[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Robert McGwier
Subject: Re: [Discuss-gnuradio] Stereo gain "hacked"
Date: Tue, 14 Feb 2006 00:13:31 -0500
User-agent: Thunderbird 1.5 (Windows/20051201)

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.


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.

Discuss-gnuradio mailing list

AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
Laziness is the number one inspiration for ingenuity.  Guilty as charged!

reply via email to

[Prev in Thread] Current Thread [Next in Thread]