[time-nuts] Audio format with embedded timestamps?

Bob Camp kb8tq at n1k.org
Thu Dec 1 13:30:19 EST 2016


If you are after one of the AM modulated versions of IRIG, an MCU with
a flash image of a 1 and a 0 as a sine wave to feed the DAC or PWM 
will get you a long way. If it’s an ARM with DMA to drive the PWM, there isn’t 
much overhead involved.  You may be looking at a < $1 chip. At that point
it’s not a real big decision. Syncing things up to a few microseconds isn’t 
all that hard in this case. 


> On Dec 1, 2016, at 1:09 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> IIn theory it is  pretty easy, first discipline the system clock using
> something like NTP then interrupt on the system clock and the interrupt
> handler runs a state machine. But if you need to output IRIG-B and when to
> generate the sine wave carier in software you end up with latency in the
> audio output because it buffers.   Baseband output of a one bit output
> could work under Linux but then you'd need external hardware modulate a
> carrier.
> I think IRIG is best done on a small microcontroiller, perhaps tuning a
> light weight RTOS where you can set up a hard 1KHz hardware interrupt.
> Either way I think yu have three threads
> 1. Read the GPS PPS and use this to phase lock some kind of oscillator.
> NTP does this.  If will lock the system clock to GPS
> 2, Generate interrupts at the IRIG frame rate from the above sync's
> oscillator/clock
> 3. output points on the carier that are modulated by the frames that are
> computed in thread #2.
> You might be able to write loop that in effect implements three threads by
> polling the system clock.  You end up with one if statement per thread that
> reads IF (time do do thread N) {run thread N}
> or you let the POSIX thread system do the work or yu are running on a small
> uP and set up hardware trimmer interrupts or let the RTOS do that for you.
> Thinking about this, IRIG-B generation would be best done inside the same
> uP that runs a GPSDO.
> On Thu, Dec 1, 2016 at 8:33 AM, Mark Sims <holrum at hotmail.com> wrote:
>> I played with trying to get Lady Heather to generate an IRIG audio
>> signal.   I never got it to work well... maybe someday...
>> -------------------
>>> One other idea:  Use a stereo audio format and record your audio to one
>> track and a time code (like IRIG) on the other track.   IRIG works even on
>> analog multitrack tape recorders.  Use it the same way on digital
>> multitrack recordings.  Google IRIG Time Code.
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> -- 
> Chris Albertson
> Redondo Beach, California
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