Re: [Discuss-gnuradio] NTSC Demo

[Prateek Dayal]

Hi,

Actually with the color subcarrier at 3.58 Mhz, the color bandwidth
should go upto 4.08 Mhz but if we try to limit signal due to this, we
run into a problem because of finite filter roll offs .. it is not
necessary to interpolate to 2x .. we could interpolate to 1.5 x and
get the work done .. I just interpolated it to 2x because it was
easier that time for me to do it ...

we can however do this ....

orig signal --> supress audio (either at baseband or passband) --> 1.5
x --> get the picture carrier to baseband --> video

in fact even for audio decoding we should be able to put a bandpass
filter and then run the quad_demod script .. only thing we will have
to compensate for the audio_carrier term in the phase but that is just
simple maths ...  this way we save the cost of downconversion ... this
filtering for audio decoding can be done with IIR filter as they save
computational cost and also there will be no effect of envelope
distortion on FM deociding ...

Regards

prateek 

On 6/21/05, Thom Rainwater <address@hidden> wrote:
> > 
> > > 
> > > > 
> > > > > 
> > > 
> > 
>  
> > If I understand it well you do the following to get rid of the
> audio_carrier:
> > interpolate 2x to avoid aliasing
> > downconvert audio-carrier to zero
> > highpass to get rid of audio-carrier
> > 
> > 
> > 
> > 
> > downconvert video-carrier to zero
> > get absolute value(get envelope)
> > decimate 2x
> > 
> > The above steps can be done in gnuradio with the components which are
> already 
> > there
> > interpolator
> > freq_translating_fir_filter (which you feed high_pass fir filter taps)
> > multiplication (or another freq_translating_fir_filter with low_pass
> filter 
> > 
> > 
> > 
> > 
> > taps, this way you get a band_filter)
> > abs
> > decimator
> > 
>  
> 
>  
>  Is there a way to get these same results w/o interpolating the entire
> signal? See attached diagram. Can we assume that the significant spectrum of
> the video component is less than 4 MHz? If so, can we take the incoming
> signal sampled at 8 MS/s complex do our audio processing to get the audio
> signal, then filter out audio component, translate the video component and
> do the video demod?
>  
>  Thanks
>  
>  Thom Rainwater
>  
>  
>  
>  
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> 
> 
> 


-- 
Prateek Dayal
B.Tech 4th Year
ECE
IIT Guwahati

www.geocities.com/pmd_iitgw