yeah , that's not a bad solution Stuart. I like the simplicity.

especially for the a NON diversity  setup.

In the diversity setup- you really never lose packets, they just go further into the noise, so for the diversity setup, lowest BW/SNR  possibly is the go (minimum data rate/ BW )

and for  non diversity setup, to make use of some of the waste (IE there isn't much penalty so you might as well make the most of the overhead penalty ) , the this&last method you describe is worthwhile.

I once built a modem that has NO preamble....

It was a audio band OFDM modem, 36 carrier, differential BPSK, . It needed TWO OFDM frames for the message

1st OFDM frame is phase reference, second OFDM frame (using the first as a differential reference)  was the message as one wide word - roughly 15 bits of data and 21 bits of parity (LDPC) ....you dont need a CRC with LDPC, it comes for free.

I used to to transmit the state of an analog audio FM link on key up, and key down...

audio OFDM because it had to go through 'any' FM radio, Use carriers spaced 62Hz apart, approx 20mS for the burst...

g





On 20/09/2016 3:29 PM, Stuart Longland wrote:
On 20/09/16 08:26, glen english wrote:
He's is an experience to share... and all this I know already but when 
you experience just how frugal codec2 is on speech bits, it is 
enlightenment.....

I am doing a project that uses Codec2 and sends data over Si4460 
transceiver chips.

one thing that has become apparent is there is not much data coming out...

I send a packet every 20mS so that lost packets don't matter so much.
But.. all there is to transmit is ... 8 bytes ! :-)   there is 12 bytes 
wrapped around it..... Ha !

which means the data rate (and BW) has to go up  to cope with the 
overhead..... and David has worked so hard to get the bits down, and I 
just throw 1.5x excess in radio overhead...
One option might be to use some forward erasure coding on each Codec2
frame to give you two 8-byte packets, then in each RF frame, you send
packet 2 (or zeros if it's the first packet) from the previous Codec2
frame concatenated with packet 1 from the current Codec2 frame.

You double your data rate, but if an RF frame gets lost, you can still
recover the full message with no delay or loss.


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