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

[al davis]

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.