 ms or so. The ticks and beeps are generated from an on board 1 kHz source and are turned on by a PPS ISR, so they’re as accurate as possible. The whole thing is basically as accurate as an aural clock can be - the latency induced by the speed of sound has far more impact than anything else.

While the audio is turned off, the clock can also do Westminster Quarters (or any other chime you wish to load in).

The µSD card is FAT formatted and the audio sample files are easy to make with ‘sox’ (raw, 1 channel, 8 kHz, 16 bit little-endian, unsigned), so there’s no reason you can’t substitute my voice with your own, or make your own chimes.

It’s available at https://www.tindie.com/products/nsayer/gps-talking-clock/

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