Re: [Discuss-gnuradio] UHD USRP Sink Stream Tagging

[Josh Blum]

On 07/17/2012 08:31 PM, King Qiu wrote:
> Hi Josh,
> I have done the same work as Alex. I modified the code in
> gr_message_source. I added a tx_sob tag and a tx_time tag to the first
> byte of the message, and a tx_eob tag to the last byte of the message.
> Assuming:
> 1. there are 100 bytes in a message(or we call it a packet)
> 2. bpsk modulation is employed
> 3. set samples_per_symbol to 2
> So there are 800 symbols or 1600 samples in the packet. 
> In the output of the gr_message_source block, the 0 byte is tagged with
> the tx_sob and the tx_time, and the 99 byte is tagged with the tx_eob. 
> 
> Then in the output of the symbol map block, the 0 symbol is tagged with
> the tx_sob and the tx_time, and the 792 symbol is tagged with the
> tx_eob. 
> 
> After pulse shaping filter, the 0 sample is tagged with the tx_sob and
> the tx_time, and the 1584 sample is tagged with the tx_eob.
> 

I think its important to note that certain gnuradio blocks do not pass
tags because it is undecided how to interpolate the sample index. Tom
would have more to say about this.

> My questions are as follow:
> 1. The 1599 sample is the last sample in the packet, but the tx_eob tag
> are added to the 1584 sample. So, what will happen to the 1585-1599
> samples in the usrp? Are they transmitted by the usrp immediately after
> the 1584 sample? 
> 

Yes, these samples will be send ASAP, and they will not have an EOB, so
the device will regard this as underflow

> 2. Is it necessary to adding the tx_eob tag exactly to the last (1599)
> sample? How can I do this?
> 

The sample index is one of the parameters of the tag

-josh