Re: [Discuss-gnuradio] wfm_tx and nbfm_tx

[Firas Abbas]

Hi Dimtris,


--- On Fri, 12/19/08, Dimitris Symeonidis <address@hidden> wrote:
> the description should be changed to show that:
> - blks2.wfm_tx can receive a maximum audio rate of <=16k
> (instead of > the declared >= 16k)
> - blks2.nbfm_tx can receive a maximum audio rate of <=
> 4.5k (instead of >= 16k)
> 
> I hope this is clear enough...
> 
> 
> Dimitris Symeonidis


I think you have mixed between audio frequency bandwidth and audio digitizing 
rate.

The following example will explain it :

Suppose that you want to transmit nbfm signal for an input audio signal with 
maximum bandwidth of 3 KHz. Suppose also that this audio signal is digitized by 
(lets say) 32K PCM encoder which results in digitized audio rate of 32K. 

To produce nbfm signal, we do the following:

1) Ensure that the incoming digitized signal (32K) does not contains any 
frequency components above 4.5 KHz (this is done by a digital FIR low pass 
filter). 

2) Software interpolate the output of the digital FIR (32 K) to a rate suitable 
for transmission (lets say 256K) with interpolation rate of 8.

3) Do an FM Pre-emphasis to this 256K signal (this is how it was done in the 
nbtx.py). The output is also 256K.

4) Feeding this 256K to a frequency modulator (VCO) which produces complex 
output signal.

5) Stream this output complex signal to the USRP with interpolation rate of 500 
to produce 128M which is fed to its DAC.


I hope this is clear now.

Best Regards,

Firas