Re: [Discuss-gnuradio] Stereo gain "hacked"

[mgray]

Here is a link to the RBDS standard:

ftp://ftp.rds.org.uk/pub/acrobat/rbds1998.pdf


And another link to the digital AM radio standard:

http://www.ibiquity.com/technology/pdf/Waveforms_AM.pdf


On Tue, 14 Feb 2006, Robert McGwier wrote:

> Al, Matt:
> 
> Thanks for your input.  I am pretty ignorant about these FM standards 
> and basically I googled my way to a reasonable spec,  drew the flow 
> graph and coded it.  The stereo demux was definitely not working with 
> DEemphasis applied to the composite signal.  And it was clear why.  The 
> taper was awful and the lower sideband was approximately 10 dB higher 
> than the USB on the L-R, DSBSC stereo signal.  I removed the deemphasis 
> and voila, stereo.  That said,  when the pilot/carrier is 20 dB out of 
> the noise in the 512 pt FFT, that should be strong enough that you do 
> not have hiss that is irritating.  But the COMPOSITE signal is what is 
> FM modulated but I bet they do not preemphasize the composite signal. 
> They might do preemphasis on the L+R baseband signal as Matt suggests 
> but I just don't know about the others.  I am trying to understand what 
> about that would make sense.  It would be very interesting to read a 
> real spec.  I have googled and not really found one a  useful one.
> 
> I hate this RDS Top Secret MONEYWORD specification.  Frank and I are 
> determined to fix their wagon.
> 
> I did not know about Achilleas work.  I will search the archive.
> 
> Bob
> 
> 
> 
> al davis wrote:
> > On Monday 13 February 2006 20:06, Matt Ettus wrote:
> >   
> >> I seem to remember that the preemphasis on stereo signals is
> >> not performed on the multiplexed signal.  It is actually
> >> performed on the audio components separately, before mixing
> >> with the stereo subcarrier. Therefore deemphasis needs to be
> >> done after the stereo part is mixed back down to baseband.
> >>
> >> To me, this is backwards, and is not useful, but I think that
> >> is the standard.  Achilleas sent an email to the list several
> >> months ago on this subject.  He also had a simple
> >> implementation that did stereo.
> >>     
> >
> > Actually it is very useful the way it is.  It would be bad to 
> > preemphasize the composite signal.
> >
> > To preemphasize the composite in effect converts the system to 
> > phase modulation.  The subcarrier would then consume most of 
> > the bandwidth, and the baseband (mono) would be drastically 
> > reduced, resulting in a significant reduction in apparent mono 
> > signal to noise ratio. Stereo signal to noise ratio would be 
> > about the same as it is now, but there would be no advantage to 
> > mono.
> >
> > In the existing system, if stereo SNR is not good enough, you 
> > can switch to mono and get almost as good SNR as if stereo 
> > wasn't there.  "Almost" means about a 6 db degradation, which 
> > is due to the fact that baseband modulation must be lowered by 
> > that much.
> >
> >
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> 
> 
>