Spent some time yesterday evening building up a stereo FM receiver in
GRC. I know there are already a few of these for broadcast FM,
but for personal education reasons, I thought I’d build something up.
Attached is my result.
It seems to work reasonably well (better than the existing example WFM
stereo receiver on the same signals).
But something I’ve discovered is that about 30% of the program material
coming out of my fave radio station doesn’t even have
any L-R signal present. I mean, I can understand during
announcements, commercials, etc. But a lot of the musical material
doesn’t have any L-R, even modern material.
But I never really knew the gory details of FM Multiplex before–didn’t
know the L-R channel was DSB-SC, for example. Never had cause
to think deeply about it. I knew there was a 19kHz pilot tone, and
that it was doubled to 38kHz (usually with a PLL), but I always thought
it was used as a switching signal of some sort.
If I get keen, I’ll extend this to include RDS decoding.
The is a lot of really cool reasons for the stereo to be the way it
is, check out Basics of Stereo Multiplexing for FM Transmission it’s an
amazing read.
Also the problem of no stereo signal is probably with modern music,
there is not much difference between L and R anymore, just a single
channel of mass produced noise, so since the’re subtractive, silence
is broadcast on the L-R signal, not like days past when Pink Floyd
would fly you though space on both channels!
On 02/19/2012 12:46 PM, Andrew D. wrote:
The is a lot of really cool reasons for the stereo to be the way it
is, check out Basics of Stereo Multiplexing for FM Transmission it’s an
amazing read.
Also the problem of no stereo signal is probably with modern music,
there is not much difference between L and R anymore, just a single
channel of mass produced noise, so since the’re subtractive, silence
is broadcast on the L-R signal, not like days past when Pink Floyd
would fly you though space on both channels!
Yes, I recognize that a lot of modern mixing leads to almost no stereo
separation.
But this is a classic rock station. I would expect the really-old
stuff (old Beetles, for
example) to be mono, but stuff produced in the 1970s and 1980s you’d
expect to
have good stereo separation.
Ironically, the advertisements have better stereo than the music
sources!!
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
I find quite the opposite, 60’s bands will sometimes have the most
stereo separation. When stereo recording came out they made the most
of it, they would put whole instruments on just one channel, it would
make you feel like you are in the middle of the band. Then the 70’s
“wall-of-sound” came out and it started to be had to pull generated
instruments to one side without ruining the effect. Now its all just
mono again.
In this digital age its kinda strange we still use analog modulation.
If it were digital, and there was not L-R information it would just
encode the one signal better, not wasting information and bandwidth
like FM.
On 02/19/2012 01:08 PM, Andrew D. wrote:
encode the one signal better, not wasting information and bandwidth
like FM.
So, on a related topic, how do commercial FM-MPX receivers maintain
proper balance between
L-R and L+R – do they just have a gain control that they tweak at
the factory and glue in place?
If the gain balance is off, separation starts to go to heck really
quickly. And obviously, you can’t use
AGC, since that would cause a lot of noise to be injected when the
L-R channel was mostly zero.
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
Up until about 20 years ago when digital stereo generation started be
perfected and for a period after that, providing good stereo separation
better than 40 dB was pretty hard. I developed an early digital stereo
generator in 1979 that provided about 55 dB but much of that was
dependent on the low pass filter that I had to have after the generator
in order to reduce third harmonic products of 38 Khz. I ended up
putting a third order Butterworth filter in the unit. I also had to
consider ringing as you really need to minimize this in order to prevent
over-modulation.
More modern stereo generators such as Aphex’s digicoder and Bob Orban’s
and Frank Foti’s boxes provide stellar stereo separation now as
everything is in the digital domain.
The main problem with lack of stereo information or L-R content is the
original material and the how the material is processed for air. Once
upon a time, music was mixed with the home stereo environment in mind.
It sounded great. Early on many companies went way overboard on stereo
mixing (ie Decca) where the staging was just too wide and very little
energy was in the L+R domain.
That changed in the 70s and 80s when more aggressive broadcast
compression/limiting was available and in use by AM and FM stations. I
have run into many a mix engineer that would add compression and
limiting in order to “try” to have better control of what the music
sounds like over-the-air. A pretty standard procedure for these folks
is to have a little bootleg transmitter at a studio with an Orban or
Omnia processor in front of it. They will play back the cut they are
working on and run out to a car and see how the mix sounds. There are
even a series of “craptactular” speakers that you can buy and sit on
your mixing desk that will mimic the low-fidelity sound systems in most
cars.
So, radio stations are pushing recording engineers to process their
music poorly and radio stations are creating their own cesspool of audio
by each trying to sound “louder” than each other. All of this means
that the less information you put in L-R the better.
Tim Pozar
Past Chief Engineer of to many stations to list here and usually ones
that were the softest on the dial.
Keep in mind that early stereo recordings were Left/Center/Right as
pan-pots were not available on some consoles. The stereo release of
Sgt. Peppers is a good example of this.
Tim
On Sun, Feb 19, 2012 at 03:19:21PM -0500, Marcus D. Leech wrote:
So, on a related topic, how do commercial FM-MPX receivers maintain
proper balance between
L-R and L+R – do they just have a gain control that they tweak at
the factory and glue in place?
If the gain balance is off, separation starts to go to heck really
quickly. And obviously, you can’t use
AGC, since that would cause a lot of noise to be injected when the
L-R channel was mostly zero.
There is a well known design trick… the level of the DSBSC
difference signal in the FM baseband was chosen such that if you sample
the FM baseband out of the FM detector - including the stereo subcarrier
energy - in phase with the suppressed 38 KHz carrier (eg on peaks or
troughs of the suppressed carrier) you get either L or R depending on
whether the samples are on carrier peaks or troughs. In effect the
difference signal sideband phasors either add (L+R) + (L-R) producing
2L or subtract it a half cycle later producing 2R.
This of course simply requires correct frequency response of the
FM detector and deemphasis network and suitable flat group delay for
good stereo separation… rather than carefully matched gain of the
baseband and subcarrier path.
–
Dave Emery N1PRE/AE, [email protected] DIE Consulting, Weston,
Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
‘For Rent’ sign still vainly flapping outside on the weed encrusted pole
- in
celebration of what could have been, but wasn’t and is not to be now
either."
Right, all instruments and voice are mono sources, so to make stereo
they just pan the instrument tracks to one side or another, with the
Beatles and the like, they had the original instruments tracks so the
could re-release them as stereo by moving the tracks around. My
example was Pink Floyd, as they would pan instruments back and forth
to give the effect of flying around though space or something, almost
like using stereo as an instrument.
But as said, nowadays all people have are crappy iPod ear-buds and car
stereos, so you cant really hear anything anyway.