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Re: [Discuss-gnuradio] Expected behavior during underruns

From: Thomas Tsou
Subject: Re: [Discuss-gnuradio] Expected behavior during underruns
Date: Wed, 15 Dec 2010 14:19:27 -0500

On Wed, Dec 15, 2010 at 2:04 PM, Josh Blum <address@hidden> wrote:
>> It would be really nice if the send-function in the API have a TDMA
>> mode where fragments of samples are dropped when an underrun occur so
>> that the majority of the samples are aligned to the
>> start_of_burst_time (see above example (1) and (3)). With this
>> feature it is not necessary for the programmer to know if an underrun
>> has occurred and how to handle this. Instead the programmer knows
>> that if an underrun occurred some samples are dropped but the rest of
>> the sample are transmitted at the expected time without any offset.
>> Is this doable?
> This should already be the case. If you timestamp every packet, then the
> underflow recovery is handled by hardware. That is, the FPGA will drop
> until the timestamp is <= current time, and then wait until the
> timestamp == current time, so realignment is automatic.

I believe you mean timestamp is >= current time. Regardless though,
this is not the case based on my observations.

> Are you timestamping every packet? This calculation should be
> last_time_spec + time_spec_t(0, num_samps_in_last_packet, host_sample_rate)

Yes. I timestamp every packet. I also mark every packet
start_of_burst. I wonder if late unmarked packets will still transmit,
though this would not remove the necessity of detecting underruns on
the host.


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