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Re: [Discuss-gnuradio] NTSC Demo

From: Prateek Dayal
Subject: Re: [Discuss-gnuradio] NTSC Demo
Date: Thu, 16 Jun 2005 23:59:58 -0700


Thanks for the comments on the code ....

1. yes we can use a FIR notch filter but we have to be careful that
the color subcarrier part of the signal (which is close to the audio
carrier) is not supressed ... I think with just a zero placed around
audio carrier this may be difficult. I will however test the
assumption and let everyone know

2. I saw the PLL code for HDTV sometime back .. I think we can adapt
that to our needs ...

3. Also we do not need a PLL for the color subcarrier ... we need it
only for the downconversion ... for the color subcarrier we just need
an estimate of the instantaneous frequency ... after that we can use
this to calculate the instataneous phase value and decode the color
.... The PLL for downconversion is however required for the proper
functioning of the comb filter ....



On 6/16/05, Martin Dvh <address@hidden> wrote:
> Hi,
> I am trying to understand the matlab code right now.
> (I hardly ever use matlab so I don't now the syntax, most of the time if I 
> want to do some math I just program it in C++)
> 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
> I think you could also just use a fir notch filter to get rid of the 
> audio-carrier in one step.
> I need to study matlab more to understand the correlators you are using.
> I also think that using a PLL for the downconversion of video and a PLL for 
> the color subcarrier is indeed essential for correct working.
> I think there are already some pll implementations in gnuradio_blocks (HDTV) 
> floating around.
> I just tried to think of my own gnuradio pll implementation and my idea is 
> the following (in pseudocode):
> pll:
> src_c=input_c
> osc_c=sig_source_c(freq,sinus,ampl,phaseoffset)
> lowpass1settings:
> can be hight cutoff and width transfer, maybe even leave away completely
> lowpass2settings:
> should be quite low cutoff, transferwidth ??
> probably use IIR here.
> this filter and gainfactor2 determine the loop stability and phasenoise
> But also the lock-in range
> loop:
> /*the congruent is not really needed because the loop will adjust it away)
> /*otherwise we can just use -freq */
> prod_c=mult(src,congruent(osc))
> envelope_method1_f=lowpass1(abs(prod_c),lowpass1settings)
> /*less accurate 
> envelope_method2_f=lowpass2(realpart(prod_c),lowpass2settings)*/
> phasediff_f=abs(lowpass(imagepart(prod_c))
> freq+=phasediff*gainfactor2
> /*or is it -= */
> The loop would continuously adjust the downconversion frequency and output 
> the video baseband envelope signal.
> (Notice this is all pseudocode)
> greetings,
> Martin
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